Lipid Metabolism

Chapter 2

Lipid Metabolism: An Overview Daniel Gyamfi, Enoch Ofori Awuah and Stephen Owusu Department of Medical Diagnostics, College of Health Sciences, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana

1. INTRODUCTION Lipid metabolism involves the synthesis of the structural and functional lipids (such as phospholipids, glycolipids, sphingolipids, cholesterol, prostaglandins, etc.) that are characteristic of individual tissues and the degradation of lipids to satisfy the metabolic needs of the body (e.g., energy production). Lipid metabolism is in a constant state of dynamic equilibrium. This means that some lipids are constantly being oxidized to meet the body’s metabolic needs, whereas others are being synthesized and stored. This chapter therefore focuses on the metabolic pathways through which the various categories of lipids are synthesized and/or degraded in the body.

2. FATTY ACID OXIDATION The breakdown of fatty acids (FAs) to generate energy is known as fatty acid oxidation or beta (b)-oxidation. It is called “b-oxidation” because most of the chemistry centers around the b-carbon of the acyl-CoA substrate.

CH3 - ………………………………..- CH2 – CH2 – CH2 – CH2 – COOH

(Omega carbon)

-Delta - Gamma - Beta -

Alpha -

Carboxyl

The fatty acid b-oxidation pathway occurs in the mitochondria and consists of a spiral of reactions, with the substrate reducing in size until the final set of reactions liberates two acetyl-CoA molecules (i.e., for even-numbered FAs). While the cytosolic short chain fatty acids (about 10 carbons or shorter) enter the mitochondria by diffusion, long-chain fatty acids require activation and translocation. The three main stages involved in the entire oxidation process are: activation of fatty acids, translocation of activated FA, and b-oxidation.

2.1 Activation of Fatty Acids Fatty acids in the cytosol are activated by conversion to coenzyme-A (CoA) thioesters, fatty acyl-CoA, by ATP-dependent enzyme acyl-CoA synthetase. The reaction is highly reversible, as ATP and the acyl-CoA thioester product both have equivalent energy levels (Ophardt, 2013). To prevent the reversibility, the reaction is coupled to pyrophosphatase, which catalyzes the hydrolysis of the pyrophosphate (PPi) released to two molecules of inorganic phosphate (Pi). A net of two ATP equivalents are consumed to activate one fatty acid to a thioester. Acyl CoA synthetase catalyzes the two-step reaction below: ATP þ FA/AMP  FA AMP  FA þ CoASH/FA  CoA þ AMP

The Molecular Nutrition of Fats. https://doi.org/10.1016/B978-0-12-811297-7.00002-0 Copyright © 2019 Elsevier Inc. All rights reserved.

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2.2 Translocation The activated fatty acid (fatty acyl CoA) combines with carnitine to create a fatty acyl-carnitine molecule, which helps to transport the long-chain fatty acid across the mitochondrial membrane via carnitine shuttle involving carnitine acyltransferase (CAT) I and II. The fatty acyl-CoA is first converted to acyl-carnitine by cytosolic enzyme CAT I, after which it can be transported into the mitochondrion. Once inside the mitochondrion, CAT II reforms the fatty acyl-CoA for the mitochondrial b-oxidation enzymes to degrade it.

2.3 The Reactions of b-Oxidation In the mitochondrial matrix, the fatty acyl-CoA undergoes a series of reactionsddehydrogenation, hydration, and oxidationdresulting in two-carbon (2C) fragments being removed from the carboxyl end of the acid to form a beta-keto acid. An additional mole of coenzyme-A is esterified to the beta-keto function, leaving acetyl-CoA and a fatty acyl-CoA of two less carbons. One round of b-oxidation involves four enzymatic steps: acylCoA dehydrogenase, enoyl-CoA hydratase, L-hydroxyacyldehydrogenase, and ketoacyl-CoA thiolase. Each round also generates one molecule of: FADH2, NADH, acetyl CoA, and fatty acyl CoA (with two carbons shorter per each round). In order to release a 2C unit from a fatty acid, an enzyme must break the bond between the a and b carbons, but cleavage of an unsubstituted carbonecarbon bond is extremely difficult (Ophardt, 2013). In order to allow the process to occur, the initial three enzymes (acyl CoA dehydrogenase, enoyl-CoA hydratase, and L-hydroxyacyldehydrogenase) must first activate the b-carbon, followed by cleavage by the thiolase (Fig. 2.1).

3. OXIDATION OF VERY LONG-CHAIN FATTY ACIDS Fatty acids above a certain chain length, usually greater than 22 carbons, cannot enter the mitochondria. These fatty acids are metabolized in the peroxisomes instead of the mitochondria (Voet et al., 2006). The peroxisomal b-oxidation pathways are basically similar to those of the mitochondria, with the main difference occurring in the initial acyl-CoA dehydrogenase reaction. FADH2 of acyl-CoA dehydrogenase in peroxisome transfers electron to O2 to yield hydrogen peroxide (H2O2) Fatty acyl CoA (initial)

Fatty acyl CoA (2C-shortened)) F

FAD

Acetyl CoA Thiolase

Acyl CoA dehydrogenase THIOLYSIS

OXIDATION

SH-CoA

Trans ∆2-enoyl CoA

β-ketoacyl CoA

NADH + H+ β-Hydroxyacyl dehydrogenase

FADH2

OXIDATION

HYDRATION H2O

Enoyl CoA hydratase

NAD + β-Hydroxyacyl CoA FIGURE 2.1 The b-oxidation of fatty acid. Fatty acyl-CoA is dehydrogenated (oxidation reaction) by FAD-dependent enzyme acyl CoA dehydrogenase to form an unsaturated molecule transD2-enoyl CoA with the release of FADH2. Enoyl CoA hydratase hydrates the double bond in transD2-enoyl CoA to form b-hydroxyacyl CoA, which is in turn converted to b-ketoacyl CoA with the introduction of ketone group by an NAD-dependent enzyme b-hydroxyacyl dehydrogenase. b-ketoacyl CoA undergoes thiolytic cleavage in presence of coenzyme A (SH-CoA) to liberate a 2-carbon fragment acetyl CoA in addition to acyl CoA molecule with two carbons less than the original. The cycle continues until all the fatty acids are completely oxidized. FAD, flavin adenine dinucleotide (oxidized); FADH2, flavin adenine dinucleotide (reduced); NADH, nicotinamide adenine dinucleotide (reduced); NADþ, nicotinamide adenine dinucleotide (oxidized).

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because there is no existence of electron transport pathway in the peroxisomes (Voet et al., 2006). Catalase then converts H2O2 into H2O and O2. The subsequent steps are identical to b-oxidation in the mitochondria.

