Nonenzymatic glycation alters protein structure and stability. A study of two eye lens crystallins.

We have investigated the effect of nonenzymatic glycation (fructation) in vitro on the structure and stability of two proteins that are glycated in vivo as a consequence of high endogenous levels of sugar. We find that whereas fructation leads to the structural destabilization of the monomeric gamma-crystallin from the core of the eye lens, it leads to an increase in stability in the multimeric alpha-crystallin of the lens cortex. Thus, while glycated gamma-crystallin shows (a) a longer wavelength of fluorescent emission, indicating a greater exposure of its aromatic side chains to the medium; (b) a reduced secondary structural content; and (c) a more facile denaturation by thermodynamic and chemical means, alpha-crystallin displays the opposite behavior. Furthermore, alpha-crystallin shows an increased tendency toward multimeric aggregation upon fructation. We interpret these differences in the broad context of the effects of neutralization of positive charges on protein structure and stability. Fructation tends to destabilize gamma-crystallin, by effecting a significant reversal in the balance of charges in the protein, at physiological pH. alpha-Crystallin is a multimeric protein whose pI is lower than its pH of optimum stability. Fructation in this case effectively neutralizes the cationic charges and promotes conformational order. This study indicates that although glycation brings about similar changes in the covalent chemical structures of proteins, its influence on the three-dimensional structures of different proteins can be different.