Characterization of a 12-Kilodalton Rhodanese Encoded by glpE of Escherichia coli and Its Interaction with Thioredoxin

Abstract

ABSTRACT Rhodaneses catalyze the transfer of the sulfane sulfur from thiosulfate or thiosulfonates to thiophilic acceptors such as cyanide and dithiols. In this work, we define for the first time the gene, and hence the amino acid sequence, of a 12-kDa rhodanese from Escherichia coli . Well-characterized rhodaneses are comprised of two structurally similar ca. 15-kDa domains. Hence, it is thought that duplication of an ancestral rhodanese gene gave rise to the genes that encode the two-domain rhodaneses. The glpE gene, a member of the sn -glycerol 3-phosphate ( glp ) regulon of E. coli , encodes the 12-kDa rhodanese. As for other characterized rhodaneses, kinetic analysis revealed that catalysis by purified GlpE occurs by way of an enzyme-sulfur intermediate utilizing a double-displacement mechanism requiring an active-site cysteine. The K m s for SSO 3 2− and CN − were 78 and 17 mM, respectively. The apparent molecular mass of GlpE under nondenaturing conditions was 22.5 kDa, indicating that GlpE functions as a dimer. GlpE exhibited a k cat of 230 s −1 . Thioredoxin 1 from E. coli , a small multifunctional dithiol protein, served as a sulfur acceptor substrate for GlpE with an apparent K m of 34 μM when thiosulfate was near its K m , suggesting that thioredoxin 1 or related dithiol proteins could be physiological substrates for sulfurtransferases. The overall degree of amino acid sequence identity between GlpE and the active-site domain of mammalian rhodaneses is limited (∼17%). This work is significant because it begins to reveal the variation in amino acid sequences present in the sulfurtransferases. GlpE is the first among the 41 proteins in COG0607 (rhodanese-related sulfurtransferases) of the database Clusters of Orthologous Groups of proteins ( http://www.ncbi.nlm.nih.gov/COG/ ) for which sulfurtransferase activity has been confirmed.