Common chelatase design in the branched tetrapyrrole pathways of heme and anaerobic cobalamin synthesis

Prosthetic groups such as heme, chlorophyll, and cobalamin (vitamin B-12) a re characterized by their branched biosynthetic pathway and unique metal in sertion steps. The metal ion chelatases can be broadly classed either as si ngle-subunit ATP-independent enzymes, such as the anaerobic cobalt chelatas e and the protoporphyrin IX (PPIX) ferrochelatase, or as heterotrimeric, AT P-dependent enzymes, such as the Mg chelatase involved in chlorophyll biosy nthesis. The X-ray structure of the anaerobic cobalt chelatase from Salmone lla typhimurium, CbiK, has been solved to 2.4 Angstrom resolution. Despite a lack of significant amino acid sequence similarity, the protein structure is homologous to that of Bacillus subtilis PPIX ferrochelatase. Both enzym es contain a histidine residue previously identified as the metal ion ligan d, but CbiK contains a second histidine in place of the glutamic acid resid ue identified as a general base in PPM: ferrochelatase. Site-directed mutag enesis has confirmed a role for this histidine and a nearby glutamic acid i n cobalt binding, modulating metal ion specificity as well as catalytic eff iciency. Contrary to the predicted protoporphyrin binding site in PPIX ferr ochelatase, the precorrin-2 binding site in CbiK is clearly defined within a large horizontal cleft between the N- and C-terminal domains. The structu ral similarity has implications for the understanding of the evolution of t his branched biosynthetic pathway.