Harpers

GLYCOPROTEINS / 521Table 47–9. Summary of main featuresof O-glycosylation.• Involves a battery of membrane-bound glycoprotein glycosyltransferasesacting in a stepwise manner; each transferaseis generally specific for a particular type of linkage.• The enzymes involved are located in various subcompartmentsof the Golgi apparatus.• Each glycosylation reaction involves the appropriatenucleotide-sugar.• Dolichol-P-P-oligosaccharide is not involved, nor are glycosidases;and the reactions are not inhibited by tunicamycin.• O-Glycosylation occurs posttranslationally at certain Ser andThr residues.N-LINKED GLYCOPROTEINS CONTAINAN Asn-GlcNAc LINKAGEN-Linked glycoproteins are distinguished by the presenceof the Asn-GlcNAc linkage (Figure 47–1). It is themajor class of glycoproteins and has been much studied,since the most readily accessible glycoproteins (eg,plasma proteins) mainly belong to this group. It includesboth membrane-bound and circulating glycoproteins.The principal difference between this and theprevious class, apart from the nature of the amino acidto which the oligosaccharide chain is attached (Asn versusSer or Thr), concerns their biosynthesis.Complex, Hybrid, & High-MannoseAre the Three Major Classesof N-Linked OligosaccharidesThere are three major classes of N-linked oligosaccharides:complex, hybrid, and high-mannose (Figure47–4). Each type shares a common pentasaccharide,Man 3 GlcNAc 2 —shown within the boxed area in Figure47–4 and depicted also in Figure 47–5—but they differin their outer branches. The presence of the commonpentasaccharide is explained by the fact that all threeclasses share an initial common mechanism of biosynthesis.Glycoproteins of the complex type generallycontain terminal NeuAc residues and underlying Galand GlcNAc residues, the latter often constituting thedisaccharide N-acetyllactosamine. Repeating N-acetyllactosamineunits—[Galβ1–3/4GlcNAcβ1–3] n (poly-N-acetyllactosaminoglycans)—are often found on N-linked glycan chains. I/i blood group substances belongto this class. The majority of complex-type oligosaccharidescontain two, three, or four outer branches (Figure47–4), but structures containing five branches have alsobeen described. The oligosaccharide branches are oftenreferred to as antennae, so that bi-, tri-, tetra-, andpenta-antennary structures may all be found. A bewilderingnumber of chains of the complex type exist, andthat indicated in Figure 47–4 is only one of many.Other complex chains may terminate in Gal or Fuc.High-mannose oligosaccharides typically have two tosix additional Man residues linked to the pentasaccharidecore. Hybrid molecules contain features of both ofthe two other classes.The Biosynthesis of N-LinkedGlycoproteins InvolvesDolichol-P-P-OligosaccharideLeloir and his colleagues described the occurrence ofa dolichol-pyrophosphate-oligosaccharide (Dol-P-Poligosaccharide),which subsequent research showedto play a key role in the biosynthesis of N-linked glycoproteins.The oligosaccharide chain of this compoundgenerally has the structure R-GlcNAc 2 Man 9 Glc 3 (R =Dol-P-P). The sugars of this compound are first assembledon the Dol-P-P backbone, and the oligosaccharidechain is then transferred en bloc to suitable Asn residuesof acceptor apoglycoproteins during their synthesison membrane-bound polyribosomes. All N-glycanshave a common pentasaccharide core structure (Figure47–5).To form high-mannose chains, only the Glcresidues plus certain of the peripheral Man residues areremoved. To form an oligosaccharide chain of the complextype, the Glc residues and four of the Manresidues are removed by glycosidases in the endoplasmicreticulum and Golgi. The sugars characteristic ofcomplex chains (GlcNAc, Gal, NeuAc) are added bythe action of individual glycosyltransferases located inthe Golgi apparatus. The phenomenon whereby theglycan chains of N-linked glycoproteins are first partiallydegraded and then in some cases rebuilt is referredto as oligosaccharide processing. Hybrid chains areformed by partial processing, forming complex chainson one arm and Man structures on the other arm.Thus, the initial steps involved in the biosynthesis ofthe N-linked glycoproteins differ markedly from thoseinvolved in the biosynthesis of the O-linked glycoproteins.The former involves Dol-P-P-oligosaccharide; thelatter, as described earlier, does not.The process of N-glycosylation can be broken downinto two stages: (1) assembly of Dol-P-P-oligosaccharideand transfer of the oligosaccharide; and (2) processingof the oligosaccharide chain.A. ASSEMBLY & TRANSFER OFDOLICHOL-P-P-OLIGOSACCHARIDEPolyisoprenol compounds exist in both bacteria andeukaryotic cells. They participate in the synthesis ofbacterial polysaccharides and in the biosynthesis of N-