4. OXIDATION OF ODD-CHAIN FATTY ACIDS Odd-numbered fatty acids are found in plants, herbivores, microorganisms, and some marine animals. These fatty acids undergo b-oxidation in the same way as even-numbered fatty acids elaborated earlier. However, the final cleavage product is a three-carbon compound, propionyl CoA, rather than acetyl CoA. Three enzymes convert the propionyl-CoA to succinyl-CoA (TCA intermediate): (1) Propionyl-CoA carboxylase (biotin-dependent) uses the energy in ATP to add a carbon, resulting in the formation of D-methylmalonyl-CoA; (2) Methylmalonyl-CoA epimerase catalyzes the next reaction by reversing the stereochemistry at the chiral carbon of D-methylmalonyl-CoA to form L-methylmalonyl-CoA; and (3) Methylmalonyl-CoA mutase converts the branched chain compound, L-methylmalonyl-CoA, into succinyl-CoA. Unlike acetyl-CoA, succinylCoA is not consumed in the TCA cycle as such odd-numbered fatty acids are gluconeogenic precursors.

5. OXIDATION OF UNSATURATED FATTY ACIDS Unsaturated fatty acid degradation demands two extra enzymes, enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase, in addition to the enzymes involved in b-oxidation pathway, but the extra enzymes’ function depends on the position of the original double bond. In situations where the original double bond is found in an odd-numbered position, normal b-oxidation will ultimately lead to the formation of a double bond at the C-3 position - D3-enoyl-CoA formation. As such, the double bond is moved to the C-2 position catalyzed by enoyl-CoA isomerase to form trans-D2-enoyl-CoA, the first b-oxidation intermediate. On the other hand, when the original double bond is on an even number, a D2, D4 conjugated intermediate will be formed from the usual b-oxidation. However, the enoyl-CoA hydratase cannot act on this conjugated substrate. Instead, 2,4-dienoyl-CoA reductase reduces the two conjugated double bonds to a single double bond at the C-3 position using electrons from NADPH. The D3 compound formed is converted to the trans-D2-enoyl-CoA by enoyl-CoA isomerase as indicated previously.

6. ENERGETICS OF FATTY ACID OXIDATION For the 16-carbon fatty acid palmitate, the activation step is followed by seven spirals of the b-oxidation pathway, resulting in the generation of eight acetyl-CoA, seven NADH, and seven FADH2 molecules. The eight cycles of the TCA cycle are required to oxidize the acetyl-CoA produced, resulting in the formation of 8 ATP, 24 NADH, and 8 FADH2. The equation for one pass, beginning with the coenzyme A ester of a 16-carbon fatty acid palmitate, is shown here: Palmitoyl  CoA þ CoA þ FAD þ NADþ þ H2 O/myristoyl  CoA þ acetyl  CoA þ FADH2 þ NADH þ Hþ In all, seven cycles through the b-oxidation pathway are required to completely oxidize one molecule of palmitoylCoA. The balanced equation for oxidizing one palmitoyl-CoA by the seven cycles of b-oxidation is: Palmitoyl  CoA þ 7 HS  CoA þ 7 FAD þ 7 NADþ þ 7 H2 O/8 Acetyl CoA þ 7 FADH2 þ 7 NADH þ 7 Hþ Considering an average production of 3 ATP per NADH and 2 ATP per FADH2 via the respiratory chain, 131 ATP molecules will be produced per palmitate. However, 2 ATP molecules are expended to activate palmitate, and therefore the net energy yield is 129 ATP as summarized in Table 2.1.

7. KETOGENESIS AND ITS IMPORTANCE Ketogenesis mainly occurs in the mitochondrial matrix of the liver through a series of enzyme-controlled reactions (Fukao et al., 2014). During fasting or starvation, blood glucose level decreases and excess acetyl-CoA from fat metabolism can be utilized to synthesize to ketone bodiesdb-hydroxybutyrate, acetoacetate, and acetone. These ketone bodies can serve as a fuel source if glucose levels are too low in the body (e.g., in times of prolonged starvation) or in uncontrolled diabetic patients where most of the circulating glucose cannot be utilized (Vance, 2008). In ketogenesis, two acetyl-CoA units first condense to form acetoacetyl-CoA catalyzed by a thiolase. The acetoacetylCoA then condenses with another acetyl-CoA to give hydroxyl methylglutaryl-CoA (HMG-CoA). This reaction is the ratelimiting step and is catalyzed by HMG-CoA synthase. HMG-CoA is a precursor of cholesterol and also an intermediate

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TABLE 2.1 Summary of the Energetics in Complete Oxidation of Palmitate Enzyme Catalyzing Oxidation Step

Number of NADH or FADH2 Formed

Number of ATP Ultimately Formed

Acyl-CoA dehydrogenase

7 FADH2a

14

b

21

b-Hydroxyacyl-CoA dehydrogenase

7 NADH

Isocitrate dehydrogenase

8 NADH

24

a-Ketoglutarate dehydrogenase

8 NADH

24

Succinyl-CoA synthetase

8

Succinate dehydrogenase

8 FADH2

16

Malate dehydrogenase

8 NADH

24

TOTAL

131

Energy expended

2

NET YIELD

129

a

1 FADH2 yields 2 ATP. 1 NADH yields 3 ATP.

b

that is subsequently converted into b-hydroxybutyrate, the primary ketone body in the blood. Thus, HMG-CoA lyase cleaves HMG-CoA to acetyl-CoA and acetoacetate. The hydroxybutyrate arises from acetoacetate by a reduction reaction. This interconversion is catalyzed by the mitochondrial enzyme 3-hydroxybutyrate dehydrogenase. Acetoacetate also continually undergoes spontaneous decarboxylation to yield acetone. Extrahepatic organs such as the brain, heart, kidney, and skeletal tissue can use ketone bodies as an alternative energy source (DiMarco and Hoppel, 1975). This keeps these organs/tissues functioning in conditions where glucose is limited. When ketones are produced faster than they can be used, they can be broken down into CO2 and acetone, which is exhaled from the body. A patient’s breath smelling sweet like alcohol (acetone) is one symptom of ketogenesis (Fukao et al., 2014). The concentration of total ketone bodies in the blood of well-fed mammals does not normally exceed 0.2 mmol/L, but in diabetic ketoacidosis, the concentration may even exceed 9.0 mmol/L (Voet et al., 2006).

