- Email: [email protected]
Intermediary metabolism
Physiology
Intermediary metabolism
such as glucose, normally involves the loss of an entire atom, usually hydrogen. The oxidation (dehydrogenation) of a compound, such as glucose, results in the production of the low-energy compound carbon dioxide and the acceptance of electrons (hydrogen atoms), ultimately by oxygen, to produce water. As hydrogen ions are released in the intermediate parts of the oxidation process they are taken up transiently by co-enzymes, most commonly flavine adenine dinucleotide (FAD, derived from vitamin B2) to form FADH2 and nicotinamide adenine dinucleotide (NAD, derived from nicotinamide, also a B-group vitamin) to form NADH. ATP is synthesized from a series of electron transport proteins situated on the inner wall of the mitochondrion, the flavoprotein-cytochrome (respiratory) chain. The electrons (hydrogen atoms) enter this enzyme chain via FADH2 and NADH. They are passed down the respiratory chain, from one enzyme to the next, each enzyme having a greater affinity for electrons than the previous one. Free energy is released and is used to drive hydrogen ions (protons) across the inner mitochondrial membrane into the intermembrane space to create an electrochemical gradient across the inner membrane. The protons then pass back down this gradient into the mitochondrion, driving a reversible ATPase in the membrane. It is this ATPase that synthesizes ATP from ADP and inorganic phosphate. Other high-energy phosphate compounds are also used as immediate sources of energy; they include creatine phosphate, large quantities of which are found in muscle, as well as other purines and pyrimidines (e.g. guanosine triphosphate, cytidine triphosphate, inosine triphosphate). 90% of whole body oxygen consumption in the basal state is mitochondrial and 80% of it is coupled to ATP synthesis. About 30% of ATP is used for protein synthesis, 25% by Na/K ATPase in cell membranes, 10% for gluconeogenesis and 3% for urea genesis. ATP is also the precursor for cyclic AMP.
Iain Campbell
Abstract Carbohydrate and fat form the immediate and long-term energy stores of the body, and protein constitutes the active (functional) cell mass and is also an energy source but, normally, a relatively minor one. All three macronutrients are interrelated. Proteins are synthesized from amino acids derived from ingested protein. Glucose and fat provide energy via ATP. The brain and red cells can only obtain their energy from glucose. Glucose is oxidized via the glycolytic and the tricarboxylic acid (Krebs) cycle pathways. Fatty acids are metabolized by the process of β-oxidation, whereby two carbon fragments are cleaved from the fatty acid chain and enter the Krebs cycle. Amino acids are deaminated to keto acids and the nitrogen moiety excreted in the urine mostly as urea. The keto acids enter the metabolic pathways at various points, mostly in the Krebs cycle. Glucose can be synthesized from lactate, glycerol and amino acids (gluconeogenesis) but not from fatty acids.
Keywords carbohydrate, fat and protein oxidation; gluconeogenesis; glycolysis; ketogenesis; tricarboxylic acid (Krebs) cycle
Energy is obtained from the oxidation of the macronutrients carbohydrate, fat and protein. The structure of these macronutrients and the whole body quantification of their metabolism are described on page 172. The end-products of fat and carbohydrate metabolism are carbon dioxide, water and energy only. The endproducts of protein metabolism are carbon dioxide, water, energy and a number of nitrogen-containing compounds excreted in the urine (mainly urea, but also creatinine, uric acid and ammonia). About 80% of urinary nitrogen is normally present as urea.
Carbohydrate, fat and protein metabolism Carbohydrate is the immediate most usable form of energy, fat is the body’s main long-term energy store and protein constitutes the structure of the body. Carbohydrate metabolism The principal carbohydrate is glucose, but other monosaccharides include fructose and galactose. Glucose is absorbed from the gastrointestinal tract via the portal vein. It passes to the liver where in times of excess, such as after a large meal, some is synthesized into glycogen. The rest passes into the general circulation where it is metabolized in the tissues, or may be stored as glycogen, particularly in muscle. In starvation, most tissues, including muscle (both skeletal and cardiac), can obtain their energy needs from fatty acids alone, but some, such as the CNS and RBCs, are obligatory users of glucose. Glucose is oxidized via two sequential metabolic pathways: the Embden–Meyerhof or glycolytic pathway, which occurs in the cytoplasm; and the Krebs’ or tricarboxylic acid (TCA) cycle, which takes place in the mitochondria (Figure 1).
