A three-dimensional homology model of the O-acetylserine sulfhydrylase-B from Salmonella typhimurium

Written By Wael Rabeh

Abstract

O-acetylserine sulfhydrylase (OASS) catalyzes the last step in the cysteine biosynthetic pathway in enteric bacteria and plants. The overall pathway involves the substitution of the beta-acetoxy group of O-acetyl-L-serine with inorganic bisulfide. Two isozymes are present in S. typhimurium, the A- and B-isozymes, expressed under aerobic and anaerobic conditions, respectively. No crystal structure is presently available for the B-isozyme. Kinetic data indicate the catalytic mechanism of OASS-B is ping-pong, as found for the A-isozyme, but kinetic parameters and substrate specificity differ. In order to estimate whether structural differences may be responsible for the kinetic differences, a homology model was built using the structure of OASS-A as the template for the OASS-B model. The beta-subunit of tryptophan synthase and cystathionine beta-synthase were used for comparison. Differences between the OASS-A structure and the homology model for OASS-B are discussed.

 
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Structure, Mechanism, and Conformational Dynamics of O-Acetylserine Sulfhydrylase from Salmonella typhimurium: Comparison of A and B Isozymes
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Mechanism of the Addition Half of the O-Acetylserine Sulfhydrylase-A Reaction