Members of this family are involved in cobalamin synthesis. The gene encoded by Swiss:P72862 has been designated cbiH but in fact represents a fusion between cbiH and cbiG. As other multi-functional proteins involved in cobalamin biosynthesis catalyse adjacent steps in the pathway, including CysG, CobL (CbiET), CobIJ and CobA-HemD, it is therefore possible that CbiG catalyses a reaction step adjacent to CbiH. In the anaerobic pathway such a step could be the formation of a gamma lactone, which is thought to help to mediate the anaerobic ring contraction process [1]. Within the cobalamin synthesis pathway CbiG catalyses the both the opening of the lactone ring and the extrusion of the two-carbon fragment of cobalt-precorrin-5A from C-20 and its associated methyl group (deacylation) to give cobalt-precorrin-5B [2]. This family is the C-terminal region, and the mid- and N-termival parts are conserved independently in other families.

1: Cobalamin (vitamin B12) biosynthesis: identification and characterization of a Bacillus megaterium cobI operon. Raux E, Lanois A, Warren MJ, Rambach A, Thermes C; Biochem J 1998;335:159-166. PMID:9742225

2: Fine-tuning our knowledge of the anaerobic route to cobalamin (vitamin B12). Roessner CA, Scott AI; J Bacteriol. 2006;188:7331-7334. PMID:16936030