Structural Insights into Component SoxY of the Thiosulfate-Oxidizing Multienzyme System of Chlorobaculum thiosulfatiphilum

Abstract

We discuss the crystal structure of component SoxY of the SoxYZ complex that is known to play a key role in the sulfur-oxidizing multienzyme system of the green sulfur bacterium Chlorobaculum thiosulfatiphilum. The protein appears to be structurally similar to a monomeric immunoglobulin-like protein that oligomerizes into a tetramer via conserved contact regions between the monomers. The tetramer is a dimer of dimers and exhibits one large hydrophobic contact region in each dimer, and two small hydrophilic interface patches between the dimers. At the tetramer interface patch, two conserved redox-active C-terminal cysteines form an intersubunit disulfide bridge. Depending on the redox state of the cysteines, the tetramer is in equilibrium with the dimers, each one of which is a candidate to covalently bind a thiosulfate molecule by means of a thiol–disulfide exchange reaction with the interprotein disulfide bonds. The significant conservation level of the interfaces, the specific interactions between the subunits in the tetramer, and the dimer–tetramer equilibrium suggest that these SoxY oligomers are biologically relevant. A possible role for these protomers in the mechanism of the Soxsystem is proposed.