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Dermatan sulphate: Structure, biosynthesis and functions
156 acid H-atoms to the structure of glucose would be made (Step 4, Fig 1).
Background
Acknowledgement
Dermatan sulphate, earlier named chondriotin sulphate B and 13-heparin, was first isolated from pig skin by Meyer and Chaffee in 1941.~ It belongs to a family of compounds called glycosaminoglycans (GAGs). GAGs contain alternating units of a hexuronic acid (o-glucuronic acid or Liduronic acid) and a hexosamine (I>glucosamine or ogalactosamine) with the exception of keratan sulphates which have no hexuronic acid. Hyaluronic acid and chondroitin sulphate contain o-glucuronic acid while dermatan sulphate, heparin and heparan sulphate contain both o-glucuronic acid and c-iduronic acid. GAGs containing exclusively c-iduronic acid have not been reported. GAGs occur as proteoglycans in connective tissue. Covalent linkage of polysaccharide to the polypeptide chain is of 3 types: (i) an O-glycosidic bond between xylose and serine, (ii) an O-glycosidic bond between galactosamine and serine or threonine, and (iii) an Nglycosidic bond between glucosamine and amide nitrogen of asparagine.
I thank Eric Bonner, Dan Krane and David Szymkowski for help with the computer-drawn figures.
Biosynthesis
Net Result The net result of these metabolic interrelationships in the liver during fasting and starvation is that the most efficient storage form of metabolic free energy (fatty acids) is mobilized and partially converted into glucose, a necessary form of metabolic free energy for certain cell types (brain, erythrocytes). This biological interconversion of free energy between the molecular forms of fatty acid and carbohydrate may even be more pronounced, since there has been recent evidence for conversion of fatty acid carbon skeleton into that of glucose via acetone metabolism. 2'3 Irrespective of future verification of such a metabolic pathway for fatty acid carbons, textbooks should give more emphasis to the contribution that fatty acid hydrogen atoms can make to net glucose synthesis in the liver by the routes presented in Figs 1 and 2.
References 1Cahill, G F (1978) Ecology o f Food and Nutrition 6, 221-230 2Argiles, J M (1986) Trends in Biochem Sci 11, 61-63 3Kosugi, K, Scofield, R F Chandramonli, V, Kumaran, K, Schumann, W C and Landau, B R (1986) J Biol Chem 261, 3952-3957
Dermatan Sulphate: Structure, Biosynthesis and Functions S K SINGLA
Department of Biochemistry Punjab Agricultural University Ludhiana-141004, India
Chain synthesis of GAGs proceeds by successive addition of single sugar residues to the core protein. UDP is the carrier of sugars in the activated form and specific glycosyl transferases catalyse these sugar transfer reactions. UDPglucose (UDPG) and UDP-N-acetylglucosamine (UDPNAG) are the starting materials and other monosaccharides are formed by modification of glucose and Nacetylglucosamine linked to UDP. For example, UDPG can be oxidised to UDP-glucuronate (UDP-GIcUA) which can further be decarboxylated to UDP-xylose. Both U D P G and UDP-NAG can undergo epimerization at C4 to form UDP-galactose (UDP-Gal) and UDP-N-acetylgalactosamine (UDP-NAGal). Various steps in biosynthesis of chondroitin sulphate (CS) and dermatan sulphate (DS) are outlined below and the disaccharide repeating unit of DS is shown in Figure 1. CH20H
A consideration of the interesting and important substance, dermatan sulphate, can form a useful and illuminating area for discussion in teaching biochemistry. The structures involved can be described during revision of carbohydrate structure, which will incidentally reveal how great a range of carbohydrate polymers may be formed from a relatively few building blocks. A consideration of the biosynthetic reactions can be used to show how polysaccharides and certain glycoproteins are made and bring out points concerning coupled reactions and the use of nucleotide diphosphate sugars. The compounds eventually formed (GAGs, see below) have, of course, extremely important structural roles in the matrix of connective tissue. Surprisingly few textbooks deal with this fascinating topic area adequately or indeed at all.
