221
biotopics The coming of age of glycobiology The foundations of carbohydrate structure, like those of protein chemistry, were laid in the organic chemistry laboratories of Europe. Sugar chemistry in the life sciences dates, in one sense, from the notable proof of the configuration of dextrorotatory (+)-glucose by the great chemist Emil Fischer in 1891, for which he received the Nobel Prize. As the discipline of biochemistry evolved, sugar chemistry became synonymous with the great discoveries of glycolysis and gluconeogenesis. The presence of carbohydrate associated with protein was .recognized from around i900, confirming earlier reports on the composition of mucins. By 1949, the covalent modification of polypeptide chains by sugar polymers was firmly established. The neuraminic acids were recognized at a late stage because, unlike all the other sugars, these had not been found as free monosaccharides. However, the focus of world interest was captured first by proteins and then by nucleic acids. Rooted in intermediary metabolism and natural product chemistry, serious study of simple sugars and sugar polymers as biological effectors or functional determinants of proteins languished.
Versatility and heterogeneity The adherents to the field seem to have been 'voices crying in the wilderness' about the functional importance of sugar polymers. One such voice (D. A. Rees, now secretary of the UK Medical Research Council), produced a popular work on sugars and sugar polymers in the mid-1960s 1. Physical chemists working at that time made important contributions to defining the regular structures of sugar polymers, and pointed the way to understanding the tremendous versatility of this class of biopolymer (there are far more ways to link together monosaccharides than there are to link amino acids to form biopolymers). The resultant gel-forming, water-holding, protease-resisting, stiff or elastic, neutral or highly charged characteristics of sugar polymers seemed sufficient to explain their presence at cell surfaces and in interstitial tissues.
Thus, polysaccharides and proteoglycans were needed for the construction of barriers for limiting cell migration, filtering or retaining biopolymers and metabolites (basement membranes, glycocalyx), and in building bone, cartilage and other structural tissues. The glycoproteins, which carry fewer and more complex oligosaccharide chains, posed a puzzle. Their heterogeneity, that was beginning to be observed, suggested a lack of specific function. At one time, many workers considered the primary role of carbohydrate in glycoproteins to be simply to shield protease-sensitive sites.
A circle of neglect These general views may well have influenced peer review to give lower priority to fundamental research on sugar polymers because these were considered 'framework' rather than 'engine' in understanding biological mechanism. There w:,.s also the serious problem of a lack of tools with which to determine the structure of complex glycoconjugates- a prerequisite for progress in modern biochemistry. Poor peer interest or approval, together with primitive tools with which to redress the situation, created a circle of neglect. Carbohydrate biochemists used to give each other pep talks at seminars and congresses about the relevance and ultimate importance of glycoconjugates. After all, cell-surface proteins and lipids are liberally decorated with discrete and specific oligosaccharide chains; their presence would seem unlikely in the absence of an important biological purpose.
Glycobiology- recognized The tide of opinion has now turned and those who have per-
sisted in the field are beginning to reap the rewards for their faith. We have a 'new' branch of bioscience: 'Giycobiology' and new terminology including terms such as 'glycoform' (see below). Work by pharmaceutical chemists on carbohydrate drugs (such as synthetic heparins, which potentiate the anticoagulant activity of antithrombin III), is increasing. We will no doubt soon be seeing 'glycomimetics' in addition to 'peptidomimetics'. The age of the glycobiologist has arrived.
The glycoeonjugates There have been years of specialization on peptides and proteins. Glycobiology is less familiar to many bioscientists - including some involved in protein characterization - hence this short review of glycoconjugates (mainly glycanprotein conjugates), is presented here. Most types of biopolymer and cell-membrane lipids may be conjugated with sugars or sugar polymers (see Table 1 for major sugars involved). There are three major types of protein glycoconjugate: (1) linkage to Ash (Nlinkage with GIcNAc); (2) linkage to Set, Thr or hydroxylysine (Olinkage with GalNAc, Xyl or Gal); and (3) linkage to lipid via a phosphoinositol glycan - the 'GPl or PIG tail' (protein-CONH-EtOP(O)20-glycan-O-inositol phospholipid).
N-linked &lFcans Much of the current knowledge of glycoconjugates derives from structural studies on the serum glycoproteins. The N-linked glycans show wide variation in sequence, glycosidic bond anomericity and substitution of the oligosaccharide chain, but may be broadly classified into three classes on the basis of structure (Fig. 1). Most serum glycoproteins carry
Table 1. The major sugars in glycoproteins Sugar
Abbreviation
Modifications
Abbreviations
D-Galactose D-Glucose L-FUcOSe D-Mannose Neuraminic acid D-Xylose
Gal GIc Fuc Man Neu Xyl
-amine, N-acetyl -amine, N-acetyl
GAIN,GalNAc GIcN, GIcNAc
N-acetyl, N-glycolyl
NeuNAc,NeuGc
(~) 1991, ElsevierScience PublishersLtd (UK) 0167- 9430/91/$2.00
TIBTECHJULY 1991 (VOL9)
222
biotopics The common 'core' structure: Fuc 1-6 (animals), 3-6 (plants) I I
Man l ~ M a Man
n 131-4 GIcNAc~1-4 GIclNkc
1-" I!
