Biologically important thiol-disulfide reactions and the role of cyst(e)ine in proteins: an evolutionary perspective.

Selected aspects of the reactions of thiols and disulfides are reviewed in an evolutionary context with special emphasis on the implications of the transition from a reducing to an oxidizing atmosphere on the earth. It is argued that thiols were important in prebiotic chemistry and in primitive metabolism but that disulfides, owing to their general instability in a reducing environment, came to be of importance as structural links in proteins only after the transition to an oxidizing atmosphere. The occurrence of glutathione is reviewed and discussed in terms of the role of glutathione in maintaining a reducing intracellular environment. The occurrence of cysteine and cystine in intracellular and extracellular proteins of bacteria and of animals is examined in terms of the redox state of the environment in which the protein functions. Thiol-disulfide changes associated with the dormant state are described and the role of cellular water content in dormancy is discussed. The potential significance of reactions between thiols and products of oxidative metabolism is discussed with special emphasis upon thiol additions to carbonyl and alpha, beta-unsaturated carbonyl groups and their possible role in steroid-receptor interaction.