dissimilatory sulfite reductase dissimilatory sulfite reductase dissimilatory sulfite reductase (ec 1.8.99.5) is an enzyme that participates in sulfur metabolism in dissimilatory sulfate reduction. dissimilatory sulfite reductaseidentifiersec no.1.8.99.5databasesintenzintenz viewbrendabrenda entryexpasynicezyme viewkeggkegg entrymetacycmetabolic pathwaypriamprofilepdb structuresrcsb pdb pdbe pdbsumgene ontologyamigo / quickgosearchpmcarticlespubmedarticlesncbiproteins 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: (1) sulfite + a -dithiol + 2 reduced acceptor + 2 h+ = hydrogen sulfide + a -disulfide + 2 acceptor + 3 h2o (overall reaction) (1a) sulfite + a -dithiol + 2 reduced acceptor + 2 h+ = a -s-sulfanyl-l-cysteine + 2 acceptor + 3 h2o (1b) a -s-sulfanyl-l-cysteine = hydrogen sulfide + a -disulfide reaction in organisms performing sulfur oxidation: (2) a -s-sulfanyl-l-cysteine + 3 acceptor + 3 h2o = sulfite + a -disulfide + 3 reduced acceptor + 2 h+ (overall reaction) (2a) a -s-sulfanyl-l-cysteine + 3 acceptor + 3 h2o = a -s-sulfo-l-cysteine + 3 reduced acceptor + h+ (2b) a -s-sulfo-l-cysteine = sulfite + a -disulfide the systematic name of this enzyme class is hydrogen-sulfide:,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.