1.8.99.5	a [DsrC protein]-S-sulfanyl-L-cysteine + 2 acceptor + 3 H2O = sulfite + a [DsrC protein]-dithiol + 2 reduced acceptor + 2 H+	(1b)	-	
1.8.99.5	a [DsrC protein]-S-sulfanyl-L-cysteine + 3 acceptor + 3 H2O = a [DsrC]-S-sulfo-L-cysteine + 3 reduced acceptor + H+	(2a)	-	
1.8.99.5	a [DsrC protein]-S-sulfanyl-L-cysteine + 3 acceptor + 3 H2O = sulfite + a [DsrC protein]-disulfide + 3 reduced acceptor + 2 H+	(2), overall reaction	-	
1.8.99.5	a [DsrC]-S-sulfo-L-cysteine = sulfite + a [DsrC protein]-disulfide	(2b)	-	
1.8.99.5	hydrogen sulfide + a [DsrC protein]-disulfide + 2 acceptor + 3 H2O = sulfite + a [DsrC protein]-dithiol + 2 reduced acceptor + 2 H+	(1), overall reaction	-	
1.8.99.5	hydrogen sulfide + a [DsrC protein]-disulfide + 2 acceptor + 3 H2O = sulfite + a [DsrC protein]-dithiol + 2 reduced acceptor + 2 H+	sulfite is bound at an oxidation state of S1IV and immediately dehydrated. All oxygen atoms are fixed in a tight hydrogen-bonding network, upon transfer of two electrons to yield an S1II state, a second oxygen atom is released as water, but held within the active-site cavity. A further two-electron reduction leads to the S0 state, weakening the remaining S-O bond. With the final transfer of two electrons, the state of sulfide (S2II) is reached and a third water is released, catalytic mechanism overview. The heterobimetallic active-site heme 2 has a Cu(I) ion juxtaposed to a heme c at a Fe-Cu distance of 4.4 A. While the combination of metals is reminiscent of respiratory heme-copper oxidases, the oxidation-labile Cu(I) centre of MccA does not seem to undergo a redox transition during catalysis. Intact MccA tightly binds SO2 at heme 2, a dehydration product of the substrate sulfite that is partially turned over due to photoreduction by X-ray irradiation, yielding the reaction intermediate SO. Structure of sulfite reduction at heme2 of MccA, sulfite binds to the resting state of the enzyme, but the active-site architecture shifts the reversible dehydration equilibrium to SO2 + H2O. Reduction by two electrons occurs through photoreduction in the X-ray beam and leads to release of a second H2O after protonation, with SO remaining bound to the active site. A tight network of hydrogen bonds surrounds the bimetal centre and the bound substrate, holding the reaction products in place. The sulfite substrate only oxidizes half the heme groups of reduced MccA, emphasizing that the total electron charge of the multiheme enzyme is of high relevance for catalysis	743347	
1.8.99.5	hydrogen sulfide + a [DsrC protein]-disulfide = a [DsrC protein]-S-sulfanyl-L-cysteine	(1a)	-