Insights into the catalytic mechanism of a type I sulfide quinone oxidoreductase (SQR) from Acidithiobacillus caldus

Acidithiobacillus caldus, a typical sulfur oxidizer, derives the majority of its energy from sulfur oxidation. And the essential enzyme for sulfide oxidation catalysis is sulfide quinone oxidoreductase (SQR), an ancient flavoprotein. Here, the catalytic mechanism of SQR generated from A. caldus was investigated (SQR(Ac)). According to phylogenetic study, SQR(Ac) (ACAty RS11315) is closely related to SQR (BAD99305) of Acidithiobacillus ferrooxidans NASF-1 and is classified as a type I Sqr enzyme. SQR(Ac) heterologously produced in Escherichia coli exhibits the distinctive absorption peaks (375, 450 nm) of the flavoproteins family of proteins in its absorption spectrum. Utilizing site-directed mutagenesis, the function of conserved cysteines in the catalytic pathway was determined. Based on the sulfide quinone redox reactions in vitro of SQR(Ac) and variations, Cys160 and Cys356 have been identified as enzyme-active residues. Mutation of another cysteine present in all type I SQRs (Cys128) decreased enzyme activity by 56%, indicating that this residue plays an important but non-essential role in enzyme function. In addition, the binding affinities of SQR(Ac), the visualization of its 3D structure, and the interaction between receptors and ligands were investigated. Finally, a suitable sulfide quinone redox catalytic mechanism for A. caldus was proposed.