Department of Biology, Osaka University^* Department of Bioscience and Biotechnology, Ritsumeikan University^**
○Kei Wada^* Jiro Harada^** Hirozo Oh-oka^* Hitoshi Tamiaki^** Keiichi Fukuyama^*

Vitamin B₁₂ (cobalamin) contains a cobalt-centered corrin ring (modified tetrapyrrole ring) that belongs to the same family of the metalloprosthetic groups as heme and chlorophyll. Vitamin B₁₂ biosynthesis represents one of the most complex metabolic pathways in nature, requiring approximately 30 enzymes to complete de novo synthesis. One of the unique features of Vitamin B₁₂ biosynthesis is the addition of eight methyl groups derived from S-adenosylmethionine to the tetrapyrrole framework during corrin construction. These methyl groups are added by the six separate methyltransferases. An S-adenosylmethionine-dependent methyltransferase CbiL catalyzes methylation at the C-20 position of the tetrapyrrole moiety. Interestingly, in subsequent step, both the methyl group and the C-20 carbon are lost during the ring contract process, extruded as acetaldehyde; the tetrapyrrole ring is converted to a corrin ring. Thus, the methylation of C-20 position is a key modification for the ring contract process, however, the methylation mechanism including the substrates recognition underlying the biosynthesis of vitamin B₁₂ is poorly understood.
Recombinant Chlorobium tepidum CbiL overproduced in Escherichia coli was purified, and crystallized by the hanging-drop vapor-diffusion method. A native data set was collected to 2.1 Å resolution using synchrotron radiation at SPring-8. The initial phase was obtained by the molecular replacement. Model building and structure refinement are currently in progress.