Formation and function of the manganese(iv)/iron(iii) cofactor in chlamydia trachomatis ribonucleotide reductase

The β₂ subunit of a class Ia or Ib ribonucleotide reductase (RNR) is activated when its carboxylate-bridged Fe₂^II/II cluster reacts with O₂ to oxidize a nearby tyrosine (Y) residue to a stable radical (Y). During turnover, the Y in β₂ is thought to reversibly oxidize a cysteine (C) in the α₂ subunit to a thiyl radical (C) by a long-distance (∼35 Å) proton-coupled electron-transfer (PCET) step. The C in α₂ then initiates reduction of the 2′ position of the ribonucleoside 5′-diphosphate substrate by abstracting the hydrogen atom from C3′. The class I RNR from Chlamydia trachomatis (Ct) is the prototype of a newly recognized subclass (Ic), which is characterized by the presence of a phenylalanine (F) residue at the site of β₂ where the essential radical-harboring Y is normally found. We recently demonstrated that Ct RNR employs a heterobinuclear Mn ^IV/Fe^III cluster for radical initiation. In essence, the Mn^IV ion of the cluster functionally replaces the Y of the conventional class I RNR. The Ct β₂ protein also autoactivates by reaction of its reduced (Mn^II/Fe^II) metal cluster with O₂. In this reaction, an unprecedented Mn^IV/Fe ^IV intermediate accumulates almost stoichiometrically and decays by one-electron reduction of the Fe^IV site. This reduction is mediated by the near-surface residue, Y222, a residue with no functional counterpart in the well-studied conventional class I RNRs. In this review, we recount the discovery of the novel Mn/Fe redox cofactor in Ct RNR and summarize our current understanding of how it assembles and initiates nucleotide reduction.