Properties of the Escherichia coli rhodanese-like protein SseA: contribution of the active-site residue Ser240 to sulfur donor recognition

The product of Escherichia coli sseA gene (SseA) was the subject of the pre sent investigation aimed to provide a tool for functional classification of the bacterial proteins of the rhodanese family. E. coli SseA contains the motif CGSGVTA around the catalytic cysteine (Cys238), In eukaryotic sulfurt ransferases this motif discriminates for 3-mercaptopyruvate:cyanide sulfurt ransferase over thiosulfate:cyanide sulfurtransferases (rhodanese), The bio chemical characterization of E. coli SseA allowed the identification of the first prokaryotic protein with a preference for 3-mercaptopyruvate as dono r substrate. Replacement of Ser240 with Ala showed that the presence of a h ydrophobic residue did not affect the binding of 3-mercaptopyruvate, but st rongly prevented thiosulfate binding. On the contrary, substitution of Ser2 40 with an ionizable residue (Lys) increased the affinity for thiosulfate. (C) 2001 Federation of European Biochemical Societies. Published by Elsevie r Science B.V. All rights reserved.