Catalytic Site Cysteines of Thiol Enzyme: Sulfurtransferases

Thiol enzymes have single- or double-catalytic site cysteine residues and are redox active. Oxidoreductases and isomerases contain double-catalytic site cysteine residues, which are oxidized to a disulfide via a sulfenyl intermediate and reduced to a thiol or a thiolate. The redox changes of these enzymes are involved in their catalytic processes. On the other hand, transferases, and also some phosphatases and hydrolases, have a single-catalytic site cysteine residue. The cysteines are redox active, but their sulfenyl forms, which are inactive, are not well explained biologically. In particular, oxidized forms of sulfurtransferases, such as mercaptopyruvate sulfurtransferase and thiosulfate sulfurtransferase, are not reduced by reduced glutathione but by reduced thioredoxin. This paper focuses on why the catalytic site cysteine of sulfurtransferase is redox active. 1. Introduction Cysteine residues in proteins maintain the protein conformation, coordinate metal(s), and regulate protein function [1–3]. Enzymes with catalytic site cysteines (Table 1) [4–42] have critical roles in biologic processes such as cell cycle regulation, apoptosis, and signal transduction [43]. Table 1: Typical thiol enzymes. A cysteine residue that easily accepts and donates (an) electron(s) is referred to as a redox-active cysteine, and has a lower pKa value than an unperturbed cysteine residue. Clairborne and colleagues extensively and successfully studied redox changes of cysteine residues and reviewed the biologic importance of redox-active cysteine [44, 45]; a redox-active cysteine is generally a thiolate at physiologic pH and is easily oxidized to a sulfenic acid. Cysteine-related enzymes are generally inhibited by mild oxidation and are reversibly reduced by thioredoxin or glutathione. The sulfenyl form is a reaction intermediate for peroxiredoxin to form disulfide [46] or protein tyrosine phosphatase 1B to form sulfenyl amide [47, 48]. The sulfenyl form is further oxidized to the sulfinyl form and/or sulfonyl form. It is noteworthy that cysteine sulfinate desulfinase catalyzes the desulfination of cysteine sulfinic acid [49, 50], which is not a reversible reaction. On the other hand, cysteine sulfinic acid reductase (sulfiredoxin) catalyzes the reduction of cysteine sulfinic acid [51, 52], although neither thioredoxin nor glutathione can reduce sulfinic acid. Thus, sulfination of cysteine residues is a reversible oxidative process under the conditions that cysteine sulfinic acid reductase can access the catalytic site cysteine of an enzyme. When the reductase cannot