FIGURE 2.

Possible mechanisms of sulfur/polysulfide respiration in Desulfurella amilsii. During chemolithotrophic growth, hydrogenases (HYD) might transfer electrons to sulfur reductase (SRE) via menaquinones (K) encoded in the genome, and protons to an encoded ATPase, creating a proton motive force. If sulfide dehydrogenase (SUDH) plays a role in sulfur respiration, its cytoplasmic nature hampers the generation of proton motive force by any conventional mechanisms and therefore, it is likely that the membrane-bound hydrogenases pump protons out of the cell to generate a gradient. In case of formate used as electron donor, the intracellular formate dehydrogenase (FDH) encoded might transfer electrons to SUDH, with NAD⁺/NADH as intermediates. Moreover, rhodanese-like proteins (TST) encoded in the genome might have a role in the process, but its performance in sulfur-respiring microorganisms is not yet clearly understood.