Structure and Kinetic Analysis of H2S Production by Human Mercaptopyruvate Sulfurtransferase

Mercaptopyruvate sulfurtransferase (MST) is a source of endogenous H 2 S, a gaseous signaling molecule implicated in a wide range of physiological processes. The contribution of MST versus the other two H 2 S generators, cystathionine β-synthase and γ-cystathionase, has been difficult to evaluate because many studies on MST have been conducted at high pH and have used varied reaction conditions. In this study, we have expressed, purified, and crystallized human MST in the presence of the substrate 3-mercaptopyruvate (3-MP). The kinetics of H 2 S production by MST from 3-MP was studied at pH 7.4 in the presence of various physiological persulfide acceptors: cysteine, dihydrolipoic acid, glutathione, homocysteine, and thioredoxin, and in the presence of cyanide. The crystal structure of MST reveals a mixture of the product complex containing pyruvate and an active site cysteine persulfide (Cys 248 -SSH) and a nonproductive intermediate in which 3-MP is covalently linked via a disulfide bond to an active site cysteine. The crystal structure analysis allows us to propose a detailed mechanism for MST in which an Asp-His-Ser catalytic triad is positioned to activate the nucleophilic cysteine residue and participate in general acid-base chemistry, whereas our kinetic analysis indicates that thioredoxin is likely to be the major physiological persulfide acceptor for MST. Background: Mercaptopyruvate sulfurtransferase (MST) generates H 2 S, a signaling molecule. Results: The detailed kinetics and crystal structure of human MST with bound substrate are reported. Conclusion: Thioredoxin is the preferred persulfide acceptor from MST. Significance: The structure provides molecular insights into activation and stabilization of MST reaction intermediates.