BIOCHEMICAL EDUCATION 16(3) 1988
-o
i
Introduction
%so/--°,ol
OH
NHCOCH3
Figure 1 Unless otherwise stated, all the monosaccharides are [3o-isomers in the following reactions:
A Chain synthesis (1) Xylosyl transfer Ser
UDP--xyLose
\
UDP
J
XyLosyL tronsferase
Xyt--Ser
•
.,.
157
Functions
(2) Two galactosyl transfers XyL--Ser Core
UDP--GaL
Pr!tein
UDP
)
4--Ga L--XyL--Ser :
GatactosyL trar~ferase I
Core Pr!t.ein [ I
UDP--GaL~,I ] GaLactosyL UDP~ transferase TT 3--GaL--GaL--XyL--Ser Core
I
Protein
(3) Glucuronate transfer UDP--GLcUA UDP 3--Gal-- GoL--XyL--Ser 3-GLcUA-GaL--Gol--Xyl--Ser Core Prlotein ~ J : Core PrJtein GLucurona~etransferose
(4) Galactosamine transfer UDP--NAGal UDP 3-GLcUA-GaL--GoL--XyL--Ser ~ j 4-NAGaL--GLcUA--Gat--GaL--XyL--Ser Core Prote n Core Proten NAGaL transferose
B Sulphatation 3'-phosphoadenosine-5'-phosphosulphate (PAPS) is the active donor of sulphate group. 2 Sulphate groups are transferred at positions 4 or 6 of galactosamine in CS and DS and at position 2 of Liduronate in DS. Sulphatation at C4 of hexosamine in CS is probably a signal for chain termination as CS chains ending in NAGal-4-SO4 are unable to accept GlcUA. C Epimerization Malmstr6m (1975) 3 has described the epimerization of o-glucuronate to L-iduronate. This illustrates the fact that epimerization at C5 changes the sugar from the o- to the L-series. In DS, O-sulphatation (in contrast to N-sulphatation in heparin and heparan sulphate) is required for C5-epimerization. It is not known whether sulphatation occurs before or after epimerization but it drives the reaction towards iduronate formation. Sulphatation before epimerization may meet the specificity requirement of the epimerase. Sulphatation after epimerization may shift the otherwise unfavourable equilibrium towards L-iduronate formation by withdrawing the initial product. DS differs from CS in two respects, namely the nature of hexuronic acid (which is L-iduronic acid in DS and o-glucuronic acid in CS) and the glycosidic linkage which is ix-l,3 in DS and 13-1,3 in CS. The second point of difference needs emphasis but is not even mentioned in some text-books. 4'5 In fact, in the book by West et al (1966) 6 the 13-anomer of L-iduronic acid is incorrectly depicted as the tx-anomer. Very few text-books mention that in ot-anomer of the L-series of monosaccharides the anomeric carbon has the hydroxyl group pointing upwards. Roden 7 has emphasised that the a-linkage of eiduronic acid is analogous to the B-linkage of the chondroitin sulphates.
