Sulfhydryl Group

(or thiol group), an SH group of organic compounds. Sulfhydryl groups have great and varied reactivity. They oxidize easily, with the formation of disulfides and sulfenic, sulfinic, or sulfonic acids, and they readily undergo alkylation, acylation, and thiol-disulfide exchange. They form mercaptides upon reacting with the ions of heavy metals, and they form mer-captals and mercaptols upon reacting with aldehydes and ketones, respectively.

Sulfhydryl groups play an important role in biochemical processes. The sulfhydryl groups of coenzyme A, lipoic acid, and 4’-phosphopantotheine participate in enzymic reactions for the formation and transfer of acyl residues that are related to lipid and carbohydrate metabolism. The sulfhydryl groups of glutathione play an important role in the neutralization of foreign organic compounds and the reduction of peroxides; they are also of major importance in the fulfillment by glutathione of its function as a coenzyme. In proteins, residues of the amino acid cysteine have sulfhydryl groups. As components of the active centers of a number of enzymes, sulfhydryl groups participate in the catalytic effect of the enzymes and in the binding of substrates, coenzymes, and metal ions. The catalytic role of the sulfhydryl groups of enzymes consists in the formation of intermediates with substrates or their residues; in some oxidative enzymes, the process consists in the transfer of electrons and protons from substrates to acceptors.

The blocking of sulfhydryl groups by means of specific reagents results in partial or complete inhibition of the activity of many enzymes. Disulfide bonds (—S—S—), which are formed upon oxidation of sulfhydryl groups during biosynthesis of proteins, play an important role in stabilizing the structures of proteins, including enzymes, antibodies, and several hormones. Cleavage of disulfide bonds leads to disruption of the native structure of the proteins and to loss of the proteins’ biological activity.