Thornton34 mentions that free thiols are unstable outside the cell, that is, cysteines predominantly occur in disulfide bridges (see also Fahey et ill.35). In the intracellular environment the thiols are kept reduced by glutathione, but once outside they are reactive and may even cause polymerizations. In our survey we grouped the proteins into three subgroups (intracellular, extracellular, and periplasmic), according to their cellular locations, and checked the occurrence of different types of cysteine in the subgroups (Table I). The cellular location shows a high correlation with the oxidation state of the cysteines, but this correlation is not exclusive. In our set, among the intra- and extracellular proteins only one contains both types of cysteine (Table I) whereas 10% of extracellular proteins contain free cysteine. Inside the cell, none of the proteins contain disulfide bridges, according to our data set. Only a few periplasmic proteins are present in the data set, but they
33 D. Bordo, K. Dijnovic, and M. Bolognesi, J. Mol. Biol. 238, 366 (1994).
35 R. C. Fahey, J. S. Hunt, and G. C. Windham, J. Mol. Evol. 10, 155 (1977).
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