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BluB cannibalizes flavin to form the lower ligand of vitamin B12

2007; Nature Portfolio; Volume: 446; Issue: 7134 Linguagem: Inglês

10.1038/nature05611

1476-4687

Michiko E. Taga, Nicholas Larsen, Annaleise R. Howard‐Jones, Christopher T. Walsh, Graham C. Walker,

Cancer Research and Treatments

Vitamin B12 (cobalamin) is one of the largest known non-polymeric natural products, and it is the only vitamin that is synthesized exclusively by microorganisms. Despite years of study, the biosynthesis of one part of the vitamin is poorly understood. Now the last unknown step in its biosynthesis is revealed. The X-ray crystal structure of BluB, an enzyme that uses molecular oxygen to cleave a flavin mononucleotide cofactor to form the lower ligand of vitamin B12, has been determined. This reaction is an example of an unusual process, the enzymatic destruction of one cofactor to synthesize another. The X-ray crystal structure of BluB is solved. Blub is an enzyme that uses molecular oxygen to cleave a flavin mononucleotide cofactor to form 5,6-dimethylbenzimidazole, which is incorporated into vitamin B12. The enzymatic destruction of one cofactor to synthesis another is quite unusual and this biochemical reaction represents the last unknown step in the biosynthesis of this vitamin. Vitamin B12 (cobalamin) is among the largest known non-polymeric natural products, and the only vitamin synthesized exclusively by microorganisms1. The biosynthesis of the lower ligand of vitamin B12, 5,6-dimethylbenzimidazole (DMB), is poorly understood1,2,3. Recently, we discovered that a Sinorhizobium meliloti gene, bluB, is necessary for DMB biosynthesis4. Here we show that BluB triggers the unprecedented fragmentation and contraction of the bound flavin mononucleotide cofactor and cleavage of the ribityl tail to form DMB and d-erythrose 4-phosphate. Our structural analysis shows that BluB resembles an NAD(P)H-flavin oxidoreductase, except that its unusually tight binding pocket accommodates flavin mononucleotide but not NAD(P)H. We characterize crystallographically an early intermediate along the reaction coordinate, revealing molecular oxygen poised over reduced flavin. Thus, BluB isolates and directs reduced flavin to activate molecular oxygen for its own cannibalization. This investigation of the biosynthesis of DMB provides clarification of an aspect of vitamin B12 that was otherwise incomplete, and may contribute to a better understanding of vitamin B12-related disease.