Proteins glycosylation, nonenzymatic

It is well known that proteins undergo nonenzymatic glycosylation reactions under hyperglycemic conditions. These reactions are referred to as glycation to distinguish them from enzymatic glycosylation catalyzed by glycosyltransferase. 

B28. Brownlee, M., Pongor, S., and Cerami, A., Covalent attachment of soluble proteins by nonenzymatically glycosylated collagen Role in the in situ formation of immune complexes. J. Exp. Med. 158, 1739-1744 (1983). 

Although nonenzymatic glycosylation may affect practically every protein in vivo, it is likely that nonenzymatic browning will occur only in proteins that have a slow turnover or none at all, such as lens crystallins, collagen, elastin and proteoglycans. In some tissues, these proteins are, in effect, "stored" for a lifetime and undergo some characteristic changes, many of which have been observed in stored and processed foodstuffs (Table II). 

The Amadori sugar-amino acid residue adducts in proteins are produced with prolonged hyperglycemia and undergo progressive nonenzymatic reactions involving dehydration, condensation, and cyclization. These compounds are collectively known as advanced glycosylation end products and are involved in the chronic complications of diabetes mellitus (cataracts and nephropathy) (Chapter 22). 

Cu,Zn-SOD is subject to nonenzymatic glycosylation (glycation). The reaction often causes inactivation of the protein through glycation of lysine residues 122 and 128, which play a role in the entrance of the substrate O into the enzyme. 

P6. Patrick, J. S., Thorpe, S. R., and Baynes, J. W., Nonenzymatic glycosylation of protein does not increase with age in normal human lenses. J. Gerontol. 45, B18-B23 (1990). 

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