BIOCHEMICAL EDUCATION 16(3) 1988
Poole (1986) 8 has reviewed the functions of various proteoglycans. Many proteoglycans containing DS are known. They differ in size and iduronate content. Small DS proteoglycans having 30-60% of their weight as core protein are the major DS containing proteoglycans. They have 1-2 DS chains and iduronate varying from 45-85% of the total hexuronate. Small DS-containing proteoglycans inhibit fibril assembly of type I and type II collagens of tendons and cartilage, respectively. 9 Low iduronate-containing DS is present in cornea. By inserting itself between the collagen fibrils it allows corneal transparency. In opaque corneal scars the space between fibrils is greater and more proteoglycan is present in this space. By regulating fibril spacing in cornea, DS plays a role in optical clarity. Large DS-containing proteoglycans have been shown to be present in fibroblasts and in follicular fluids but their functions are yet to be established. References 1Meyer, K and Chaffee, E (1941) J Biol Chem 138,491 2Robbins, P W and Lipmann, F (1957) J Biol Chem 229, 837-851 3Malmstr6m, A, Franson, L A, Hook, M and Lindahl, U (1975). J Biol Chem 250, 3419-3425 4Stryer, L (1981) 'Biochemistry' (Second Edition) W H Freeman & Co, New York, p 201 5White, A, Handler, P, Smith, E, Hill, R L and Lehman, I E (1978) 'Principles of Biochemistry' (Sixth Edition) McGraw-Hill Kogkusha, p 1149 6West, E S, Todd, W R, Mason, H S and Bruggen, J T V (1966) 'Textbook of Biochemistry', The MacMillan Company, New York, p 252 7Roden, L (1981) In 'The Biochemistry of Glycoproteins and Proteoglycans', edited by Lennarz, W L, Plenum Press, New Work, pp 352-354 Spoole, A R (1986) Biochem J 236, 1-14 9Vogel, K G, Paulsson, M and Heinegard, D (1984) Biochern J 223, 587-597
Book Review Chemical Synthesis in Molecular Biology Edited by H Blocker, R Frank and H-J Fritz. pp 222. VCH Publishers, Weinheim, FRG. 1987. DM 128 ISBN 3-527-26564-3 This book deals, mainly from a synthetic organic chemical viewpoint, with topics in molecular biology including proteinDNA interaction, total synthesis of somatomedin C gene, DNA repair, peptide synthesis, site-directed mutagenesis and the oligosaccharide chains of glycoproteins. The major impression given is that this volume is somewhat superfluous. The contributions on peptide synthesis, anti-peptide antibodies and protein engineering lack both novelty and interest since these areas have been better covered elsewhere (even by the same authors). One paper on inhibitors of oligosaccharide processing has been reprinted from a journal. The papers on synthetic DNA chemistry are, in the main, only of marginal interest to biochemists. G E Blair
Background
Acknowledgement
Dermatan sulphate, earlier named chondriotin sulphate B and 13-heparin, was first isolated from pig skin by Meyer and Chaffee in 1941.~ It belongs to a family of compounds called glycosaminoglycans (GAGs). GAGs contain alternating units of a hexuronic acid (o-glucuronic acid or Liduronic acid) and a hexosamine (I>glucosamine or ogalactosamine) with the exception of keratan sulphates which have no hexuronic acid. Hyaluronic acid and chondroitin sulphate contain o-glucuronic acid while dermatan sulphate, heparin and heparan sulphate contain both o-glucuronic acid and c-iduronic acid. GAGs containing exclusively c-iduronic acid have not been reported. GAGs occur as proteoglycans in connective tissue. Covalent linkage of polysaccharide to the polypeptide chain is of 3 types: (i) an O-glycosidic bond between xylose and serine, (ii) an O-glycosidic bond between galactosamine and serine or threonine, and (iii) an Nglycosidic bond between glucosamine and amide nitrogen of asparagine.
I thank Eric Bonner, Dan Krane and David Szymkowski for help with the computer-drawn figures.
Biosynthesis
Net Result The net result of these metabolic interrelationships in the liver during fasting and starvation is that the most efficient storage form of metabolic free energy (fatty acids) is mobilized and partially converted into glucose, a necessary form of metabolic free energy for certain cell types (brain, erythrocytes). This biological interconversion of free energy between the molecular forms of fatty acid and carbohydrate may even be more pronounced, since there has been recent evidence for conversion of fatty acid carbon skeleton into that of glucose via acetone metabolism. 2'3 Irrespective of future verification of such a metabolic pathway for fatty acid carbons, textbooks should give more emphasis to the contribution that fatty acid hydrogen atoms can make to net glucose synthesis in the liver by the routes presented in Figs 1 and 2.
