Ribonucleotide reductase--structural studies of a radical enzyme.

Ribonucleotide reductase contains a stable organic free radical essential for its activity located on a tyrosine residue in the small subunit of the enzyme called R2. The substrate binding site is, however, found in the catalytic subunit called R1. A long-range protein-mediated radical transfer pathway appears to be responsible for the delivery of the radical from the tyrosine in R2 to the substrate on R1. The active site is located deep inside the protein in a very stable beta/alpha-barrel structure and a hydrogen bonded system leads from the surface to Cys439 at the active site which is in excellent position to remove a hydrogen from the 3' of the ribose of a bound substrate nucleotide.