Measurement of thiol-disulfide interchange reactions and thiol pKa values

Abstract

Thiol-disulfide interchange (SH/S2 interchange) reactions involving proteins are important in a number of biochemical processes including formation and cleavage of structural cystines, control of enzyme activities by reversible redox reactions of enzyme thiols and disulfides, and redox processes requiring thiols. The reaction is mechanistically simple: it involves initial ionization of thiol to thiolate anion, followed by nucleophilic attack of thiolate anion on the sulfur-sulfur bond of the disulfide. There are three types of parameters that must be determined to characterize fully a SH/S2 interchange reaction: (1) the rates at which the displacement steps occur, (2) the values of pKa of the participating thiols, and (3) the positions of the equilibria between the thiol (thiolate) and disulfide species. The chapter discusses the characteristics of each of these parameters and describes methods of determining them. The methods are useful for biologically relevant thiols and cysteine groups in proteins.