References 1Cahill, G F (1978) Ecology o f Food and Nutrition 6, 221-230 2Argiles, J M (1986) Trends in Biochem Sci 11, 61-63 3Kosugi, K, Scofield, R F Chandramonli, V, Kumaran, K, Schumann, W C and Landau, B R (1986) J Biol Chem 261, 3952-3957
Dermatan Sulphate: Structure, Biosynthesis and Functions S K SINGLA
Department of Biochemistry Punjab Agricultural University Ludhiana-141004, India
Chain synthesis of GAGs proceeds by successive addition of single sugar residues to the core protein. UDP is the carrier of sugars in the activated form and specific glycosyl transferases catalyse these sugar transfer reactions. UDPglucose (UDPG) and UDP-N-acetylglucosamine (UDPNAG) are the starting materials and other monosaccharides are formed by modification of glucose and Nacetylglucosamine linked to UDP. For example, UDPG can be oxidised to UDP-glucuronate (UDP-GIcUA) which can further be decarboxylated to UDP-xylose. Both U D P G and UDP-NAG can undergo epimerization at C4 to form UDP-galactose (UDP-Gal) and UDP-N-acetylgalactosamine (UDP-NAGal). Various steps in biosynthesis of chondroitin sulphate (CS) and dermatan sulphate (DS) are outlined below and the disaccharide repeating unit of DS is shown in Figure 1. CH20H
A consideration of the interesting and important substance, dermatan sulphate, can form a useful and illuminating area for discussion in teaching biochemistry. The structures involved can be described during revision of carbohydrate structure, which will incidentally reveal how great a range of carbohydrate polymers may be formed from a relatively few building blocks. A consideration of the biosynthetic reactions can be used to show how polysaccharides and certain glycoproteins are made and bring out points concerning coupled reactions and the use of nucleotide diphosphate sugars. The compounds eventually formed (GAGs, see below) have, of course, extremely important structural roles in the matrix of connective tissue. Surprisingly few textbooks deal with this fascinating topic area adequately or indeed at all.
BIOCHEMICAL EDUCATION 16(3) 1988
-o
i
Introduction
%so/--°,ol
OH
NHCOCH3
Figure 1 Unless otherwise stated, all the monosaccharides are [3o-isomers in the following reactions:
A Chain synthesis (1) Xylosyl transfer Ser
UDP--xyLose
\
UDP
J
XyLosyL tronsferase
Xyt--Ser
•
.,.
157
Functions
(2) Two galactosyl transfers XyL--Ser Core
UDP--GaL
Pr!tein
UDP
)
4--Ga L--XyL--Ser :
GatactosyL trar~ferase I
Core Pr!t.ein [ I
UDP--GaL~,I ] GaLactosyL UDP~ transferase TT 3--GaL--GaL--XyL--Ser Core
I
Protein
(3) Glucuronate transfer UDP--GLcUA UDP 3--Gal-- GoL--XyL--Ser 3-GLcUA-GaL--Gol--Xyl--Ser Core Prlotein ~ J : Core PrJtein GLucurona~etransferose
(4) Galactosamine transfer UDP--NAGal UDP 3-GLcUA-GaL--GoL--XyL--Ser ~ j 4-NAGaL--GLcUA--Gat--GaL--XyL--Ser Core Prote n Core Proten NAGaL transferose
B Sulphatation 3'-phosphoadenosine-5'-phosphosulphate (PAPS) is the active donor of sulphate group. 2 Sulphate groups are transferred at positions 4 or 6 of galactosamine in CS and DS and at position 2 of Liduronate in DS. Sulphatation at C4 of hexosamine in CS is probably a signal for chain termination as CS chains ending in NAGal-4-SO4 are unable to accept GlcUA. C Epimerization Malmstr6m (1975) 3 has described the epimerization of o-glucuronate to L-iduronate. This illustrates the fact that epimerization at C5 changes the sugar from the o- to the L-series. In DS, O-sulphatation (in contrast to N-sulphatation in heparin and heparan sulphate) is required for C5-epimerization. It is not known whether sulphatation occurs before or after epimerization but it drives the reaction towards iduronate formation. Sulphatation before epimerization may meet the specificity requirement of the epimerase. Sulphatation after epimerization may shift the otherwise unfavourable equilibrium towards L-iduronate formation by withdrawing the initial product. DS differs from CS in two respects, namely the nature of hexuronic acid (which is L-iduronic acid in DS and o-glucuronic acid in CS) and the glycosidic linkage which is ix-l,3 in DS and 13-1,3 in CS. The second point of difference needs emphasis but is not even mentioned in some text-books. 4'5 In fact, in the book by West et al (1966) 6 the 13-anomer of L-iduronic acid is incorrectly depicted as the tx-anomer. Very few text-books mention that in ot-anomer of the L-series of monosaccharides the anomeric carbon has the hydroxyl group pointing upwards. Roden 7 has emphasised that the a-linkage of eiduronic acid is analogous to the B-linkage of the chondroitin sulphates.
