Identification of two inactive forms of the central sulfur cycle protein SoxYZ of Paracoccus pantotrophus

The central protein of the sulfur-oxidizing enzyme system of Paracoccus pantotrophus, SoxYZ, reacts with three different Sox proteins. Its active site Cys110Y is on the carboxy-terminus of the SoxY subunit. SoxYZ “as isolated” consisted mainly of the catalytically inactive SoxY-Y(Z)2 heterotetramer linked by a Cys110Y-Cys110Y interprotein disulfide. Sulfide activated SoxYZ “as isolated” 456-fold, reduced the disulfide, and yielded an active SoxYZ heterodimer. The reductant tris(2-carboxyethyl)phosphine (TCEP) inactivated SoxYZ. This form was not re-activated by sulfide, which identified it as a different inactive form. In analytical gel filtration, the elution of “TCEP-treated” SoxYZ was retarded compared to active SoxYZ, indicating a conformational change. The possible enzymes involved in the re-activation of each inactive form of SoxYZ are discussed.