Energy release Energy released by oxidation is not used directly by the cells but goes into the formation of bonds between phosphoric acid residues and certain organic compounds and is thus ‘stored’ as discrete packets mainly of adenosine triphosphate (ATP). When these bonds are hydrolysed, the energy is released for use in muscle contraction, membrane transport, protein synthesis, etc. ‘Oxidation’ involves the loss of electrons, but organic molecules do not give up electrons easily and oxidation of a complex molecule,
Iain Campbell, MD, FRCA, is Consultant Anaesthetist at the University Hospitals of South Manchester NHS Trust and visiting Professor of Human Physiology at Liverpool John Moores University. He qualified from Guy’s Hospital Medical School, London, and trained in anaesthesia in Zimbabwe, Southend, Montreal and Leeds.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:4
Glycolysis is the first stage of glucose breakdown; it is a sixstage metabolic pathway in which metabolism of one molecule of glucose results in the production of two molecules of pyruvate, which enter the TCA cycle. Absorbed glucose is phosphorylated 177
© 2008 Elsevier Ltd. All rights reserved.
Physiology
Pentose monophosphate shunt is an alternative pathway for glucose metabolism. This starts with G6P and its main function is to supply pentose sugars, essential in the synthesis of nucleotides and nucleic acids, as well as reduced nicotinamide-adenine dinucleotide phosphate (NADPH), a phosphorylated form of NADH used in a number of biosynthetic pathways. Excess ribose-5-phosphate not required for nucleotide synthesis returns to the glycolytic pathway as fructose-6-phosphate and phosphoglycer-aldehyde. The pentose shunt does not use ATP or oxygen. Whether glucose is metabolized via this route or via the glycolytic pathway appears to depend on whether the cell is engaged in biosynthesis (e.g. fatty acid synthesis).
Carbohydrate, fat and protein metabolism Glycogen 4
5
NADPH 1 2
Glucose
Glucose-6-phosphate
Ribose
3
Pentose shunt Glycolysis
Glycerol 7
Gluconeogenesis
Lactate
8
Oxaloacetate
TCA cycle is a sequence of reactions that result in the production of most of the ATP derived from both fat and carbohydrate oxidation. The reactions occur within the matrix of the mitochondrion. With carbohydrates, pyruvate is transported across the inner mitochondrial membrane and is then oxidatively decarboxylated to acetyl CoA. This reacts with the 4-carbon acid oxaloacetate to produce the 6-carbon tricarboxylic acid citrate. In a subsequent series of seven reactions, oxaloacetate is regenerated and two carbon dioxide molecules are produced. A number of other intermediate compounds are generated (oxoglutarate, malate, fumarate) but the oxidation of the acetyl groups to carbon dioxide is coupled to the reduction of NAD and FAD to NADH and FADH2, which enter the flavoprotein-cytochrome chain in the mitochondrion to produce ATP. Entry of glucose into the cells is via a number of specialist receptors. Some, such as GLUT4 in muscle, are insulin dependent, others, such as GLUT2 in the liver, are not and glucose follows the concentration gradient. Hexokinase activity is not affected by insulin but glucokinase activity is and insulin stimulates both glycolysis and glycogen synthesis. Glycogen is a branched glucose polymer and is synthesized from glucose by glycogen synthase. Glycogen is a short-term store or buffer for glucose and, in the short term, maintains blood glucose levels in the absence of a glucose intake. Glycogen breakdown to glucose is performed by glycogen phosphorylase, which is converted from its inactive b form to its active a form via cyclic AMP, and protein kinase A, which catalyses the phosphorylation and activation of phosphorylase. Protein kinase also inhibits glycogen synthase, which is active in its dephosphorylated form and inactive when phosphorylated. The balance of glycogen synthesis (glycogenesis) and glycogen breakdown (glycogenolysis) is determined by the balance of the anabolic hormone insulin and the catabolic hormones glucagon, epinephrine and norepinephrine. The balance of insulin and glucagon controls the normal cycle of glycogen synthesis/ breakdown, the catecholamines come into play during ‘stress’. Muscle does not contain G6P, therefore muscle glycogen does not contribute to blood glucose levels. When muscle glycogen is mobilized, the glucose is metabolized by glycolysis. In severe exercise or ‘shock’ the rate of glycolysis exceeds the oxygen supply and the rate at which pyruvate enters the TCA cycle. Pyruvate goes to form lactic acid, catalysed by the enzyme lactate dehydrogenase. Pyruvate receives hydrogen ions from NADH with the consequent development of a metabolic acidosis. When the oxygen supply is restored to balance the rate of substrate utilization (the ‘shock’ is treated, or the intensity of exercise diminishes) pyruvate is resynthesized.
