Taxonomic distribution, structure/function relationship and metabolic context of the two families of sulfide dehydrogenases: SQR and FCSD

Hydrogen sulfide (H₂S) is a versatile molecule with different functions in living organisms: it can work as a metabolite of sulfur and energetic metabolism or as a signaling molecule in higher Eukaryotes. H₂S is also highly toxic since it is able to inhibit heme cooper oxygen reductases, preventing oxidative phosphorylation. Due to the fact that it can both inhibit and feed the respiratory chain, the immediate role of H₂S on energy metabolism crucially relies on its bioavailability, meaning that studying the central players involved in the H₂S homeostasis is key for understanding sulfide metabolism. Two different enzymes with sulfide oxidation activity (sulfide dehydrogenases) are known: flavocytochrome c sulfide dehydrogenase (FCSD), a sulfide:cytochrome c oxidoreductase; and sulfide:quinone oxidoreductase (SQR). In this work we performed a thorough bioinformatic study of SQRs and FCSDs and integrated all published data. We systematized several properties of these proteins: (i) nature of flavin binding, (ii) capping loops and (iii) presence of key amino acid residues. We also propose an update to the SQR classification system and discuss the role of these proteins in sulfur metabolism.