<p>Sirohaem synthase (CysG), a multifunctional enzyme of the sirohaem and cobalamin (vitamin B12) biosynthesis pathways, represents a fusion between uroporphyrin-III C-methyltransferase (SUMT) and precorrin-2 oxidase/chelatase. Therefore, in some bacteria, all four reactions of sirohaem biosynthesis are catalysed by one multifunctional enzyme, sirohaem synthase [<cite idref="PUB00015677"/>].</p><p>Sirohaem and cobalamin are related macrocyclic structures derived from uroporphyrinogen III by C-methylation of the tetrapyrrole framework. All biologically important modified tetrapyrroles (including also haem and chlorophyll) share a common biosynthetic pathway up to the synthesis of the first macrocyclic intermediate, uroporphyrinogen III [<cite idref="PUB00014667"/>]. Then, SUMT (corresponding to the C-terminal (CysGA) domain of CysG ([<cite idref="PUB00015848"/>]) catalyses C-methylation of uroporphyrinogen III (<db_xref db="EC" dbkey="2.1.1.107"/>). It transfers two methyl groups from S-adenosyl-L-methionine to the C-2 and C-7 atoms of uroporphyrinogen III to yield precorrin-2 via the intermediate formation of precorrin-1. SUMT is the first enzyme committed to the biosynthesis of either sirohaem or cobalamin rather than haem, and precorrin-2 is the last common intermediate in the biosynthesis of corrinoids such as cobalamin, sirohaem and coenzyme F430 [<cite idref="PUB00014667"/>]. SUMT belongs to the domain superfamily of tetrapyrrole (corrin/porphyrin) methylases (<db_xref db="INTERPRO" dbkey="IPR000878"/>), which includes methylases that use S-AdoMet in the methylation of diverse substrates. A number of other methylases in the cobalamin biosynthesis pathway also belong to this domain superfamily. Stand-alone forms of SUMT are in <db_xref db="INTERPRO" dbkey="IPR012203"/> and <db_xref db="INTERPRO" dbkey="IPR012066"/>, and bifunctional uroporphyrin-III methyltransferase/uroporphyrinogen-III synthases are in <db_xref db="INTERPRO" dbkey="IPR012225"/>.</p><p>In sirohaem biosynthesis, the next two steps are beta-NAD(⁺)-dependent dehydrogenation of precorrin-2 to generate sirohydrochlorin followed by ferrochelation to yield sirohaem [<cite idref="PUB00014702"/>, <cite idref="PUB00014703"/>]. Both of these steps are performed by precorrin-2 oxidase/ferrochelatase. In sirohaem synthase CysG, it corresponds to the N-terminal (CysGB) domain [<cite idref="PUB00014702"/>, <cite idref="PUB00014703"/>]. Stand-alone forms of precorrin-2 oxidase/ferrochelatase are in <db_xref db="PIRSF" dbkey="PIRSF004999"/>. Ferrochelation can also be performed by CbiK (<db_xref db="PIRSF" dbkey="PIRSF033579"/>) [<cite idref="PUB00014379"/>] or by SirB [<cite idref="PUB00015809"/>] or CbiX [<cite idref="PUB00014361"/>] (<db_xref db="PIRSF" dbkey="PIRSF004877"/>).</p> <p>In the anaerobic cobalamin biosynthesis (e.g., in Salmonella typhimurium), a cobaltochelatase produces cobalt-precorrin-2. This cobaltochelation can be performed by CbiK [<cite idref="PUB00014379"/>] or by CbiX [<cite idref="PUB00015833"/>, <cite idref="PUB00014361"/>], but also by CysGB homologues [<cite idref="PUB00015704"/>], even though this is not their primary function [<cite idref="PUB00014703"/>]. Therefore, CysGB can essentially duplicate the function of an unrelated chelatase, CbiK [<cite idref="PUB00015704"/>, <cite idref="PUB00014379"/>]. Note that in the aerobic cobalamin biosynthesis, cobalt insertion occurs in a later, different step (see <db_xref db="PIRSF" dbkey="PIRSF031715"/>).</p> <p>Precorrin-2 oxidase/chelatases, represented by the N-terminal domain of <db_xref db="PIRSF" dbkey="PIRSF036426"/> and by <db_xref db="PIRSF" dbkey="PIRSF004999"/>, are not similar in sequence or structural fold to any other known chelatases or oxidases [<cite idref="PUB00014459"/>, <cite idref="PUB00014702"/>]. Analysis of mutant proteins suggests that both catalytic activities share a single active site cleft formed between the N-terminal NAD-binding subdomain and the central subdomain [<cite idref="PUB00014459"/>]. Therefore, they can be considered as the third class of cobalt chelatases, in addition to class I (ATP-dependent, aerobic pathway <db_xref db="PIRSF" dbkey="PIRSF031715"/>) and class II (ATP-independent, anaerobic pathway <db_xref db="PIRSF" dbkey="PIRSF033579"/>) [<cite idref="PUB00014361"/>, <cite idref="PUB00006361"/>]. As with the class II chelatases, they do not require ATP for activity. However, they are not structurally similar to class II chelatases, and it is likely that they have arisen by the acquisition of a chelatase function within a dehydrogenase catalytic framework [<cite idref="PUB00014459"/>, <cite idref="PUB00014361"/>].</p> Sirohaem synthase