Pyruvate Fatty acids
6
Acetyl CoA TCA cycle
Deamination Key enzymes 1 Hexokinase 2 Glucokinase 3 Glucose-6-phosphatase 4 Glycogen synthase 5 Glycogen phosphorylase
Amino acids 6 7
8
Pyruvate dehydrogenase Phosphoenol-pyruvate carboxykinase Lactate dehydrogenase
Figure 1
in the tissues by hexokinase (and in the liver by glucokinase) to glucose-6-phosphate (G6P), which is then either converted to glucose-1-phosphate prior to glycogen synthesis or enters the glycolytic pathway. During the course of glycolysis the 6-carbon molecule is split into two 3-carbon fragments (glyceraldehyde-3phosphate and dihydroxyacetone). Dihdroxyacetone phosphate undergoes isomerization to glyceraldehyde-3-phosphate and it is this 3-carbon molecule that eventually forms pyruvate, but dihydroxyacetone can also be converted to glycerol and thus provides a link with fat metabolism, though, in terms of energy flux, this pathway is minor. Most of the reactions in glycolysis are reversible, but some are not and these irreversible reactions direct the flow of metabolites through the system. Reversal of these steps in the pathway requires different enzymes. • The conversion of glucose to G6P by hexokinase (or glucokinase). This is reversed by glucose-6-phosphatase. • The conversion of fructose-6-phosphate to fructose 1:6 diphosphate in the middle of the glycolytic pathway by phosphofructokinase. This is reversed by fructose 1:6 diphosphatase. • The conversion of pyruvate (3 carbons) to acetyl CoA (2 carbons; 1 molecule of carbon dioxide is released) by pyruvate dehydrogenase for entry into the TCA cycle. This step cannot be reversed. Four molecules of ATP are produced in glycolysis, but two are used in the process, so the net gain in glycolysis is two molecules of ATP. 36 molecules are eventually synthesized from the complete oxidation of a molecule of glucose via the glycolytic pathway and the TCA cycle, so these two represent about 5% of the total amount of energy ultimately available.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:4
178
© 2008 Elsevier Ltd. All rights reserved.
Physiology
is in equilibrium with ammonia. Most of the NH4+ formed by deamination in the liver is converted to urea. Removal of the amino group from amino acids produces the corresponding ketoacids, most of which undergo oxidative metabolism via pyruvate or the various intermediates in the TCA cycle. Some amino acids are ketogenic in that their ketoacids are converted to acetoacetate, but most are glucogenic and are converted to glucose via gluconeogenesis.
Fat metabolism Fatty acids are also metabolized in the mitochondrion via the TCA cycle. Two carbon fragments at a time are split off the fatty acid chain from the carboxyl end (see page 172) by the process of β oxidation to form acetyl CoA, which then enters the TCA cycle. Short and medium-length fatty acid chains can enter the mitochondrion directly, but the long chain molecules need to be bound to carnitine before they can cross the inner mitochondrial membrane. When large quantities of acetyl CoA from fat become available, ketone bodies (acetoacetic acid and β-hydroxybutyric acid) are formed. They are synthesized in the liver from free fatty acids and are seen most commonly in starvation, but pathologically in diabetes. In starvation they are used by many tissues in place of glucose, but in diabetic keto-acidosis, in which they are produced in huge quantities, they contribute to a metabolic acidosis. Acetoacetic acid is formed from the condensation of two molecules of acetyl CoA. It is converted to β-hydroxybutyrate and to acetone (excreted in the breath). Fatty acids can be synthesized (lipogenesis) from acetyl CoA in the presence of excess glucose, which may occur if a high glucose intravenous feed is given in excess of requirements. High insulin concentrations in response to the glucose load stimulate the process. Lipogenesis occurs principally in the liver and in adipose tissue and takes place in the cytosol. NADPH derived from the pentose phosphate pathway is involved as the source of hydrogen. Fatty acid synthesis stops when the carbon chain reaches 16C and, in adipose tissue depots, fatty acids combine with glycerol to form triglyceride. Whether or not lipogenesis takes place is determined also by the patient’s nutritional status and is most evident in the wellnourished individual with a high proportion of carbohydrate in the diet. In the presence of a fat intake of more than 10–15% of the diet there is little conversion of carbohydrate to fat.
