Crystal structure of CbiL (CT0388), a methyltransferase involved in anaerobic biosynthesis of vitamin B12

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.