Substitution of manganese for iron in ribonucleotide reductase from Escherichia coli. Spectroscopic and crystallographic characterization.

Each polypeptide chain of protein R2, the small subunit of ribonucleotide reductase from Escherichia coli, contains a stable tyrosyl radical and two antiferromagnetically coupled oxo-bridged ferric ions. A refined structure of R2 has been recently obtained. R2 can be converted into apoR2 by chelating out the metal cofactor and scavenging the radical. This study shows that apoR2 has a very strong affinity for four stable Mn2+ ions. The manganese-containing form of R2, named Mn-R2, has been studied by EPR spectroscopy and x-ray crystallography. It contains two binuclear manganese clusters in which the two manganese ions occupy the natural iron-binding sites and are only bridged by carboxylates from glutamates 115 and 238. This in turn explains why the spin-exchange interaction between the two ions is very weak and why Mn-R2 is EPR active. Mn-R2 could provide a model for the native diferrous form of protein R2, and a detailed molecular mechanism for the reduction of the iron center of protein R2 is proposed.