Structural and biochemical analysis of a type II free methionine‐R‐sulfoxide reductase from Thermoplasma acidophilum (774.7)

Abstract

Free methionine‐R‐sulfoxide reductase (fRMsr) enzymes only reduce the free form of methionine‐R‐sulfoxide (Met‐R‐O). These enzymes can be grouped into two types with respect to the number of conserved Cys residues in the active sites. Type I fRMsr enzymes, which have three conserved Cys residues, have been characterized biochemically and structurally. However, the structural and biochemical properties of type II enzymes, which have two conserved Cys residues, are unknown. Here, we present the crystal structures of type II fRMsr from Thermoplasma acidophilum in native form and in a complex with Met‐R‐O. T. acidophilum fRMsr functions as an antioxidant protein. Based on biochemical and growth complementation assays, Cys84 is demonstrated to be the catalytic Cys. Structural analyses also support the catalytic function of Cys84 and provide insights into substrate recognition and orientation, conformational changes in the active site during substrate binding, and the role of active site residues in substrate binding and catalysis. A model for the catalytic mechanism of type II T. acidophilum fRMsr is proposed, in which a resolving Cys is not involved and the catalytic Cys84 sulfenic acid is directly reduced by the reductant thioredoxin.