The active form of Chlamydia trachomatis ribonucleotide reductase R2 protein contains a heterodinuclear Mn(IV)/Fe(III) cluster with S = 1 ground state

Abstract

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.

Figure 1

4.2-K/zero-field Mössbauer spectrum of the final product of the reaction of Mn^II/Fe^II Ct-R2 with O₂ (hashed marks). The solid line is the spectrum of the final product of the reaction of Fe^II/Fe^II Ct-R2 with O₂ (25% of the total intensity).

Figure 2

4.2-K Mössbauer spectra of Mn^IV/Fe^III-R2 derived by component analysis of the experimental spectra. The solid lines are simulations according to the spin Hamiltonian given in the Supporting Information and the following parameters: S_Total = 1, D_S=1 = −1.9 cm⁻¹, (E/D)_S=1 = 0.33, δ = 0.52 mm/s, ΔE_Q = 1.32 mm/s, η = −2.6, (A/g_Nβ_N)_Fe = (−40.2, −38.9, −38.0) T.

Scheme 1

Radical-generating cofactor of Ct RNR (left) and conventional class I RNR (right).