Gluconeogenesis Gluconeogenesis occurs in the liver and the kidney as part of the normal adaptation to starvation and in response to stress. The driving force is the fall in insulin and the rise in the counterregulatory hormones (glucagon, adrenaline, norepinephrine, cortisol) and, in trauma and sepsis, the release of cytokines that affect peripheral and hepatic metabolism. All of the TCA cycle intermediates potentially act as gluconeogenic precursors but, as stated earlier, the reaction catalysed by pyruvate dehydrogenase that takes pyruvate into the TCA cycle is irreversible. The key enzyme for gluconeogenesis is phosphoenolpyruvate carboxy-kinase, which converts oxaloacetate to phosphoenolpyruvate, the final product of the glycolytic pathway before pyruvate. Glucose is then synthesized via reversal of the various steps in glycolysis, bypassing the ‘irreversible’ ones referred to earlier. Amino acids are transferred to the liver and kidney, about 50% of them as alanine (the amino acid corresponding to pyruvate) and glutamine (the amino acid corresponding to α-ketoglutarate, a constituent of the TCA cycle). Transamination takes place in the periphery, mostly by transfer of amino groups from the branch-chain amino acids, valine, leucine and isoleucine. Other gluconeogenic precursors are lactate and glycerol. Lactate is synthesized to glucose in the liver and glycerol via dihydroxyacetone phosphate, one of the constituents of the glycolytic pathway. ◆
Protein metabolism Proteins are made up of chains of amino acids (see page 172). Interconversions can take place between amino acids and products of fat and carbohydrate metabolism involving transfer of the amino groups to other ketoacids by transamination, or oxidative deamination can take place with the formation of NH4+ which
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:4
Further reading Frayn KN. Metabolic regulation. A human perspective, 2nd edn. Oxford: Blackwell Science, 2003. Ganong WR. Review of medical physiology, 21st edn. New York: Lange Medical Books/McGraw-Hill, 2005.
179
© 2008 Elsevier Ltd. All rights reserved.
Intermediary metabolism
such as glucose, normally involves the loss of an entire atom, usually hydrogen. The oxidation (dehydrogenation) of a compound, such as glucose, results in the production of the low-energy compound carbon dioxide and the acceptance of electrons (hydrogen atoms), ultimately by oxygen, to produce water. As hydrogen ions are released in the intermediate parts of the oxidation process they are taken up transiently by co-enzymes, most commonly flavine adenine dinucleotide (FAD, derived from vitamin B2) to form FADH2 and nicotinamide adenine dinucleotide (NAD, derived from nicotinamide, also a B-group vitamin) to form NADH. ATP is synthesized from a series of electron transport proteins situated on the inner wall of the mitochondrion, the flavoprotein-cytochrome (respiratory) chain. The electrons (hydrogen atoms) enter this enzyme chain via FADH2 and NADH. They are passed down the respiratory chain, from one enzyme to the next, each enzyme having a greater affinity for electrons than the previous one. Free energy is released and is used to drive hydrogen ions (protons) across the inner mitochondrial membrane into the intermembrane space to create an electrochemical gradient across the inner membrane. The protons then pass back down this gradient into the mitochondrion, driving a reversible ATPase in the membrane. It is this ATPase that synthesizes ATP from ADP and inorganic phosphate. Other high-energy phosphate compounds are also used as immediate sources of energy; they include creatine phosphate, large quantities of which are found in muscle, as well as other purines and pyrimidines (e.g. guanosine triphosphate, cytidine triphosphate, inosine triphosphate). 90% of whole body oxygen consumption in the basal state is mitochondrial and 80% of it is coupled to ATP synthesis. About 30% of ATP is used for protein synthesis, 25% by Na/K ATPase in cell membranes, 10% for gluconeogenesis and 3% for urea genesis. ATP is also the precursor for cyclic AMP.
