Formation and Function of the Manganese(IV)/Iron(III) Cofactor in Chlamydia trachomatis Ribonucleotide Reductase

The beta(2) subunit of a class la or Ib ribonucleotide reductase (RNR) is activated when its carboxylate-bridged Fe-2(II/II) cluster reacts with O-2 to oxidize a nearby tyrosine (Y) residue to a stable radical (Y-center dot). During turnover, the Y-center dot in beta(2) is thought to reversibly oxidize a cysteine (C) in the alpha(2) subunit to a thiyl radical (C-center dot) by a long-distance (similar to 35 angstrom) proton-coupled electron-transfer (PCET) step. The C-center dot in alpha(2) 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 beta(2) 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-center dot of the conventional class I RNR. The Ct beta(2) protein also autoactivates by reaction of its reduced (Mn-II/Fe-II) metal cluster with O-2. 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.