Radical mechanisms in adenosylcobalamin-dependent enzymes.

Adenolsylcobalamin-dependent enzymes catalyze free radical mediated reactions of their substrates. Stereochemical methods have been used to establish the nature of the primary radical initiation step in ribonucleoside triphosphate reductase. Kinetic isotope effects have been used to establish a kinetic coupling between cobalt-carbon bond cleavage and hydrogen atom abstraction from the substrate. Isotope effects have also been used to identify rate-limiting steps with wild type and mutant forms of the enzymes and in model reactions to assess tunneling contributions to hydrogen atom transfer steps. Computational methods have been employed to explore the pathways for functional group migration in the radical pathways. Analogs of substrates and of adenosylcobalamin have been used to explore the fidelity of the enzyme active sites and the radical pathways.