Iain Campbell
Abstract Carbohydrate and fat form the immediate and long-term energy stores of the body, and protein constitutes the active (functional) cell mass and is also an energy source but, normally, a relatively minor one. All three macronutrients are interrelated. Proteins are synthesized from amino acids derived from ingested protein. Glucose and fat provide energy via ATP. The brain and red cells can only obtain their energy from glucose. Glucose is oxidized via the glycolytic and the tricarboxylic acid (Krebs) cycle pathways. Fatty acids are metabolized by the process of β-oxidation, whereby two carbon fragments are cleaved from the fatty acid chain and enter the Krebs cycle. Amino acids are deaminated to keto acids and the nitrogen moiety excreted in the urine mostly as urea. The keto acids enter the metabolic pathways at various points, mostly in the Krebs cycle. Glucose can be synthesized from lactate, glycerol and amino acids (gluconeogenesis) but not from fatty acids.
Keywords carbohydrate, fat and protein oxidation; gluconeogenesis; glycolysis; ketogenesis; tricarboxylic acid (Krebs) cycle
Energy is obtained from the oxidation of the macronutrients carbohydrate, fat and protein. The structure of these macronutrients and the whole body quantification of their metabolism are described on page 172. The end-products of fat and carbohydrate metabolism are carbon dioxide, water and energy only. The endproducts of protein metabolism are carbon dioxide, water, energy and a number of nitrogen-containing compounds excreted in the urine (mainly urea, but also creatinine, uric acid and ammonia). About 80% of urinary nitrogen is normally present as urea.
Carbohydrate, fat and protein metabolism Carbohydrate is the immediate most usable form of energy, fat is the body’s main long-term energy store and protein constitutes the structure of the body. Carbohydrate metabolism The principal carbohydrate is glucose, but other monosaccharides include fructose and galactose. Glucose is absorbed from the gastrointestinal tract via the portal vein. It passes to the liver where in times of excess, such as after a large meal, some is synthesized into glycogen. The rest passes into the general circulation where it is metabolized in the tissues, or may be stored as glycogen, particularly in muscle. In starvation, most tissues, including muscle (both skeletal and cardiac), can obtain their energy needs from fatty acids alone, but some, such as the CNS and RBCs, are obligatory users of glucose. Glucose is oxidized via two sequential metabolic pathways: the Embden–Meyerhof or glycolytic pathway, which occurs in the cytoplasm; and the Krebs’ or tricarboxylic acid (TCA) cycle, which takes place in the mitochondria (Figure 1).
Energy release Energy released by oxidation is not used directly by the cells but goes into the formation of bonds between phosphoric acid residues and certain organic compounds and is thus ‘stored’ as discrete packets mainly of adenosine triphosphate (ATP). When these bonds are hydrolysed, the energy is released for use in muscle contraction, membrane transport, protein synthesis, etc. ‘Oxidation’ involves the loss of electrons, but organic molecules do not give up electrons easily and oxidation of a complex molecule,
Iain Campbell, MD, FRCA, is Consultant Anaesthetist at the University Hospitals of South Manchester NHS Trust and visiting Professor of Human Physiology at Liverpool John Moores University. He qualified from Guy’s Hospital Medical School, London, and trained in anaesthesia in Zimbabwe, Southend, Montreal and Leeds.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:4
Glycolysis is the first stage of glucose breakdown; it is a sixstage metabolic pathway in which metabolism of one molecule of glucose results in the production of two molecules of pyruvate, which enter the TCA cycle. Absorbed glucose is phosphorylated 177
© 2008 Elsevier Ltd. All rights reserved.
Physiology
Pentose monophosphate shunt is an alternative pathway for glucose metabolism. This starts with G6P and its main function is to supply pentose sugars, essential in the synthesis of nucleotides and nucleic acids, as well as reduced nicotinamide-adenine dinucleotide phosphate (NADPH), a phosphorylated form of NADH used in a number of biosynthetic pathways. Excess ribose-5-phosphate not required for nucleotide synthesis returns to the glycolytic pathway as fructose-6-phosphate and phosphoglycer-aldehyde. The pentose shunt does not use ATP or oxygen. Whether glucose is metabolized via this route or via the glycolytic pathway appears to depend on whether the cell is engaged in biosynthesis (e.g. fatty acid synthesis).
