Radical Use of Rossmann and TIM Barrel Architectures for Controlling Coenzyme B12 Chemistry

The ability of enzymes to harness free-radical chemistry allows for some of the most amazing transformations in nature, including reduction of ribonucleotides and carbon skeleton rearrangements. Enzyme cofactors involved in this chemistry can be large and complex, such as adenosylcobalamin (coenzyme B12), simpler, such as S-adenosylmethionine and an iron-sulfur cluster (i.e., poor man's B12), or very small, such as one nonheme iron atom coordinated by protein ligands. Although the chemistry catalyzed by these enzyme-bound cofactors is unparalleled, it does come at a price. The enzyme must be able to control these radical reactions, preventing unwanted chemistry and protecting the enzyme active site from damage. Here, we consider a set of radical folds: the (β/α)8 or TIM barrel, combined with a Rossmann domain for coenzyme B12-dependent chemistry. Using specific enzyme examples, we consider how nature employs the common TIM barrel fold and its Rossmann domain partner for radical-based chemistry.