Of glycosylated proteins

Interferons [alFN, piFN and ylFN]. Interferons are a family of glycosylated proteins and are cytokines which are produced a few hours after cells have been infected with a virus. Interferons protect cells from viral infections and have antiviral activities at very low concentrations ( 3 x 10 M, less than 50 molecules are apparently sufficient to protect a single cell). Double stranded RNA are very efficient inducers of IFNs. There are three main types of IFNs. The aIFNs are synthesised in lymphocytes and the piFNs are formed in infected fibroblasts. The a and P families are fairly similar consisting of ca 166 to 169 amino acids. Although ylFNs are also small glycosylated proteins (ca 146 amino acids), they are different because they are not synthesised after viral infections but are produced by lymphocytes when stimulated by mitogens (agents that induced cell division). 

Other adhesion receptors that are structurally and functionally related include the receptors for fibronectin, vitronectin, platelet glycoproteins 13b and Ilia and the VLA (very-late antigen) series. All molecules involved in adhesion recognise the RGD motif and require the divalent cations Ca2+ and Mg2+ for binding. All are dimers of glycosylated proteins with relative molecular masses 95-190 kDa. There is also some sequence homology between the /J-chain (CD18) and one chain of the fibronectin receptor. 

However, certain limitations do exist that need to be considered. Although enzymes necessary for post-translational modifications can be added, in principle there is currently no productive system available for the preparation of glycosylated proteins, although some interesting results have already been obtained [161]. Also, the expression of functional membrane proteins in quantities necessary for structural analysis will be a challenging task for the future. 

Figure 1. Amadori rearrangement of glycosylated proteins. Integration of known reactions leading to browning in sugar-amine systems. (Reproduced from Ref. 101. Copyright 1953, American Chemical Society.)...

McVerry, B. A., Fisher, C., Hopp, A., and Huehns, E. R., Production of pseudodiabetic renal glomerular changes in mice after repeated injections of glycosylated proteins. Lancet 1, 738-740 (1980). 

N-Acetylneuraminic acid aldolase catalyzes the cleavage of N-acetylneuraminic acid (Neu5Ac) to N-acetylmannosamine (ManNAc) and pyruvate (Pyr). The reverse reaction can be employed to synthesize N-acetylneuraminic acid, which plays an important physiological role as a terminal sugar residue of glycosylated proteins 1481 (Eq. (3)). 

Protein glycosylation is a PTM of eukaryotic proteins and does not occur in prokaryotes. The carbohydrate groups are highly variable with significant effects on protein folding, stability, and activity. The importance of HPCE for the analysis of glycosylated proteins is described in Chapter 22. 

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