Dissimilatory sulfite reductase
Dissimilatory sulfite reductase () is an enzyme that participates in sulfur metabolism in dissimilatory sulfate reduction. The enzyme is essential in prokaryotic sulfur-based energy metabolism, including sulfate/sulfite reducing organisms, sulfur-oxidizing bacteria, and organosulfonate reducers. In sulfur reducers it catalyses the reduction of sulfite to sulfide (reaction 1), while in sulfur oxidizers it catalyses the opposite reaction (reaction 2). The reaction involves the small protein DsrC, which is present in all the organisms that contain dissimilatory sulfite reductase. During the process an intramolecular trisulfide is formed between two L-cysteine residues of DsrC and the sulfur  atom from sulfite. This trisulfide can be reduced by a number of proteins including DsrK and TcmB. Reaction in organisms performing dissimilatory sulfate reduction: Reaction in organisms performing sulfur oxidation: The systematic name of this enzyme class is hydrogen-sulfide:[DsrC sulfur-carrier protein],acceptor oxidoreductase. This enzyme is different from EC 1.8.1.2, assimilatory sulfite reductase (NADPH), and EC 1.8.7.1, assimilatory sulfite reductase (ferredoxin), which are involved in sulfate assimilation.