Ribonucleotide reductase mechanism. (1) An electron is transferred from a cysteine residue on R1 to a tyrosine radical on R2, generating a highly reactive cysteine thiyl radical. (2) This radical abstracts a hydrogen atom from C-3′ of the ribose unit. (3) The radical at C-3′ releases OH^- from the C-2′ carbon atom. Combined with a proton from a second cysteine residue, the OH^– is eliminated as water. (4) A hydride ion is transferred from a third cysteine residue with the concomitant formation of a disulfide bond. (5) The C-3′ radical recaptures the originally abstracted hydrogen atom. (6) An electron is transferred from R2 to reduce the thiyl radical, which also accepts a proton. The deoxyribonucleotide is free to leave R1. The disulfide formed in the active site must be reduced to begin another cycle.