Harpers

GLYCOPROTEINS / 515Table 47–1. Some functions servedby glycoproteins.FunctionGlycoproteinsStructural molecule CollagensLubricant andMucinsprotective agentTransport molecule Transferrin, ceruloplasminImmunologic molecule Immunoglobulins, histocompatibilityantigensHormoneChorionic gonadotropin, thyroidstimulatinghormone (TSH)EnzymeVarious, eg, alkaline phosphataseCell attachment- Various proteins involved in cell-cellrecognition site (eg, sperm-oocyte), virus-cell,bacterium-cell, and hormone-cellinteractionsAntifreezeCertain plasma proteins of coldwater fishInteract with specific Lectins, selectins (cell adhesioncarbohydrates lectins), antibodiesReceptorVarious proteins involved in hormoneand drug actionAffect folding of Calnexin, calreticulincertain proteinsTable 47–2. Some functions of theoligosaccharide chains of glycoproteins. 1• Modulate physicochemical properties, eg, solubility, viscosity,charge, conformation, denaturation, and bindingsites for bacteria and viruses• Protect against proteolysis, from inside and outside of cell• Affect proteolytic processing of precursor proteins tosmaller products• Are involved in biologic activity, eg, of human chorionicgonadotropin (hCG)• Affect insertion into membranes, intracellular migration,sorting and secretion• Affect embryonic development and differentiation• May affect sites of metastases selected by cancer cells1 Adapted from Schachter H: Biosynthetic controls that determinethe branching and heterogeneity of protein-bound oligosaccharides.Biochem Cell Biol 1986;64:163.Table 47–3. Some important methods used tostudy glycoproteins.MethodUsePeriodic acid-Schiff reagent Detects glycoproteins as pinkbands after electrophoretic separation.Incubation of cultured cells Leads to detection of a radiowithglycoproteins as active sugar after electrophoradioactivebands retic separation.Treatment with appropriate Resultant shifts in electrophoendo-or exoglycosidase retic migration help distinguishor phospholipases among proteins with N-glycan,O-glycan, or GPI linkages andalso between high mannoseand complex N-glycans.Sepharose-lectin column To purify glycoproteins or glychromatographycopeptides that bind the particularlectin used.Compositional analysis Identifies sugars that the glyfollowingacid hydrolysis coprotein contains and theirstoichiometry.Mass spectrometryProvides information on molecularmass, composition, sequence,and sometimes branchingof a glycan chain.NMR spectroscopyTo identify specific sugars, theirsequence, linkages, and theanomeric nature of glycosidiclinkages.Methylation (linkage) To determine linkages betweenanalysissugars.Amino acid or cDNA Determination of amino acidsequencingsequence.EIGHT SUGARS PREDOMINATEIN HUMAN GLYCOPROTEINSAbout 200 monosaccharides are found in nature; however,only eight are commonly found in the oligosaccharidechains of glycoproteins (Table 47–4). Most ofthese sugars were described in Chapter 13. N-Acetylneuraminicacid (NeuAc) is usually found at the terminiof oligosaccharide chains, attached to subterminalgalactose (Gal) or N-acetylgalactosamine (GalNAc)residues. The other sugars listed are generally found inmore internal positions. Sulfate is often found in glycoproteins,usually attached to Gal, GalNAc, or GlcNAc.