The bacterial Sox (sulfur oxidizing) system enables the utilization of inorganic sulfur compounds in energy metabolism. SoxY plays a key role, as it functions as the sulfur substrate-binding protein that offers its sulfur substrate, which is covalently bound to a conserved C-terminal cysteine, to another oxidizing Sox enzyme. Structural domains comprising this superfamily share the structure of the SoxY domain, which presents an Ig-like fold and is found in the sulfur oxidation protein SoxY PMID:26655737,PMID:17522046.

Many times it has been reported that SoxY strictly interacts with SoxZ to form a SoxYZ heterodimer. However, recent studies have shown this stable complex is not an obligate, and that the individual proteins may exist on their own. SoxY exhibits a dimer/tetramer equilibrium that is dependent on the redox state of the cysteines and on the type of sulfur substrate component bound to them. The SoxY tetramer consists of two dimeric beta-sandwiches that interact with each other via two small interface patches at the tips of the dimers PMID:17327392