JoAnne stubbe has proposed the following catalytic mechanism for E. Coli ribonucleotide reductase (Fig. 23-10):
1. Ribonucleotide trductase’s free radical (X.) abstracts an H atom from C3' of the substrate in the reaction’s rate-determinig step.
2. and3 Acid-catalyzed cleavage of the C2'̶ OH group’s unshared electron pair stabilizes the C2' แation. This accounts for the radical’s catalytic role.
3.
4. The radical-cation intermediate is reduced by the enzyme’s redox-active sulfhydryl pair to yield a 3'- deoxyribonucleotide radical and a protein disulfide group (this group must eventually be reduced to regenerate the enzyme’s activity).