8. FATTY ACID SYNTHESIS Excess dietary carbohydrate (glucose) and amino acids can be utilized to synthesize triacylglycerol in a process known as lipogenesis. Acetyl CoA, synthesized from glucose or amino acids, is used to produce fatty acids, which are subsequently esterified to form triacylglycerol. In humans, fatty acid synthesis occurs mainly in the liver and adipose tissue, in addition to the mammary glands during lactation (Voet et al., 2006). Fatty acid synthesis is similar to b-oxidation, however, acetyl groups are incorporated into a growing chain, and also reduction reactions are involved, which are the reversal oxidation reactions in the fatty acid spiral (Stryer, 1995). However, the mechanism of the pathway is distinctly different from being simply the reverse of b-oxidation in that it occurs in the cytoplasm; it utilizes NADPH as the reducing agent and acyl carrier protein instead of coenzyme A as well as a multienzyme complex called fatty acid synthase (FAS), an enzyme complex performing seven different functional activities in a single polypeptide chain. During fatty acid synthesis, acetyl-CoA supplies the carbon atoms while ATP and NADPH provide the requisite energy and reducing equivalents, respectively. The acetyl CoA, obtained from the degradation of glucose, amino acids, or ketone bodies, is unable to pass through the inner mitochondrial membrane to the cytosol, and, therefore, a bypass route is utilized where acetyl-CoA condenses with oxaloacetate catalyzed by citrate synthase to form citrate. Citrate can move freely across the inner mitochondrial membrane into the cytosol via either citrate/phosphate antiport or a citrate/malate antiport. Once in the cytosol, citrate is cleaved by ATP citrate lyase into acetyl-CoA and oxaloacetate. Then, the cytosolic acetyl-CoA is converted to malonyl CoA through carboxylation reaction catalyzed by acetyl CoA carboxylase. The malonyl CoA formed is used to synthesize the fatty acid chain catalyzed by FAS involving a repetitive condensation of 2C units (derived from malonyl-CoA), with each passage through the cycle resulting in the fatty acyl chain extended by two carbons.

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FA synthesis begins from the methyl end and proceeds toward the carboxylic acid end. Thus, C16 and C15 are added first and C2 and C1 are added last. For instance, the overall synthesis of palmitate from acetyl-CoA requires 14 NADPH and 7 ATP, as shown in the following reaction: Acetyl CoA þ 7 Malonyl CoA þ 14 NADPH þ 14 Hþ Y Palmitate þ 7 CO2 þ 14 NADPþ þ 8 HS  CoA þ 6 H2 O Once palmitate (C16:0) has been formed, it can undergo a number of modifications to produce longer and unsaturated fatty acids via chain elongation and desaturation, respectively, or it can undergo esterification into acylglycerols and cholesteryl esters. The free palmitate is first activated to acyl CoA (palmityl CoA) before it can be used to undergo any other metabolic pathway.

8.1 Fatty Acid Elongation Fatty acids longer than palmitic acid are produced by fatty acid chain elongation enzymes called elongases. These elongases lengthen palmitate to form many of the other fatty acids. Elongases are found in the mitochondria and endoplasmic reticulum (ER), and they usually act through the carboxy terminus. Two-carbon units are added from malonyl CoA (C units donor) during the elongation process involving enzymatic steps that are essentially the same as those carried out by FAS.

8.2 Fatty Acid Desaturation Plants, animals, and some prokaryotes can produce unsaturated fatty acids from saturated fatty acids by inserting a double bond (C]C) in the preformed saturated FA substrates. The double bond can be formed by removing two hydrogen atoms from the saturated FA in a process called desaturation. The desaturation process is catalyzed by a complex of three membranebound nonheme iron enzymes, namely, desaturase, NADH-cytochrome b5 reductase, and cytochrome b5, all located in the endoplasmic reticulum. In the desaturation pathway, O2 acts as the ultimate electron acceptor to introduce the double bonds.

8.3 Essential Fatty Acid Synthesis In most mammals, fatty acyl-CoA desaturase can introduce double bonds at the D4, D5, D6, and D9 positions but never beyond the D9 position. Hence, mammals cannot synthesize all the fatty acids containing a double bond at positions beyond C-9 and therefore have to be supplied in the diet (Chatterjea and shinde, 2011). Examples include linoleic acid (C18:2n-6, D9,12) and linolenic (C18:3n-3, D9,12,15). Linoleic acid has double bonds at the C-9 and C-12 (¼D9,12) positions, whereas linolenic acid has double bonds at C-9, C-12, C-15 (¼D9,12,15) positions. These polyunsaturated FAs are termed essential fatty acids (EFAs). Unlike mammals, plants have the ability to produce the EFAs and thus serve as a major source of these fatty acids. However, the polyunsaturated 20-carbon arachidonic acid (D5,8,11,14) is not found in plants, and can only be synthesized by mammals from linoleic acid (Rodrigo and Alfonso, 2013). EFAs act as precursors for the synthesis of eicosanoids, which are described latter in this chapter.

9. REGULATION OF FATTY ACID OXIDATION AND SYNTHESIS The balance between lipogenesis and lipolysis is a product of continuous neurohumoral regulation, which is dependent on feeding/fasting cycling and immediate energy requirements of the body (Greenberg et al., 2001). FA synthesis and oxidation are reciprocally regulated both in the fasted and fed states, with the preferential pathway depending upon the body’s metabolic state or requirements. Fed/postprandial state: In this state, the use of carbohydrates as fuel, glycogenesis and FA synthesis are favored. These occur as a result of increase in the level of insulin (secreted by b-pancreatic cells) in the blood. Insulin increases the transport of glucose into the cell (e.g., in adipose tissue) via glucose transporter 4 (GLUT4). It also stimulates glycolysis but inhibits gluconeogenesis and hydrolysis of stored triacylglycerol (TAG)dlipolysis; as such, no fatty acid is released from the adipose tissue (Berg et al., 2002). In addition, insulin stimulates fatty acid and TAG synthesis in the liver. It stimulates the enzyme lipoprotein lipase, which causes fatty acids to be removed from very low-density lipoproteins and those stored in the adipose tissue (Greenberg et al., 2001). Besides, insulin increases the availability of both pyruvate (for fatty acid synthesis) and glycerol 3-phosphate (for esterification of the newly formed fatty acids). Also, it causes the

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dephosphorylation of acetyl-CoA carboxylase, favoring TAG synthesis. This leads to increased formation of malonyl-CoA from acetyl-CoA and, consequently, the conversion of carbohydrates to TAG (Berg et al., 2002; Witters and Kemp, 1992). Fasted state/Starvation: Glucagon and epinephrine released during starvation and exercise trigger cyclic AMPdependent phosphorylation of acetyl-CoA carboxylase (i.e., enzyme inactivation) that shifts the equilibrium toward inhibition of lipogenesis and favoring of fatty acid oxidation. Most of the acetyl CoA is therefore shunted into the TCA for energy production. Thus, they cause a mass action drive of b-oxidation in the liver by stimulating hormone-sensitive lipase to release more FAs. Thus, in the fasted state, fat reserves serve as fuel during or after depletion of glycogen.

