Abstract
The biosynthesis of vitamin B12 involves many enzymatic steps and two evolutionarily divergent pathways. Although the pathways differ in their requirement for molecular oxygen and the timing of cobalt insertion, the precorrin-8x methylmutase genes cobH and cbiC share greater than 30% identity, and are designated as the CobH family.1 These enzymes catalyze the methyl isomerization of metal-free (CobH) and cobalt bound (CbiC) precorrin-8x to hydrogenobyrinic acid (HBA). We have determined the 2.1-A crystal structure of precorrin-8x methylmutase from the facultative anaerobe Thermoplasma acidophilum (Ta0654). Although cobalamin synthesis has not been investigated in T. acidophilum, the structure bears a striking resemblance to CobH from Pseudomonas denifitricans (Pa2905), to which it shares 33% sequence identity. The structure–function relationships for Pa2905 have been characterized, including the role of catalytic residues and crystallographic observation of the product HBA in the active site formed by dimerization.2–4
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