RESUMO
The reaction catalyzed by E. coli ribonucleotide reductase (RNR) composed of α and ß subunits that form an active α2ß2 complex is a paradigm for proton-coupled electron transfer (PCET) processes in biological transformations. ß2 contains the diferric tyrosyl radical (Y122·) cofactor that initiates radical transfer (RT) over 35 Å via a specific pathway of amino acids (Y122· â [W48] â Y356 in ß2 to Y731 â Y730 â C439 in α2). Experimental evidence exists for colinear and orthogonal PCET in α2 and ß2, respectively. No mechanistic model yet exists for the PCET across the subunit (α/ß) interface. Here, we report unique EPR spectroscopic features of Y356·-ß, the pathway intermediate generated by the reaction of 2,3,5-F3Y122·-ß2/CDP/ATP with wt-α2, Y731F-α2, or Y730F-α2. High field EPR (94 and 263 GHz) reveals a dramatically perturbed g tensor. [1H] and [2H]-ENDOR reveal two exchangeable H bonds to Y356·: a moderate one almost in-plane with the π-system and a weak one. DFT calculation on small models of Y· indicates that two in-plane, moderate H bonds (rO-H â¼1.8-1.9 Å) are required to reproduce the gx value of Y356· (wt-α2). The results are consistent with a model, in which a cluster of two, almost symmetrically oriented, water molecules provide the two moderate H bonds to Y356· that likely form a hydrogen bond network of water molecules involved in either the reversible PCET across the subunit interface or in H+ release to the solvent during Y356 oxidation.
Assuntos
Escherichia coli/enzimologia , Ribonucleotídeo Redutases/química , Espectroscopia de Ressonância de Spin Eletrônica , Transporte de Elétrons , Escherichia coli/química , Ligação de Hidrogênio , Modelos Moleculares , Subunidades Proteicas/química , Água/químicaRESUMO
Ribonucleotide reductases (RNRs) catalyze the conversion of ribonucleotides to deoxyribonucleotides in all living organisms. The catalytic cycle of E. coli RNR involves a long-range proton-coupled electron transfer (PCET) from a tyrosyl radical (Y122Ë) in subunit ß2 to a cysteine (C439) in the active site of subunit α2, which subsequently initiates nucleotide reduction. This oxidation occurs over 35 Å and involves a specific pathway of redox active amino acids (Y122 â [W48?] â Y356 in ß2 to Y731 â Y730 â C439 in α2). The mechanisms of the PCET steps at the interface of the α2ß2 complex remain puzzling due to a lack of structural information for this region. Recently, DFT calculations on the 3-aminotyrosyl radical (NH2Y731Ë)-α2 trapped by incubation of NH2Y731-α2/ß2/CDP(substrate)/ATP(allosteric effector) suggested that R411-α2, a residue close to the α2ß2 interface, interacts with NH2Y731Ë and accounts in part for its perturbed EPR parameters. To examine its role, we further modified NH2Y731-α2 with a R411A substitution. NH2Y731Ë/R411A generated upon incubation of NH2Y731/R411A-α2/ß2/CDP/ATP was investigated using multi-frequency (34, 94 and 263 GHz) EPR, 34 GHz pulsed electron-electron double resonance (PELDOR) and electron-nuclear double resonance (ENDOR) spectroscopies. The data indicate a large conformational change in NH2Y731Ë/R411A relative to the NH2Y731Ë single mutant. Particularly, the inter-spin distance from NH2Y731Ë/R411A in one αß pair to Y122Ë in a second αß pair decreases by 3 Å in the presence of the R411A mutation. This is the first experimental evidence for the flexibility of pathway residue Y731-α2 in an α2ß2 complex and suggests a role for R411 in the stacked Y731/Y730 conformation involved in collinear PCET. Furthermore, NH2Y731Ë/R411A serves as a probe of the PCET process across the subunit interface.
RESUMO
The radical concentrations and g factors of stable organic radicals in different lignin preparations were determined by X-band EPR at 9 GHz. We observed that the g factors of these radicals are largely determined by the extraction process and not by the botanical origin of the lignin. The parameter mostly influencing the g factor is the pH value during lignin extraction. This effect was studied in depth using high-field EPR spectroscopy at 263 GHz. We were able to determine the gxx, gyy, and gzz components of the g tensor of the stable organic radicals in lignin. With the enhanced resolution of high-field EPR, distinct radical species could be found in this complex polymer. The radical species are assigned to substituted o-semiquinone radicals and can exist in different protonation states SH3+, SH2, SH1-, and S2-. The proposed model structures are supported by DFT calculations. The g principal values of the proposed structure were all in reasonable agreement with the experiments.
Assuntos
Espectroscopia de Ressonância de Spin Eletrônica/métodos , Lignina/química , Benzoquinonas/química , Lignina/análise , Modelos Moleculares , Prótons , Madeira/químicaRESUMO
Ribonucleotide reductases (RNRs) catalyze the conversion of ribonucleotides to deoxyribonucleotides in all organisms. In all Class Ia RNRs, initiation of nucleotide diphosphate (NDP) reduction requires a reversible oxidation over 35 Å by a tyrosyl radical (Y122â¢, Escherichia coli) in subunit ß of a cysteine (C439) in the active site of subunit α. This radical transfer (RT) occurs by a specific pathway involving redox active tyrosines (Y122 â Y356 in ß to Y731 â Y730 â C439 in α); each oxidation necessitates loss of a proton coupled to loss of an electron (PCET). To study these steps, 3-aminotyrosine was site-specifically incorporated in place of Y356-ß, Y731- and Y730-α, and each protein was incubated with the appropriate second subunit ß(α), CDP and effector ATP to trap an amino tyrosyl radical (NH2Yâ¢) in the active α2ß2 complex. High-frequency (263 GHz) pulse electron paramagnetic resonance (EPR) of the NH2Yâ¢s reported the gx values with unprecedented resolution and revealed strong electrostatic effects caused by the protein environment. (2)H electron-nuclear double resonance (ENDOR) spectroscopy accompanied by quantum chemical calculations provided spectroscopic evidence for hydrogen bond interactions at the radical sites, i.e., two exchangeable H bonds to NH2Y730â¢, one to NH2Y731⢠and none to NH2Y356â¢. Similar experiments with double mutants α-NH2Y730/C439A and α-NH2Y731/Y730F allowed assignment of the H bonding partner(s) to a pathway residue(s) providing direct evidence for colinear PCET within α. The implications of these observations for the PCET process within α and at the interface are discussed.