Carbohydrate, fat and protein metabolism Glycogen 4
5
NADPH 1 2
Glucose
Glucose-6-phosphate
Ribose
3
Pentose shunt Glycolysis
Glycerol 7
Gluconeogenesis
Lactate
8
Oxaloacetate
TCA cycle is a sequence of reactions that result in the production of most of the ATP derived from both fat and carbohydrate oxidation. The reactions occur within the matrix of the mitochondrion. With carbohydrates, pyruvate is transported across the inner mitochondrial membrane and is then oxidatively decarboxylated to acetyl CoA. This reacts with the 4-carbon acid oxaloacetate to produce the 6-carbon tricarboxylic acid citrate. In a subsequent series of seven reactions, oxaloacetate is regenerated and two carbon dioxide molecules are produced. A number of other intermediate compounds are generated (oxoglutarate, malate, fumarate) but the oxidation of the acetyl groups to carbon dioxide is coupled to the reduction of NAD and FAD to NADH and FADH2, which enter the flavoprotein-cytochrome chain in the mitochondrion to produce ATP. Entry of glucose into the cells is via a number of specialist receptors. Some, such as GLUT4 in muscle, are insulin dependent, others, such as GLUT2 in the liver, are not and glucose follows the concentration gradient. Hexokinase activity is not affected by insulin but glucokinase activity is and insulin stimulates both glycolysis and glycogen synthesis. Glycogen is a branched glucose polymer and is synthesized from glucose by glycogen synthase. Glycogen is a short-term store or buffer for glucose and, in the short term, maintains blood glucose levels in the absence of a glucose intake. Glycogen breakdown to glucose is performed by glycogen phosphorylase, which is converted from its inactive b form to its active a form via cyclic AMP, and protein kinase A, which catalyses the phosphorylation and activation of phosphorylase. Protein kinase also inhibits glycogen synthase, which is active in its dephosphorylated form and inactive when phosphorylated. The balance of glycogen synthesis (glycogenesis) and glycogen breakdown (glycogenolysis) is determined by the balance of the anabolic hormone insulin and the catabolic hormones glucagon, epinephrine and norepinephrine. The balance of insulin and glucagon controls the normal cycle of glycogen synthesis/ breakdown, the catecholamines come into play during ‘stress’. Muscle does not contain G6P, therefore muscle glycogen does not contribute to blood glucose levels. When muscle glycogen is mobilized, the glucose is metabolized by glycolysis. In severe exercise or ‘shock’ the rate of glycolysis exceeds the oxygen supply and the rate at which pyruvate enters the TCA cycle. Pyruvate goes to form lactic acid, catalysed by the enzyme lactate dehydrogenase. Pyruvate receives hydrogen ions from NADH with the consequent development of a metabolic acidosis. When the oxygen supply is restored to balance the rate of substrate utilization (the ‘shock’ is treated, or the intensity of exercise diminishes) pyruvate is resynthesized.
Pyruvate Fatty acids
6
Acetyl CoA TCA cycle
Deamination Key enzymes 1 Hexokinase 2 Glucokinase 3 Glucose-6-phosphatase 4 Glycogen synthase 5 Glycogen phosphorylase
Amino acids 6 7
8
Pyruvate dehydrogenase Phosphoenol-pyruvate carboxykinase Lactate dehydrogenase
Figure 1
in the tissues by hexokinase (and in the liver by glucokinase) to glucose-6-phosphate (G6P), which is then either converted to glucose-1-phosphate prior to glycogen synthesis or enters the glycolytic pathway. During the course of glycolysis the 6-carbon molecule is split into two 3-carbon fragments (glyceraldehyde-3phosphate and dihydroxyacetone). Dihdroxyacetone phosphate undergoes isomerization to glyceraldehyde-3-phosphate and it is this 3-carbon molecule that eventually forms pyruvate, but dihydroxyacetone can also be converted to glycerol and thus provides a link with fat metabolism, though, in terms of energy flux, this pathway is minor. Most of the reactions in glycolysis are reversible, but some are not and these irreversible reactions direct the flow of metabolites through the system. Reversal of these steps in the pathway requires different enzymes. • The conversion of glucose to G6P by hexokinase (or glucokinase). This is reversed by glucose-6-phosphatase. • The conversion of fructose-6-phosphate to fructose 1:6 diphosphate in the middle of the glycolytic pathway by phosphofructokinase. This is reversed by fructose 1:6 diphosphatase. • The conversion of pyruvate (3 carbons) to acetyl CoA (2 carbons; 1 molecule of carbon dioxide is released) by pyruvate dehydrogenase for entry into the TCA cycle. This step cannot be reversed. Four molecules of ATP are produced in glycolysis, but two are used in the process, so the net gain in glycolysis is two molecules of ATP. 36 molecules are eventually synthesized from the complete oxidation of a molecule of glucose via the glycolytic pathway and the TCA cycle, so these two represent about 5% of the total amount of energy ultimately available.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:4
178
© 2008 Elsevier Ltd. All rights reserved.