10. TRIACYLGLYCEROL BIOSYNTHESIS When carbohydrate, fat, or protein is taken in amounts exceeding the body’s energy demand, the excess is stored mainly in the adipose tissue as white depot fatdTAG. The other tissue that could store TAG under conditions of excess fats is the liver. The body depends on these fat reserves for energy and they enable the body to survive periods of prolonged fasting or starvation. Triacylglycerol is comprised of three fatty acyl molecules esterified to a glycerol backbone at the sn-1, sn-2, and sn-3 positions as mentioned earlier. The assembly of TAG occurs in the ER. The glycerol backbone, glycerol-3-phosphate (G3P), can be formed in two ways. In the liver, it is derived from glycerol by the action of glycerol kinase or the glycolytic intermediate dihydroxyacetone phosphate (DHAP) in a reaction catalyzed by the cytosolic NAD-linked glycerol-3-phosphate dehydrogenase. In the adipose tissue, the glycerol backbone is derived from DHAP only. G3P therefore serves as an important intermediate linking carbohydrate and lipid metabolic pathways. Fatty acid, in order to be attached to a glycerol backbone, has to be activated into fatty acyl CoA (Vance and Vance, 2002). This activation process is catalyzed by the enzyme fatty acyl CoA synthetase, located in the lumen of ER, in the mitochondrial outer membrane and matrix, and also in the membrane of peroxisomes. Following activation, the first fatty acyl CoA molecule is acylated to the hydroxyl group at sn-1 (C-1) of G3P by the enzyme glycerol 3-phosphate acyltransferase. This enzyme is the point of regulation in TAG synthesis. Further addition of a second fatty acyl CoA unit at C-2 by acyl-CoA:lyso-phosphatidic acid acyltransferase yields diacylglycerol-3-phosphate, commonly known as phosphatidic acid (PA). Before the final acylation, there is the removal of the phosphate group from PA catalyzed by phosphatidic acid phosphatase to generate 1,2-diacylglycerol. The two intermediates, phosphatidate and diacylglycerol (DAG), are important substrates for the synthesis of membrane phospholipids. DAG is then ultimately converted into triacylglycerols by transesterification with a third fatty acyl-CoA molecule at C-3. This final acyl-CoA-dependent acylation is catalyzed by the enzyme acyl-CoA:diacylglycerol acyltransferase. It should be noted that the type of fatty acids attached to the glycerol backbone could be of any type depending on the dietary intake. For example, glycerol backbone could have a first carbon attached with saturated fatty acid like palmitate, second carbon attached with a monounsaturated fatty acid like oleic acid, and third carbon attached with a polyunsaturated fatty acid like a-linolenic acid.

11. BIOSYNTHESIS OF MEMBRANE PHOSPHOLIPIDS 11.1 Glycerophospholipids Glycerophospholipids generally contain a glycerol, two fatty acids, a phosphate group, and an organic molecule (alcohol). In eukaryotic cells, phospholipids are synthesized from PA and DAG, intermediates that occur at a branch point in TAG biosynthesis. The synthesis of phospholipids occurs primarily at the surface of the smooth endoplasmic reticulum (van Meer et al., 2008). DAG, derived from PA, can be converted to cytidine diphosphate-diacylglycerol and used to synthesize many classes of phospholipids. PA thus forms the backbone for the synthesis of other phospholipid molecules. Prior to this reaction, one of the hydroxyl groups (usually from PA) is first activated by condensation of phosphatidate with the nucleotide, cytidine diphosphate (CDP), with the elimination of pyrophosphate (utilizing cytidine triphosphate [CTP] as an energy source). An activated phosphatidate, CDP-diacylglycerol, results from this reaction. Cytidine monophosphate (CMP) is then displaced in a nucleophilic attack by the other unactivated hydroxyl group to give rise to a particular phospholipid class.

11.1.1 Synthesis of Phosphatidylserine, Phosphatidylglycerol, Cardiolipin, and Phosphatidylinositol All the above phospholipid classes are formed from the activated CDP-diacylglycerol in enzyme-catalyzed displacement reactions. Displacement of CMP through a nucleophilic attack, by the hydroxyl group of serine or by the C-1 hydroxyl

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group of glycerol-3-phosphate, yields phosphatidylserine or phosphatidylglycerol-3-phosphate (PG-3-P), respectively. Phosphatidylserine synthetase and PG-3-phosphate synthetase catalyze each of the respective reactions. PG-3-P is processed further by cleavage of the phosphate monoester (with the release of Pi) in a reaction catalyzed by PG-3-phosphate phosphatase, to form phosphatidylglycerol. Both phosphatidylserine and phosphatidylglycerol can serve as intermediates in the synthesis of other membrane lipids by modification of the head group. In bacteria, the membrane lipid, cardiolipin (diphosphatidylglycerol), is synthesized through the condensation of two molecules of phosphatidylglycerol leading to the elimination of one glycerol moiety. For phosphatidylinositol (PI), it is synthesized by the condensation of CDPdiacylglycerol and L-myo-inositol (a cyclic poly alcohol), with the removal of a CMP molecule catalyzed by phosphatidylinositol synthase. In eukaryotic cells, the phosphorylation of the inositol ring of phosphatidylinositol by a series of kinases results in the production of other derivatives such as phosphatidylinositol phosphate (PIP), phosphatidylinositol bisphosphate (PIP2), and phosphatidylinositol trisphosphate (PIP3). These derivatives are commonly known as inositides or phosphoinositides, which are involved in lipid signaling, cell signaling, and membrane trafficking (Kohjiro et al., 2002).

11.1.2 Synthesis of Phosphatidylethanolamine Phosphatidylethanolamine (PE) or cephalin is formed by the decarboxylation of the serine moiety in phosphatidylserine by phosphatidylserine decarboxylase. PE is common in mitochondrial membranes (Vance, 2008). PE can also be synthesized from diacylglycerol via the CDP-ethanolamine pathway, using ethanolamine as the substrate (Fig. 2.2). In this pathway, ethanolamine is first phosphorylated in the cytosol by ethanolamine kinase to form phosphoethanolamine, the head group of PE. Phosphoethanolamine is then activated through a condensation reaction with CTP to form CDP-phosphoethanolamine. The reaction is mediated by the cytosolic enzyme, CTP:phosphoethanolamine cytidylyltransferase. CTP:phosphoethanolamine cytidylyltransferase is the rate-limiting enzyme in the CDP-ethanolamine pathway, which depends on the availability of both CDP-ethanolamine and diacylglycerol. Finally, the activated phosphoethanolamine attaches to the sn-3 position of diacylglycerol to form phosphatidylethanolamine catalyzed by CDPethanolamine:1,2-diacylglycerol ethanolamine phosphotransferase located in the endoplasmic reticulum (Henneberry et al., 2002; Horibata and Hirabayashi, 2007).

ATP Ethanolamine

ADP Phosphoethanolamine

Ethanolamine kinase CTP CTP:Phosphatidylethanolamine cytidyltransferase PPi anola CDP-ethanolamine 1,2-diacylglycerol

CDP-ethanolamine:1,2-diacylglycerol phosphoethanolamine transferase

CMP

Phosphatidylethanolamine FIGURE 2.2 Synthesis of Phosphatidylethanolamine synthesis via CDP-ethanolamine pathway. Phosphorylation of ethanolamine catalyzed by ethanolamine kinase yields phosphoethanolamine, which is activated by cytidine triphosphate (CTP) to form CDP-phosphoethanolamine in a reaction catalyzed by CTP:Phosphatidylethanolamine cytidyltransferase. Then, CDP-phosphoethanolamine condenses with 1,2-diacylglycerol to generate phosphatidylethanolamine. ADP, adenosine diphosphate; ATP, adenosine triphosphate; CDP, cytidine diphosphate; CMP, cytidine monophosphate; PPi, pyrophosphate.

