The Active Form of Chlamydia trachomatis Ribonucleotide Reductase R2 Protein Contains a Heterodinuclear Mn(IV)/Fe(III) Cluster with S = 1 Ground State

The class I ribonucleotide reductase from Chlamydia trachomatis uses a stable Mn IV /Fe III cofactor to initiate nucleotide reduction by a free-radical mechanism. The enzyme provides the first example both of a Mn-dependent ribonucleotide reductase and of a Mn/Fe redox cofactor. In this work, we have used variable-field Mössbauer spectroscopy to demonstrate that the active cofactor has an S = 1 ground state due to antiferromagnetic coupling between the Mn IV ( S Mn = 3/2) and high-spin Fe III ( S Fe = 5/2) sites.