A conventional class I (subclass a or b) ribonucleotide reductase (RNR) employs a tyrosyl radical (Y • ) in its R2 subunit for reversible generation of a 3′-hydrogen-abstracting cysteine radical in its R1 subunit by proton-coupled electron transfer (PCET) through a network of aromatic amino acids spanning the two subunits. The class Ic RNR from the human pathogen Chlamydia trachomatis (Ct) uses a Mn IV /Fe III cofactor (specifically, the Mn IV ion) in place of the Y • for radical initiation. Ct R2 is activated when its Mn II /Fe II form reacts with O 2 to generate a Mn IV /Fe IV intermediate, which decays by reduction of the Fe IV site to the active Mn IV /Fe III state. Here we show that the reduction step in this sequence is mediated by residue Y222. Substitution of Y222 with F retards the intrinsic decay of the Mn IV /Fe IV intermediate by ∼10-fold and diminishes the ability of ascorbate to accelerate the decay by ∼65-fold but has no detectable effect on the catalytic activity of the Mn IV / Fe III -R2 product. By contrast, substitution of Y338, the cognate of the subunit interfacial R2 residue in the R1 ⇔ R2 PCET pathway of the conventional class I RNRs [Y356 in Escherichia coli (Ec) R2], has almost no effect on decay of the Mn IV /Fe IV intermediate but abolishes catalytic activity. Substitution of W51, the Ct R2 cognate of the cofactor-proximal R1 ⇔ R2 PCET pathway residue in the conventional class I RNRs (W48 in Ec R2), both retards reduction of the Mn IV /Fe IV intermediate and abolishes catalytic activity. These observations imply that Ct R2 has evolved branched pathways for electron relay to the cofactor during activation and catalysis. Other R2s predicted also to employ the Mn/Fe cofactor have Y or W (also competent for electron relay) aligning with Y222 of Ct R2. By contrast, many R2s known or expected to use the conventional Y • -based system have redox-inactive L or

We recently showed that the class Ic ribonucleotide reductase (RNR) from the human pathogen Chlamydia trachomatis ( Ct) uses a Mn (IV)/Fe (III) cofactor in its R2 subunit to initiate catalysis [Jiang, W., Yun, D., Saleh, L., Barr, E. W., Xing, G., Hoffart, L. M., Maslak, M.-A., Krebs, C., and Bollinger, J. M., Jr. (2007) Science 316, 1188-1191]. The Mn (IV) site of the novel cofactor functionally replaces the tyrosyl radical used by conventional class I RNRs to initiate substrate radical production. As a first step in evaluating the hypothesis that the use of the alternative cofactor could make the RNR more robust to reactive oxygen and nitrogen species [RO(N)S] produced by the host's immune system [Högbom, M., Stenmark, P., Voevodskaya, N., McClarty, G., Gräslund, A., and Nordlund, P. (2004) Science 305, 245-248], we have examined the reactivities of three stable redox states of the Mn/Fe cluster (Mn (II)/Fe (II), Mn (III)/Fe (III), and Mn (IV)/Fe (III)) toward hydrogen peroxide. Not only is the activity of the Mn (IV)/Fe (III)-R2 intermediate stable to prolonged (>1 h) incubations with as much as 5 mM H 2O 2, but both the fully reduced (Mn (II)/Fe (II)) and one-electron-reduced (Mn (III)/Fe (III)) forms of the protein are also efficiently activated by H 2O 2. The Mn (III)/Fe (III)-R2 species reacts with a second-order rate constant of 8 +/- 1 M (-1) s (-1) to yield the Mn (IV)/Fe (IV)-R2 intermediate previously observed in the reaction of Mn (II)/Fe (II)-R2 with O 2 [Jiang, W., Hoffart, L. M., Krebs, C., and Bollinger, J. M., Jr. (2007) Biochemistry 46, 8709-8716]. As previously observed, the intermediate decays by reduction of the Fe site to the active Mn (IV)/Fe (III)-R2 complex. The reaction of the Mn (II)/Fe (II)-R2 species with H 2O 2 proceeds in three resolved steps: sequential oxidation to Mn (III)/Fe (III)-R2 ( k = 1.7 +/- 0.3 mM (-1) s (-1)) and Mn (IV)/Fe (IV)-R2, followed by decay of the intermediate to the active Mn (IV)/Fe (III)-R2 product. The efficient reaction of both reduced forms with H 2O 2 contrasts with previous observations on the conventional class I RNR from Escherichia coli, which is efficiently converted from the fully reduced (Fe 2 (II/II)) to the "met" (Fe 2 (III/III)) form [Gerez, C., and Fontecave, M. (1992) Biochemistry 31, 780-786] but is then only very inefficiently converted from the met to the active (Fe 2 (III/III)-Y (*)) form [Sahlin, M., Sjöberg, B.-M., Backes, G., Loehr, T., and Sanders-Loehr, J. (1990) Biochem. Biophys. Res. Commun. 167, 813-818].

Use of a magnetic targeted microbubble system results in greater attachment to endothelial VCAM-1 in atherosclerotic aortas in conditions of high shear stress and improved detection of early inflammatory changes of atherosclerosis.

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