Physiology
is in equilibrium with ammonia. Most of the NH4+ formed by deamination in the liver is converted to urea. Removal of the amino group from amino acids produces the corresponding ketoacids, most of which undergo oxidative metabolism via pyruvate or the various intermediates in the TCA cycle. Some amino acids are ketogenic in that their ketoacids are converted to acetoacetate, but most are glucogenic and are converted to glucose via gluconeogenesis.
Fat metabolism Fatty acids are also metabolized in the mitochondrion via the TCA cycle. Two carbon fragments at a time are split off the fatty acid chain from the carboxyl end (see page 172) by the process of β oxidation to form acetyl CoA, which then enters the TCA cycle. Short and medium-length fatty acid chains can enter the mitochondrion directly, but the long chain molecules need to be bound to carnitine before they can cross the inner mitochondrial membrane. When large quantities of acetyl CoA from fat become available, ketone bodies (acetoacetic acid and β-hydroxybutyric acid) are formed. They are synthesized in the liver from free fatty acids and are seen most commonly in starvation, but pathologically in diabetes. In starvation they are used by many tissues in place of glucose, but in diabetic keto-acidosis, in which they are produced in huge quantities, they contribute to a metabolic acidosis. Acetoacetic acid is formed from the condensation of two molecules of acetyl CoA. It is converted to β-hydroxybutyrate and to acetone (excreted in the breath). Fatty acids can be synthesized (lipogenesis) from acetyl CoA in the presence of excess glucose, which may occur if a high glucose intravenous feed is given in excess of requirements. High insulin concentrations in response to the glucose load stimulate the process. Lipogenesis occurs principally in the liver and in adipose tissue and takes place in the cytosol. NADPH derived from the pentose phosphate pathway is involved as the source of hydrogen. Fatty acid synthesis stops when the carbon chain reaches 16C and, in adipose tissue depots, fatty acids combine with glycerol to form triglyceride. Whether or not lipogenesis takes place is determined also by the patient’s nutritional status and is most evident in the wellnourished individual with a high proportion of carbohydrate in the diet. In the presence of a fat intake of more than 10–15% of the diet there is little conversion of carbohydrate to fat.
Gluconeogenesis Gluconeogenesis occurs in the liver and the kidney as part of the normal adaptation to starvation and in response to stress. The driving force is the fall in insulin and the rise in the counterregulatory hormones (glucagon, adrenaline, norepinephrine, cortisol) and, in trauma and sepsis, the release of cytokines that affect peripheral and hepatic metabolism. All of the TCA cycle intermediates potentially act as gluconeogenic precursors but, as stated earlier, the reaction catalysed by pyruvate dehydrogenase that takes pyruvate into the TCA cycle is irreversible. The key enzyme for gluconeogenesis is phosphoenolpyruvate carboxy-kinase, which converts oxaloacetate to phosphoenolpyruvate, the final product of the glycolytic pathway before pyruvate. Glucose is then synthesized via reversal of the various steps in glycolysis, bypassing the ‘irreversible’ ones referred to earlier. Amino acids are transferred to the liver and kidney, about 50% of them as alanine (the amino acid corresponding to pyruvate) and glutamine (the amino acid corresponding to α-ketoglutarate, a constituent of the TCA cycle). Transamination takes place in the periphery, mostly by transfer of amino groups from the branch-chain amino acids, valine, leucine and isoleucine. Other gluconeogenic precursors are lactate and glycerol. Lactate is synthesized to glucose in the liver and glycerol via dihydroxyacetone phosphate, one of the constituents of the glycolytic pathway. ◆
Protein metabolism Proteins are made up of chains of amino acids (see page 172). Interconversions can take place between amino acids and products of fat and carbohydrate metabolism involving transfer of the amino groups to other ketoacids by transamination, or oxidative deamination can take place with the formation of NH4+ which
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:4
Further reading Frayn KN. Metabolic regulation. A human perspective, 2nd edn. Oxford: Blackwell Science, 2003. Ganong WR. Review of medical physiology, 21st edn. New York: Lange Medical Books/McGraw-Hill, 2005.
179
© 2008 Elsevier Ltd. All rights reserved.