Ultrafast Studies of the Electronic Structure and Dynamics of B12 Cofactors

摘要

Adenosylcobalamin (AdoCbl, Figure 1) dependent enzymes catalyze a variety of chemically unusual carbon skeletal rearrangement reactions 1. The catalysis proceeds via the generation of organic free radicals formed through homolysis of the active Co-C bond to produce cob(II)amin (Cbl(II)) and ana denosyl radical. When bound to protein, the homolysis rate is observed to increase 10I2-fold compared with free AdoCbl in solution 2. However, the influence of the protein and the cofactor environment in general on priming the Co-C bond for homolysis is not well understood, despite experimental and theoretical studies directed at this problem. Glutamate mutase, which catalyzes the carbon-skeletal rearrangement of L-glutamate to L-threo-3-methylaspartate,is one of the best-characterized AdoCbl dependent enzymes. Crystal structures with both the coenzyme and substrate bound in the active site have been solved for glutamate mutase 3,4. This system provides a useful paradigm for protein-cofactor interactions and for the enzymatic generation and control of free radicals.