CobN subunit of cobaltochelatase, bchH and chlH subunits of magnesium chelatases, and similar proteins.
Cobaltochelatase is a complex enzyme that catalyzes the insertion of cobalt into hydrogenobyrinic acid a,c-diamide, resulting in cobyrinic acid, as demonstrated for Pseudomonas denitrificans. This is an essential step in the bacterial synthesis of cobalamine (B12). The insertion of cobalt requires a complex composed of three polypeptides, cobN, cobS, and cobT. Also included in this family are protoporphyrin IX magnesium chelatases involved in the synthesis of chlorophyll and bacteriochlorophyll, specifically the large (chlH or bchH) subunits.They are thought to bind both the protoporphyrin and the magnesium ion. Hydrolysis of ATP by the smaller subunits in the complex may trigger a conformational change that results in the insertion of the ion into the protoporphyrin scaffold. Cryo electron microscopy studies have suggested that a distinct bchH C-terminal domain may bind tightly to the N-terminal domain upon substrate binding, requiring a substantial conformational change of the bchH subunit. It has also been suggested that chlH of higher plants binds abscisic acid via a C-terminal domain and plays a role in abscisic acid signaling, and that the protein spans the chloroplast envelope, with the C-terminus exposed to the cytosol.
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