Multiheme selenoenzyme essential for elemental sulfur respiration.

Elemental sulfur reduction is a key process in anaerobic ecosystems and the global sulfur cycle. Although elemental sulfur serves as a terminal electron acceptor in microbial respiration, the molecular basis is unclear. Here, we identify a conserved multiheme cytochrome c selenoprotein essential for sulfur reduction in a sulfur-respiring bacterium. Structural and biochemical analyses show that the enzyme forms a tetramer, with each subunit containing five hemes and one selenocysteine residue. The enzyme catalyzes polysulfide reduction at an active site, where a cysteine coordinates the heme iron, while selenocysteine is essential for catalysis. Genetic analyses show that both residues are critical for sulfur respiration in vivo. These findings reveal a selenium-sulfur-dependent catalysis on a heme center for polysulfide reduction, expanding our understanding of microbial energy metabolism.