BIOCHEMICAL EDUCATION 16(3) 1988
Poole (1986) 8 has reviewed the functions of various proteoglycans. Many proteoglycans containing DS are known. They differ in size and iduronate content. Small DS proteoglycans having 30-60% of their weight as core protein are the major DS containing proteoglycans. They have 1-2 DS chains and iduronate varying from 45-85% of the total hexuronate. Small DS-containing proteoglycans inhibit fibril assembly of type I and type II collagens of tendons and cartilage, respectively. 9 Low iduronate-containing DS is present in cornea. By inserting itself between the collagen fibrils it allows corneal transparency. In opaque corneal scars the space between fibrils is greater and more proteoglycan is present in this space. By regulating fibril spacing in cornea, DS plays a role in optical clarity. Large DS-containing proteoglycans have been shown to be present in fibroblasts and in follicular fluids but their functions are yet to be established. References 1Meyer, K and Chaffee, E (1941) J Biol Chem 138,491 2Robbins, P W and Lipmann, F (1957) J Biol Chem 229, 837-851 3Malmstr6m, A, Franson, L A, Hook, M and Lindahl, U (1975). J Biol Chem 250, 3419-3425 4Stryer, L (1981) 'Biochemistry' (Second Edition) W H Freeman & Co, New York, p 201 5White, A, Handler, P, Smith, E, Hill, R L and Lehman, I E (1978) 'Principles of Biochemistry' (Sixth Edition) McGraw-Hill Kogkusha, p 1149 6West, E S, Todd, W R, Mason, H S and Bruggen, J T V (1966) 'Textbook of Biochemistry', The MacMillan Company, New York, p 252 7Roden, L (1981) In 'The Biochemistry of Glycoproteins and Proteoglycans', edited by Lennarz, W L, Plenum Press, New Work, pp 352-354 Spoole, A R (1986) Biochem J 236, 1-14 9Vogel, K G, Paulsson, M and Heinegard, D (1984) Biochern J 223, 587-597
Book Review Chemical Synthesis in Molecular Biology Edited by H Blocker, R Frank and H-J Fritz. pp 222. VCH Publishers, Weinheim, FRG. 1987. DM 128 ISBN 3-527-26564-3 This book deals, mainly from a synthetic organic chemical viewpoint, with topics in molecular biology including proteinDNA interaction, total synthesis of somatomedin C gene, DNA repair, peptide synthesis, site-directed mutagenesis and the oligosaccharide chains of glycoproteins. The major impression given is that this volume is somewhat superfluous. The contributions on peptide synthesis, anti-peptide antibodies and protein engineering lack both novelty and interest since these areas have been better covered elsewhere (even by the same authors). One paper on inhibitors of oligosaccharide processing has been reprinted from a journal. The papers on synthetic DNA chemistry are, in the main, only of marginal interest to biochemists. G E Blair