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SECTION | I General and Introductory Aspects

ATP

ADP Phosphocholine

Choline Choline kinase

CTP CTP:Phosphatidylcholine cytidyltransferase PPi P cho CDP-choline 1,2-diacylglycerol CDP-choline:1,2-diacylglycerol phosphocholine transferase

CMP

Phosphatidylcholine FIGURE 2.3 Synthesis of Phosphatidylcholine synthesis via CDP-choline pathway. Phosphorylation of choline catalyzed by choline kinase yields phosphocholine, which is activated by cytidine triphosphate (CTP) to form CDP-phosphocholine in a reaction catalyzed by CTP:Phosphatidylcholine cytidyltransferase. Then, CDP-phosphocholine condenses with 1,2-diacylglycerol to produce phosphatidylcholine. ADP, adenosine diphosphate; ATP, adenosine triphosphate; CDP, cytidine diphosphate; CMP, cytidine monophosphate; PPi, pyrophosphate.

11.1.3 Synthesis of Phosphatidylcholine or Lecithin Phosphatidylcholine (PC) or lecithin is synthesized via the CDP-choline pathway or phosphatidylethanolamine N-methyltransferase (PEMT) pathway (Henneberry et al., 2002). In the CDP-choline pathway, choline upon entering the cell is quickly phosphorylated by choline kinase and converted to phosphocholine. The next enzyme in the pathway, CTP:phosphocholine cytidylyltransferase, then catalyzes the condensation of a CTP molecule and phosphocholine to form CDP-phosphocholine, with the loss of a pyrophosphate group. The activated phosphocholine, CDP-phosphocholine, is finally added to diacylglycerol catalyzed by CDPcholine:1,2-diacylgylcerol cholinephosphotransferase to complete the synthesis of PC (Fig. 2.3). This reaction occurs at the surface of the endoplasmic reticulum (Henneberry et al., 2002). Considering PEMT pathway, it occurs primarily in the liver and involves three repeated methylation reactions that convert phosphatidylethanolamine to PC. PEMT is an intrinsic membrane protein located in the endoplasmic reticulum and contains four membrane-spanning domains that transfer methyl group from S-adenosylmethionine to phosphatidylethanolamine in each of the reactions. Three molecules of S-adenosylhomocysteine are generated for each PC molecule produced.

11.2 Sphingolipids In all sphingolipids, the fatty acid chain is attached to the sphingosine backbone by amide bond to form ceramide (N-acetyl fatty acyl derivative of sphingosine). Ceramide thus acts as precursor for the synthesis of other sphingolipid subclasses such as globoside and ganglioside (glycosphingolipids).

11.2.1 Synthesis of Ceramide Synthesis of ceramide takes place in the ER and involves the activities of membrane-bound enzymes (Tafesse et al., 2006). Condensation of serine and palmitoyl CoA by serine palmitoyl transferase (3-ketosphinganine synthase) produces 3-ketosphinganine (Fig. 2.4). This is followed by NADPH-dependent reduction of the ketone group of 3-ketosphinganine to form sphinganine. The reaction is catalyzed by the enzyme 3-ketosphinganine reductase. In the next step, sphinganine is condensed with an acetyl CoA molecule to form N-acylsphinganine (dihydroceramide) in a reaction catalyzed by

Degradation and Synthesis of Lipids Chapter | 2

O II CoA–S–C–CH2-CH2-(CH2)12CH3 Palmitoyl-CoA

3-ketosphingamine synthase

+

25

COOI CH2–CH–N+(CH3)3 Serine

CO2 + CoASH

3-ketosphingamine NADPH + H+ 3-ketosphingamine reductase NADP+ Sphingamine (Dihydrosphingamine) SCoA Acyl-CoA transferase CoASH N-acylsphingamine(Dihydroceramide) FAD Dihydroceramide reductase FADH2 N-acylsphingosine (Ceramide) FIGURE 2.4 The synthesis of ceramide. Ceramide (N-acyl derivative of sphingosine) is formed from condensation of palmitoyl-CoA and the amino acid serine. The product of this reaction, 3-ketosphingamine, is reduced to form sphingamine. The acylation of sphingamine yields dihydroceramide, which is finally converted to ceramide, in a reduction/desaturation reaction. Ceramide is the structural precursor from which glycosphingolipids are derived. NADPþ, nicotinamide adenine dinucleotide phosphate (oxidized); NADPH, nicotinamide adenine dinucleotide phosphate (reduced); FAD, flavin adenine dinucleotide (oxidized); FADH2, flavin adenine dinucleotide (reduced).

sphinganine N-acyltransferase or dihydroceramide desaturase. N-acylsphinganine is finally oxidized by N-acylsphinganine reductase to form ceramide (N-acylsphingosine), using FAD as a cofactor. Ceramide is the intermediate from which other sphingolipid subclasses are synthesized (Tafesse et al., 2006). Ceramide is membrane bound, and it is transported from the ER to the Golgi apparatus where it can be modified into sphingomyelins and glycosphingolipids. The cell carries this out by means of vesicular transport or through the cytosolic protein, ceramide transfer protein (CERT) (Hanada et al., 2009).

11.2.2 The Synthesis of Sphingomyelin Sphingomyelin is synthesized from condensation of N-acylsphingosine (ceramide) and phosphatidylcholine with the removal of diacylglycerol. Phosphatidylcholine acts as the head-group donor in the reaction.

11.3 Glycolipids Glycolipids are lipids that contain a sugar residue. The sugar can be a monosaccharide, oligosaccharide, or polysaccharide (Halter et al., 2007). In many cases, the sugar and fatty acid residues are attached to a glycerol or sphingosine backbone to form glyceroglycolipids or glycosphingolipids, respectively. There are four main classes of glycosphingolipids: cerebrosides, sulfatides, globosides, and gangliosides. Glycolipids are assembled in the Golgi apparatus by various glycosyltransferases and embedded in the surface of a vesicle. The vesicle is then transported to the cell membrane where it fuses with the cell membrane and is exocytosed out of the cell (D’Angelo et al., 2008).

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11.3.1 The Biosynthesis of Cerebrosides Cerebrosides, the simplest neutral glycolipids/glycosphingolipids, have a single sugar that is linked to ceramide. Cerebrosides are thus ceramide monosaccharides. Most common examples are galactosylceramide (galactocerebroside) and glucosylceramide (glucocerebroside) with b-D-galactose and b-D-glucose as the monosaccharide unit, respectively. Prior to the biosynthesis, the sugar is activated through condensation with uridine diphosphate (UDP). Cerebroside ¼ Ceramide þ UDP  hexose b-D-galactose of galactocerebrosides can sometimes be sulfated at C-3 position to form ionic compounds referred to as sulfatides.

