Structural Identification of the interactions of SoxA and SoxX during the oxidation of sulfur anions via the novel global sulfur oxidizing (Sox) operon

Abstract

In this study, we have tried to elucidate the structural basis of the involvements of SoxA and SoxX in the binding and in electron transport during oxidation of sulfur anions via the novel global sulfur oxidation cycle. SoxA is known to be a di-heme cytochrome c protein, which helps in binding sulfur anions with SoxY. SoxX, a mono-heme cytochrome c protein, binds to SoxA forming a complex, which helps in efficient transport of electrons during oxidation of sulfur anions. We employed homology modeling to construct the three-dimensional structures of the SoxA and SoxX from Pseudaminobacter salicylatoxidans and established the geometry of the cytochrome c region and that of the active site of SoxAX complex. The docking studies with SoxA and SoxX allowed us to identify the details of SoxA-SoxX interactions. The structural basis of the formation of the hetero-dimeric complex of SoxA-SoxX were also demonstrated to predict the biochemical pathway of sulfur anion binding and transport of electrons via these proteins in the novel global sulfur cycle as SoxAX complex is the initiator of the sulfur anion oxidation pathway. Since there have been no previous reports regarding the structural biology of these proteins, results from this study will be important for the understanding of the three dimensional structures of SoxA and SoxX as well as to elucidate the structural basis of their mode of actions.