Two distinct mechanisms of inactivation of the class Ic ribonucleotide reductase from Chlamydia trachomatis by hydroxyurea: implications for the protein gating of intersubunit electron transfer.
Catalysis by a class I ribonucleotide reductase (RNR) begins when a cysteine (C) residue in the alpha(2) subunit is oxidized to a thiyl radical (C(*)) by a cofactor approximately 35 A away in the beta(2) subunit. In a class Ia or Ib RNR, a stable tyrosyl radical (Y(*)) is the C oxidant, whereas a Mn(IV)/Fe(III) cluster serves this function in the class Ic enzyme from Chlamydia trachomatis (Ct). It is thought that, in either case, a chain of Y residues spanning the two subunits mediates C oxidation by forming transient "pathway" Y(*)s in a multistep electron transfer (ET) process that is "gated" by the protein so that it occurs only in the ready holoenzyme complex. The drug hydroxyurea (HU) inactivates both Ia/b and Ic beta(2) subunits by reducing their C oxidants. Reduction of the stable cofactor Y(*) (Y122(*)) in Escherichia coli class Ia beta(2) is faster in the presence of alpha(2) and a substrate (CDP), leading to speculation that HU might intercept a transient ET pathway Y(*) under these turnover conditions. Here we show that this mechanism is one of two that are operant in HU inactivation of the Ct enzyme. HU reacts with the Mn(IV)/Fe(III) cofactor to give two distinct products: the previously described homogeneous Mn(III)/Fe(III)-beta(2) complex, which forms only under turnover conditions (in the presence of alpha(2) and the substrate), and a distinct, diamagnetic Mn/Fe cluster, which forms approximately 900-fold less rapidly as a second phase in the reaction under turnover conditions and as the sole outcome in the reaction of Mn(IV)/Fe(III)-beta(2) only. Formation of Mn(III)/Fe(III)-beta(2) also requires (i) either Y338, the subunit-interfacial ET pathway residue of beta(2), or Y222, the surface residue that relays the "extra electron" to the Mn(IV)/Fe(IV) intermediate during activation of beta(2) but is not part of the catalytic ET pathway, and (ii) W51, the cofactor-proximal residue required for efficient ET between either Y222 or Y338 and the cofactor. The combined requirements for the catalytic subunit, the substrate, and, most importantly, a functional surface-to-cofactor electron relay system imply that HU effects the Mn(IV)/Fe(III) --> Mn(III)/Fe(III) reduction by intercepting a Y(*) that forms when the ready holoenzyme complex is assembled, the ET gate is opened, and the Mn(IV) oxidizes either Y222 or Y338.
Jiang
Wei
W
Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
Xie
Jiajia
J
Varano
Paul T
PT
Krebs
Carsten
C
Bollinger
J Martin
JM
D000494:Allosteric Regulation; D002384:Catalysis; D002692:Chlamydia trachomatis / Q000201:enzymology*; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005290:Ferric Compounds / Q000737:chemistry; D007700:Kinetics; D008345:Manganese / Q000737:chemistry; D012264:Ribonucleotide Reductases / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000378:metabolism
D004791:Enzyme Inhibitors; D005290:Ferric Compounds; D008345:Manganese; D012264:Ribonucleotide Reductases
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