11.3.2 The Biosynthesis of Globosides and Gangliosides Globosides and gangliosides (the most complex) are complex glycosphingolipids that contain a ceramide and an oligosaccharide fragment. They are particularly abundant at nerve endings and at specific hormone receptor sites on cell surfaces and therefore play an important role in molecular recognition. Gangliosides alone make up to about 6% of the gray matter in the brain, and there are over 60 different types known; GM1, GM2, and GM3 are the common types. Both globosides and gangliosides are synthesized from a common precursor, lactosyl ceramide (LacCer; b-D-galactosyl(1/4)-b-D-glucosylceramide). Ceramide is transported from the ER to the proximal Golgi apparatus by the cytosolic protein CERT (Hanada et al., 2009), after which it is glucosylated and then b-galactosylated to form lactosylceramide. Gangliosides are then synthesized from lactosylceramide by sequential activities of distinct glycosyltransferases and sialyltransferases. These enzymes are bound to the membranes of the Golgi apparatus such that their arrangement corresponds to the order of addition of the various carbohydrate units. For example, glycosyltransferases and sialyltransferases (such as CMP:Lac Cer a-2,3 sialyltransferase or GM3 synthase), which catalyze the production of the comparatively simple ganglioside GM3, are situated in the proximal (cis) region of the Golgi body, while those that catalyze the last steps of ganglioside synthesis are found at the distal or trans region. GM1 and GM2 are subsequently formed from GM3 by the action of appropriate synthases. Also, UDP-activated sugar serves as source of the glucosyl and galactosyl residues in the synthesis of the glycosphingolipids. It is essential to note that the difference between globosides and gangliosides lies in the fact that globosides are neutral while gangliosides have a net-negative charge at pH 7.0 (acidic) due to the presence of N-acetyl neuraminic acid or sialic acid on one or more of its terminal sugar units.

11.4 Breakdown of Phospholipids and Glycolipids In the body, the rates of synthesis and degradation of different phospholipids or glycolipids are regulated in response to various physiological cues and external stresses. Deposition of lipid products tends to pose much more of a problem due to their nonpolar nature. They are not readily excreted out of the body, unlike hydrophilic substances. Cells and tissues therefore employ “opposing” enzymes capable of breaking down the generated lipid product. Hydrolytic products of this catalysis, such as free fatty acids, lysophospholipids, diacylglycerol, PA, and phosphorylated or free base (e.g., choline, ethanolamine, serine, and inositol) depend on the specific enzyme and the substrate (Aron and Mehendra, 2009). Generally, phospholipids are degraded by phospholipases through hydrolytic cleavage of carboxy- and phosphodiester bonds to give the individual componentsdFFAs, G3P, and phosphorylated or free bases. The enzymes are categorized into phospholipases A1, A2, C, or D depending on the site of hydrolysis. Phospholipases A1 and A2 are involved in the hydrolysis of the sn-1 and sn-2 acyl ester bond, respectively. On the other hand, the hydrolytic activities of phospholipases C and D occur at the “glycerolephosphate” bond and at the “glycerol phosphateeorganic alcohol” bond, respectively. The catabolism of sphingolipids happens through the hydrolysis of the sphingosine head groups by the sphingomyelinase family. The main products of sphingolipid catabolism are ceramide (N-acylsphingosine) and sphingosine, both of which are later hydrolyzed into a common intermediate, sphingosine-1-phosphate. The lysosomal enzyme ceramidase deacylates ceramide species to form sphingosine. Following the deacylation of ceramide, both the “parent” sphingosine and “ceramide-derived” sphingosine are converted to sphingosine-1-phosphate through phosphorylation by sphingosine kinases, which are localized in the cytosol. Sphingosine-1-phosphate is then catabolized by sphingosine-1-phosphate lyase (S1P phosphatases) in the ER to give hexadecenal and phosphoethanolamine. Hexadecenal dehydrogenase then converts the hexadecenal into hexadecenoic acid and then into palmitoyl-CoA. Unlike glycerolipids and sterols, the degradation pathways are particularly important for sphingolipid homeostasis because excess sphingolipids are not stored in lipid droplets (Walther and Farese, 2012).

Degradation and Synthesis of Lipids Chapter | 2

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Glycolipids are also degraded through sequential removal of sugar units in the oligosaccharide group, by a set of highly specific lysosomal glycohydrolases. These enzymes mainly remove the head group modifications.

11.5 Defects in the Metabolism of Sphingolipids and Glycolipids Several human genetic diseases result from abnormal metabolism or accumulation of phospholipids and glycolipids in the body. For instance, if their biosynthesis remains normal and degradation is impaired, then aberrant lipids and their partial degradation products accumulate in the cells and tissues (Penno et al., 2010). Many autosomal inherited diseases termed lysosomal storage diseases (also referred to as lipid storage diseases or lipidoses) result from this metabolic dysfunction. Deficiencies in lysosomal enzymes or mutations in genes coding for these catabolic enzymes underlie the development of and the symptoms observed in many of these diseases. A lipid storage disease may be gangliosidosis or sphingolipidoses both of which are usually neurodegenerative in nature (Albinet et al., 2013). Examples of common lipid storage diseases include: Tay-Sachs disease, NiemannePick disease, Gaucher disease, Fabry disease, and Krabbe disease (Table 2.2).

12. SYNTHESIS OF EICOSANOIDS The major precursor of eicosanoids, arachidonic acid, is synthesized from the essential linoleic acid by elongation and desaturation reactions. Arachidonic acid may also be released through hydrolysis of membrane phospholipids, particularly phosphatidylinositol. The released free arachidonic acid is used for the synthesis of various eicosanoids by two metabolic pathways (Fig. 2.5): the linear and cyclic pathways. The linear pathway leads to the formation of leukotrienes and lipoxins via the intermediate hydroperoxyeicosatetraenoic acids (HPETEs). The synthesis involves a series of reactions catalyzed by different lipoxygenase isozymes. In the cyclic pathway, arachidonic acid is metabolized by specific synthases to form prostaglandins (H2, D2, and E2), prostacyclins, and thromboxanes. The enzyme endoperoxide synthase/prostaglandin H synthase (consisting of cyclooxygenase and hydroperoxidase) first converts arachidonic acid to prostaglandin H2 (PGH), which in turn serves as the precursor of other prostaglandinsdprostaglandin D2, prostaglandin E2, prostacyclin, and thromboxanes. Prostaglandin H synthase (PGHS) has two isoforms, cyclooxygenase (COX)-1 and COX-2. The enzymes that catalyze the synthesis of eicosanoids are major drug targets. For example, the nonsteroidal antiinflammatory drugs such as aspirin (acetylsalicylic acid), ibuprofen, and acetaminophen reduce pain, fever, and inflammation by inhibiting the synthesis of prostaglandins and thromboxanes from acetylsalicylic acid. Aspirin particularly inhibits cyclooxygenase activity of PGHS by acetylating the enzyme.