GIcNAc~1-4 (animals),Xyl 1~1-2 (plants)
(I) Oligomannose (11) Hybrid (111)Complex
Core + oligomannose extensions
Core + GIcNAc,Gal, Fuc and NeuAc on Man ocl-3 arm only Core + GIcNAc, Gal, Fuc and NeuAc on Man ocl-3 and Man o
biotopics The coming of age of glycobiology The foundations of carbohydrate structure, like those of protein chemistry, were laid in the organic chemistry laboratories of Europe. Sugar chemistry in the life sciences dates, in one sense, from the notable proof of the configuration of dextrorotatory (+)-glucose by the great chemist Emil Fischer in 1891, for which he received the Nobel Prize. As the discipline of biochemistry evolved, sugar chemistry became synonymous with the great discoveries of glycolysis and gluconeogenesis. The presence of carbohydrate associated with protein was .recognized from around i900, confirming earlier reports on the composition of mucins. By 1949, the covalent modification of polypeptide chains by sugar polymers was firmly established. The neuraminic acids were recognized at a late stage because, unlike all the other sugars, these had not been found as free monosaccharides. However, the focus of world interest was captured first by proteins and then by nucleic acids. Rooted in intermediary metabolism and natural product chemistry, serious study of simple sugars and sugar polymers as biological effectors or functional determinants of proteins languished.
Versatility and heterogeneity The adherents to the field seem to have been 'voices crying in the wilderness' about the functional importance of sugar polymers. One such voice (D. A. Rees, now secretary of the UK Medical Research Council), produced a popular work on sugars and sugar polymers in the mid-1960s 1. Physical chemists working at that time made important contributions to defining the regular structures of sugar polymers, and pointed the way to understanding the tremendous versatility of this class of biopolymer (there are far more ways to link together monosaccharides than there are to link amino acids to form biopolymers). The resultant gel-forming, water-holding, protease-resisting, stiff or elastic, neutral or highly charged characteristics of sugar polymers seemed sufficient to explain their presence at cell surfaces and in interstitial tissues.
Thus, polysaccharides and proteoglycans were needed for the construction of barriers for limiting cell migration, filtering or retaining biopolymers and metabolites (basement membranes, glycocalyx), and in building bone, cartilage and other structural tissues. The glycoproteins, which carry fewer and more complex oligosaccharide chains, posed a puzzle. Their heterogeneity, that was beginning to be observed, suggested a lack of specific function. At one time, many workers considered the primary role of carbohydrate in glycoproteins to be simply to shield protease-sensitive sites.
A circle of neglect These general views may well have influenced peer review to give lower priority to fundamental research on sugar polymers because these were considered 'framework' rather than 'engine' in understanding biological mechanism. There w:,.s also the serious problem of a lack of tools with which to determine the structure of complex glycoconjugates- a prerequisite for progress in modern biochemistry. Poor peer interest or approval, together with primitive tools with which to redress the situation, created a circle of neglect. Carbohydrate biochemists used to give each other pep talks at seminars and congresses about the relevance and ultimate importance of glycoconjugates. After all, cell-surface proteins and lipids are liberally decorated with discrete and specific oligosaccharide chains; their presence would seem unlikely in the absence of an important biological purpose.
Glycobiology- recognized The tide of opinion has now turned and those who have per-
sisted in the field are beginning to reap the rewards for their faith. We have a 'new' branch of bioscience: 'Giycobiology' and new terminology including terms such as 'glycoform' (see below). Work by pharmaceutical chemists on carbohydrate drugs (such as synthetic heparins, which potentiate the anticoagulant activity of antithrombin III), is increasing. We will no doubt soon be seeing 'glycomimetics' in addition to 'peptidomimetics'. The age of the glycobiologist has arrived.
The glycoeonjugates There have been years of specialization on peptides and proteins. Glycobiology is less familiar to many bioscientists - including some involved in protein characterization - hence this short review of glycoconjugates (mainly glycanprotein conjugates), is presented here. Most types of biopolymer and cell-membrane lipids may be conjugated with sugars or sugar polymers (see Table 1 for major sugars involved). There are three major types of protein glycoconjugate: (1) linkage to Ash (Nlinkage with GIcNAc); (2) linkage to Set, Thr or hydroxylysine (Olinkage with GalNAc, Xyl or Gal); and (3) linkage to lipid via a phosphoinositol glycan - the 'GPl or PIG tail' (protein-CONH-EtOP(O)20-glycan-O-inositol phospholipid).
N-linked &lFcans Much of the current knowledge of glycoconjugates derives from structural studies on the serum glycoproteins. The N-linked glycans show wide variation in sequence, glycosidic bond anomericity and substitution of the oligosaccharide chain, but may be broadly classified into three classes on the basis of structure (Fig. 1). Most serum glycoproteins carry
Table 1. The major sugars in glycoproteins Sugar
Abbreviation
Modifications
Abbreviations
D-Galactose D-Glucose L-FUcOSe D-Mannose Neuraminic acid D-Xylose
Gal GIc Fuc Man Neu Xyl
-amine, N-acetyl -amine, N-acetyl
GAIN,GalNAc GIcN, GIcNAc
N-acetyl, N-glycolyl
NeuNAc,NeuGc
(~) 1991, ElsevierScience PublishersLtd (UK) 0167- 9430/91/$2.00
TIBTECHJULY 1991 (VOL9)
222
biotopics The common 'core' structure: Fuc 1-6 (animals), 3-6 (plants) I I
Man l ~ M a Man
n 131-4 GIcNAc~1-4 GIclNkc
1-" I!
GIcNAc~1-4 (animals),Xyl 1~1-2 (plants)
(I) Oligomannose (11) Hybrid (111)Complex
Core + oligomannose extensions
Core + GIcNAc,Gal, Fuc and NeuAc on Man ocl-3 arm only Core + GIcNAc, Gal, Fuc and NeuAc on Man ocl-3 and Man o
