EVOLUTIONARY DIVERGENCE OF THE HAEM AND COBALAMIN BIOSYNTHETIC PATHWAYS: THE X-RAY STRUCTURE OF THE SALMONELLA TYPHIMURIUM COBALT CHELATASE AND ITS HOMOLOGY TO FERROCHELATASE.

Heidi Schubert¹, Keith Wilson¹, Evelyne Raux² and Martin Warren²

¹Department of Chemistry, University of York, York YO1 5DD, England
²Department of Opthamology, University College London, Bath Street, London EC1V 9EL schubert@yorvic.york.ac.uk

Keywords: Cobalamin, Vitamin B₁₂, metal ion chelation, chelatase

The complex biosynthetic pathways of the tetrapyrrole cofactors such as haem, cobalamin (vitamin B₁₂), chlorophyll and sirohaem all originate from the precursor uroporphyrinogen III¹. This intermediate defines the branch point of the individual pathways which can require up to an additional 21 steps. Though the pathways have branched and evolved separately each contains a chelatase to insert a metal ion in to the tetrapyrrole: ferrochelatase (Fe-haem), cobalt chelatase (Co-cobalamin), magnesium chelatase (Mg-chlorophyll) and sirohaem synthase (Fe-sirohaem).

In haem biosynthesis the insertion of Fe²⁺ ion is the last step, forming haem from protoporphyrin IX. Two biosynthetic pathways exist for the formation of cobalamin²: an aerobic pathway which inserts cobalt late in synthesis before the formation of cobyrinic acid and an anaerobic pathway which inserts cobalt early in synthesis at the stage of precorrin-2 to generate cobalt-precorrin-2³. The anaerobic cobalt chelatase and the ferrochelatase are monomeric enzymes of 30KDa Molecular weight with very low (<10%) sequence similarity. The aerobic cobalt chelatase is a trimeric enzyme with limited similarity to a similar complex in the magnesium chelatase.

The X-ray structures of the anaerobic cobalt chelatase, CbiK, and the B. subtilis ferrochelatase have been solved and show a remarkable degree of structural similarity. Haem and Vitamin B₁₂ biosynthesis have believed to diverged several billion years ago⁴ and homology between the chelatase structures supports a divergent evolution and connection between the biosynthetic pathways.

A description of the cobalt chelatase structure and a comparison to ferrochelatase is presented.

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