13. CHOLESTEROL BIOSYNTHESIS Cholesterol can be synthesized endogenously and also obtained from diet. The liver is the main site of cholesterol synthesis in mammals, although the reproductive organs, adrenal gland, and intestine also form significant amounts. These organs/ tissues involved in sterol chemistry are thought to have cells with a well-developed smooth ER, having a very large collective membrane surface area (Berg et al., 2002). Cholesterol and all other sterols are derived from a common structural precursor, sterane (cyclopentanoperhydrophenanthrene), which has four aromatic rings known as A, B, C, and D rings. Enzymes involved in this biosynthetic pathway are found in the cytoplasm, ER, and peroxisomes. Cholesterol synthesis starts with the 2C acetate group of acetyl-CoA, transported from mitochondria via citrate transport system/dicarboxylic acid transporter as mentioned previously. Also, as with fatty acids, all the reduction reactions of cholesterol biosynthesis use NADPH as a cofactor. More on the detailed sequence of reactions involved in cholesterol synthesis (Fig. 2.6) and regulatory mechanisms in addition to its degradation products have been intensely studied by Satyanarayana and Chakrapani (2006).

14. CONCLUSION Metabolism of lipids is very essential in meeting the body’s energy requirements especially in situations where glucose is limited. Lipids also maintain the structural integrity and modulate the fluidity of the lipid bilayer of membranes in the body. These functions are achieved by metabolic pathways that are specific for the different lipid classes. In fact, the pathways, whether anabolic or catabolic, are highly regulated such that a steady state is maintained between the rate of synthesis and degradation of a particular class. However, aberrant lipid products may be formed and deposited in the body tissues if this balance is disturbed or when a particular pathway is defective, leading to lipidoses.

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Lipid Storage Disease

Enzyme Defect/Deficiency

Principal Accumulating Substance

Major Symptoms

Tay-Sachs disease or GM2 gangliosidosis (autosomal recessive)

Defect in the lysosomal enzyme hexosaminidase A due to mutations in the HEX A gene

GM2 gangliosides accumulate in nerve cells in the brain and in the spleen

Degeneration of the nervous system, progressive mental retardation, paralysis, blindness

Generalized GM1 gangliosidosis

Deficiency of lysosomal b-galactosidase enzyme

GM1 accumulates in the nervous system

Mental retardation and enlargement of the liver

NiemannePick Disease (NPD type A & NPD type B) (autosomal recessive)

NPD type A e complete absence of sphingomyelinase (aSMase) gene product NPD type B (less severe) e partial deficiency of the enzyme aSMase

Accumulation of sphingomyelin in brain, spleen, and liver

Mental retardation, retinal cherry red spots, hepatosplenomegaly, lung disease, and premature death In NPD B, there is absence of neuropathic symptoms but visceral involvement, e.g., lung disease and hepatosplenomegaly may still occur

Gaucher disease (autosomal recessive)

b-glucosidase (glucocerebrosidase)

Glucocerebrosides

Hepatosplenomegaly, mental retardation, and long-bone degeneration

Farber lipogranulomatosis (autosomal recessive)

Ceramidase

Ceramide

Painful and progressively deformed joints, skin nodules, death within few years of life

Fabry disease (X-linked recessive)

Deficiency of a-galactosidase A

Globotriaosylceramide/trihexaosylceramide accumulates in vascular endothelium and renal tissue

Kidney failure, skin rashes, pain in lower extremities

Krabbe disease or globoid cell leukodystrophy (autosomal recessive)

Deficiency or decreased activity of the enzyme b-galactocerebrosidase

Galactocerebrosides accumulate in the brain

Loss of myelin, mental retardation, increased irritability, spastic paralyses, visual failure

Metachromatic leukodystrophy (sulfatide lipidoses)

Deficiency of arylsulfatase A

Sulfatides

Mental retardation, epilepsy, seizures

SECTION | I General and Introductory Aspects

TABLE 2.2 Common Lipid Storage Diseases

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Phospholipids

Phospholipase C

Phospholipase P A2 Lipooxygenase Lip pooxyg ygenase

Aracchido hidonic acid

HPETE

Linear pathway Li th

Cyclic pathway se PGH2 synthase

H2O

Prostaglandin glandi H2 (PGH2)

H2O

Leukotriene B4 ukotr Leukotriene A4 (LTA4) (LTB4) LTA4 hydrolase Glutathione(GSH) GSH transferase G

TBX synthase

Prostacyclin synthase

PGD synthase

PGE synthase

ukotri Leukotriene C4 (LTC4) -glutamyl transferase Glutamic acid kotrie D4 (LTD4) Leukotriene Dipeptidase Glycine Prostaglandin E2 (PGE2)

Prostaglandin D2 (PGD2)

Prostacyclin (PGI2)

Thromboxane A2 (TBXA2)

kotrie E4 (LTE4) Leukotriene

FIGURE 2.5 The synthesis of eicosanoids via cyclic and linear pathways of arachidonic acid metabolism. Eicosanoids are synthesized from arachidonic acid (AA) released from phospholipids by the activation of phospholipases A2 and C. The linear pathway of AA metabolism leads to the formation of leukotrienes (LT) (LTA4, LTB4, LTC4, LTD4, LTE4) via the intermediate called hydroperoxyeicosatetraenoic acids (HPETEs). In the cyclic pathway, AA is converted to prostaglandin (PG) H2 from which PGE2, PGD2, prostacyclin (PGI2) and thromboxane (TBX) A2 are synthesized.

SUMMARY POINTS l

l l

l

l

l

l

l

The chapter focuses on the metabolic pathways through which the various categories of lipids are synthesized and degraded in the body. Lipids are available to the body either from ingested diet or synthesized de novo in the liver. Fats/lipids ingested in the diet, in the form of triglycerides (TGs) are broken down into free fatty acids by pancreatic lipases. The free fatty acids are oxidized in the liver into two-carbon acetyl CoA molecules, which can then enter the tricarboxylic acid cycle to generate energy. The fatty acids may also be used to synthesize structural and functional membrane lipids such as phospholipids, glycolipids, sphingolipids, and cholesterol, or signaling molecules like prostaglandins, leukotrienes, etc. Similarly, these lipids formed from fatty acids can be degraded or hydrolyzed into basic components as part of the body’s homeostasis and recycling mechanisms. Moreover, when there is an oversupply of dietary carbohydrate, the excess is converted to triacylglycerols and stored mainly in the adipose tissue for use during periods of energy deprivation. All these lipid metabolic pathways involve highly regulated enzyme-catalyzed reactions by hormones and other regulatory substances and hence defects in any of them may be associated with a wide range of health problems.

