Jiang Wei, Yun Danny, Saleh Lana, Bollinger J Martin, Krebs Carsten
Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
Biochemistry. 2008 Dec 30;47(52):13736-44. doi: 10.1021/bi8017625.
The beta(2) subunit of a class Ia 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()). During turnover, the Y() in beta(2) is thought to reversibly oxidize a cysteine (C) in the alpha(2) subunit to a thiyl radical (C()) by a long-distance ( approximately 35 A) proton-coupled electron-transfer (PCET) step. The C() 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(*) 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.
I类或Ib类核糖核苷酸还原酶（RNR）的β(2)亚基在其羧酸盐桥连的Fe(2)(II/II)簇与O(2)反应，将附近的酪氨酸（Y）残基氧化为稳定自由基（Y()）时被激活。在周转过程中，β(2)中的Y()被认为通过长距离（约35 Å）质子耦合电子转移（PCET）步骤将α(2)亚基中的半胱氨酸（C）可逆地氧化为硫自由基（C()）。然后α(2)中的C()通过从C3'提取氢原子引发核糖核苷5'-二磷酸底物2'位的还原。沙眼衣原体（Ct）的I类RNR是新识别的亚类（Ic）的原型，其特征是在β(2)位点存在苯丙氨酸（F）残基，而通常在该位点发现携带必需自由基的Y。我们最近证明Ct RNR使用异双核Mn(IV)/Fe(III)簇进行自由基引发。本质上，簇中的Mn(IV)离子在功能上取代了传统I类RNR的Y(*)。Ct β(2)蛋白也通过其还原的（Mn(II)/Fe(II)）金属簇与O(2)反应而自动激活。在该反应中，一种前所未有的Mn(IV)/Fe(IV)中间体几乎化学计量地积累，并通过Fe(IV)位点的单电子还原而衰减。这种还原由近表面残基Y222介导，Y222是在深入研究的传统I类RNR中没有功能对应物的残基。在本综述中，我们讲述了Ct RNR中新型Mn/Fe氧化还原辅因子的发现，并总结了我们目前对其组装和引发核苷酸还原方式的理解。