KEY FACTS OF FATTY ACID OXIDATION l l

Fatty acids are oxidized in the mitochondria of the liver in a spiral of reactions commonly referred to as b-oxidation. The carnitine shuttle system transports long-chain fatty acids (in the form of fatty acyl CoA) from the cytosol into the mitochondria, whereas shorter chains enter via diffusion.

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SECTION | I General and Introductory Aspects

Condensation 3 Acetyl CoA

HMG CoA H HMG CoA reductase Committed step & major point of regulation C Mevalonate valon

Isopentenyl pyrophosphate (C5)

Isopentenyl pyrophosphate isomerase

Dimethylallyl pyrophosphate (C5) p Geranyl-pyrophosphate synthase Geranyl pyrophosphate (C10) yroph Farnesyl diphosphate synthase Farnesyl pyrophosphate (C15) yyroph p

Farnesyl pyrophosphate (C15)

Squalene synthase Squalene (C30)

Lanosterol noste

Cholesterol oleste FIGURE 2.6 A schematic representation of the overview of cholesterol metabolism. In the initial stage, three acetyl-CoA units condense to form a six-carbon intermediate, hydroxymethylglutaryl-CoA (HMG-CoA), which is reduced by HMG-CoA reductase to form mevalonate, the committed step in the pathway of cholesterol synthesis. Mevalonate is converted through a series of reactions to activated isoprene units (isopentenyl pyrophosphate and dimethylallyl pyrophosphate). The next stage involves polymerization of six isoprene units to form the 30-carbon linear intermediate, squalene, which is cyclized to form lanosterol, the first sterol in the pathway. The postlanosterol pathway involves a series of reactions leading finally to cholesterol formation.

l

l l

l

l

The b-oxidation of fatty acids occurs by the removal of two carbons in the form of acetyl CoA at a time such that fatty acyl CoA substrate for the next round of the cycle is shortened by two carbons. The acetyl CoA molecules generated are further oxidized in the tricarboxylic acid (TCA) cycle to generate energy. The complete oxidation of fatty acids provides high caloric content, about 9 kcal/g, compared with 4 kcal/g for the breakdown of carbohydrates and proteins. Fatty acids above a certain chain length, usually greater than 22 carbons, cannot enter the mitochondria, and are therefore metabolized in the peroxisome. Odd-chain fatty acid, the final oxidation product is three-carbon molecule, propionyl CoA rather than acetyl CoA. The propionyl CoA is further converted to succinyl CoA (TCA cycle intermediate) by three enzymes.

LIST OF ABBREVIATIONS AA Arachidonic acid ADP Adenosine diphosphate AMP Adenosine monophosphate ATP Adenosine triphosphate cAMP Adenosine 3,5-cyclic monophosphate (cyclic AMP) CAT Carnitine acyltransferase

Degradation and Synthesis of Lipids Chapter | 2

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CDP Cytidine diphosphate CERT Ceramide transfer protein CMP Cytidine monophosphate CoASH Coenzyme A (reduced) COX Cyclooxygenase CTP Cytidine triphosphate DAG Diacylglycerol DHAP Dihydroxyacetone phosphate EFA Essential fatty acid ER Endoplasmic reticulum FA Fatty acid FFA Free fatty acid FAD Flavin adenine dinucleotide (oxidized) FADH2 Flavin adenine dinucleotide (reduced) FAS Fatty acid synthase G3P Glycerol-3-phosphate GLUT4 Glucose transporter type 4 HMG-CoA Hydroxyl methylglutaryl-CoA HPETEs Hydroperoxyeicosatetraenoic acids LDL Low-density lipoprotein LT Leukotriene NADD Nnicotinamide adenine dinucleotide (oxidized) NADH Nicotinamide adenine dinucleotide (reduced) NADP Nicotinamide adenine dinucleotide phosphate (oxidized) NADPH Nicotinamide adenine dinucleotide phosphate (reduced) PA Phosphatidic acid PE Phosphoethanolamine PG Prostaglandin PGHS Prostaglandin H synthetase PI Phosphatidylinositol PPi Pyrophosphate TAG Triacylglycerol TCA Tricarboxylic acid UDP Uridine diphosphate VLDL Very low-density lipoprotein

REFERENCES Albinet, V., Marie-Lise, B., Bedia, C., Sabourdy, F., Garcia, V., Segui, B., Andrieu-Abadie, N., Hornemann, T., Levade, T., 2013. Genetic disorders of simple sphingolipid metabolism. In: Gulbins, E., Petrache, I. (Eds.), Sphingolipids: Basic Science and Drug Development. Handbook of Experimental Pharmacology, vol. 215. Springer, Vienna. Aron, B.F., Mehendra, J., 2009. Phospholipases: Degradation of Phospholipids in Membranes and Emulsions. Wiley Online Library. Available at: https:// onlinelibrary.wiley.com/doi/10.1002/9780470015902.a0001394.pub2. (Accessed 30 March 2018). Berg, J.M., Tymoczko, J.L., Stryer, L., 2002. Biochemistry, fifth ed. W H Freeman, New York. Chatterjea, M.N., Shinde, R., 2011. Textbook of Medical Biochemistry, eighth ed. JP Medical Ltd. D’Angelo, G., Vicinanza, M., DeMatteis, M.A., 2008. Lipid-transfer proteins in biosynthetic pathways. Curr. Opin. Cell Biol. 20, 360e370. DiMarco, J.P., Hoppel, C., 1975. Hepatic mitochondrial function in ketogenic states. Diabetes, starvation and after growth hormone administration. J. Clin. Invest. 55, 1237. Fukao, T., Mitchell, G., Sass, J.O., Hori, T., Orii, K., Aoyama, Y., 2014. Ketone body metabolism and its defects. J. Inherit. Metab. Dis. 37 (4), 541e551. Greenberg, A.S., Shen, W.J., Murilo, K., Patel, S., Sandra, C., 2001. Stimulation of lipolysis and hormone-sensitive lipase via the extracellular signalregulated kinase pathway. J. Biol. Chem. 276 (48), 45456e45461. Halter, D., Neumann, S., van Dijk, S.M., Wolthoorn, J., de Maziere, A.M., Vieira, O.V., Mattjus, P., Klumperman, J., van Meer, G., Sprong, H., 2007. Pre-and post-Golgi translocation of glucosylceramide in glycosphingolipid synthesis. J. Cell Biol. 179, 101e115. Hanada, K., Kumagai, K., Tomishige, N., Yamaji, T., 2009. CERTemediated trafficking of ceramide. Biochim. Biophys. Acta 1791 (7), 684e691. Henneberry, A.L., Wright, M.M., McMaster, C.R., 2002. The major sites of cellular phospholipid synthesis and molecular determinants of fatty acid and lipid head group specificity. Mol. Biol. Cell 13, 3148e3161. Horibata, Y., Hirabayashi, Y., 2007. Identification and characterization of human ethanolaminephosphotransferase. J. Lipid Res. 48 (3), 503e508.

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