Cytochrome b559 of photosystem II

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Insights into substrate binding to the oxygen-evolving complex of photosystem II from ammonia inhibition studies

© 2014 American Chemical Society. Water oxidation in Photosystem II occurs at the oxygen-evolving complex (OEC), which cycles through distinct intermediates, S0-S4. The inhibitor ammonia selectively binds to the S2 state at an unresolved site that is not competitive with substrate water. By monitoring the yields of flash-induced oxygen production, we show that ammonia decreases the net efficiency of OEC turnover and slows the decay kinetics of S2 to S1. The temperature dependence of biphasic S2 decay kinetics provides activation energies that do not vary in control and ammonia conditions. We interpret our data in the broader context of previous studies by introducing a kinetic model for both the formation and decay of ammonia-bound S2. The model predicts ammonia binds to S2 rapidly (t1/2 = 1 ms) with a large equilibrium constant. This finding implies that ammonia decreases the reduction potential of S2 by at least 2.7 kcal mol-1 (\u3e120 mV), which is not consistent with ammonia substitution of a terminal water ligand of Mn(IV). Instead, these data support the proposal that ammonia binds as a bridging ligand between two Mn atoms. Implications for the mechanism of O-O bond formation are discussed

Electron spin relaxation of CuA and cytochrome a in cytochrome c oxidase: comparison to heme, copper, and sulfur radical complexes

Journal ArticleThe method of continuous saturation has been used to measure the electron spin relaxation parameter T1T2 at temperatures between 10 and 50 K for a variety of S = % species including: CuA and cytochrome a of cytochrome c oxidase, the type1 copper in several blue copper proteins, the type 2 copper in laccase, inorganic Cu(I1) complexes, sulfur radicals, and low spin heme proteins. The temperature dependence and the magnitude of T1T2 for all of the species examined are accounted for by assuming that the Van Vleck Raman process dominates the electron spin-lattice relaxation

Structure of cytochrome a3-Cua3 couple in cytochrome c oxidase as revealed by nitric oxide binding studies

The addition of NO to oxidized cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) causes the appearance of a high-spin heme electron paramagnetic resonance (EPR) signal due to cytochrome a3. This suggests that NO coordinates to Cu{a3}+2 and breaks the antiferromagnetic couple by forming a cytochrome a3+3-Cu{a3}+2-NO complex. The intensity of the high-spin cytochrome a3 signal depends on the method of preparation of the enzyme and maximally accounts for 58% of one heme. The effect of N3- on the cytochrome a3+3-Cu{a3}+2-NO complex is to reduce cytochrome a3 to the ferrous state, and this is followed by formation of a new complex that exhibits EPR signals characteristic of a triplet species. On the basis of optical and EPR results, a NO bridge between cytochrome a3+2 and Cu{a3}+2 is proposed-i.e., cytochrome a3+2-NO-Cu{a3}+2. The half-field transition observed at g = 4.34 in the EPR spectrum of this triplet species exhibits resolved copper hyperfine splittings with |A{}| = 0.020 cm-1, indicating that the Cu{a3}+2 in the cytochrome a3+2-NO-Cu{a3}+2 complex is similar to a type 2 copper site

Photosynthetic water oxidation: Binding and activation of substrate waters for O-O bond formation

© 2015 The Royal Society of Chemistry. Photosynthetic water oxidation occurs at the oxygen-evolving complex (OEC) of Photosystem II (PSII). The OEC, which contains a Mn4CaO5 inorganic cluster ligated by oxides, waters and amino-acid residues, cycles through five redox intermediates known as Si states (i = 0-4). The electronic and structural properties of the transient S4 intermediate that forms the O-O bond are not well understood. In order to gain insight into how water is activated for O-O bond formation in the S4 intermediate, we have performed a detailed analysis of S-state dependent substrate water binding kinetics taking into consideration data from Mn coordination complexes. This analysis supports a model in which the substrate waters are both bound as terminal ligands and react via a water-nucleophile attack mechanism

Electron Spin Relaxation of Cu_A and Cytochrome a in Cytochrome c Oxidase. Comparison to heme, copper, and sulfur radical complexes

The method of continuous saturation has been used to measure the electron spin relaxation parameter T_(1)T_(2) at temperatures between 10 and 50 K for a variety of S = 1/2 species including: Cu_A and cytochrome a of cytochrome c oxidase, the type 1 copper in several blue copper proteins, the type 2 copper in laccase, inorganic Cu(II) complexes, sulfur radicals, and low spin heme proteins. The temperature dependence and the magnitude of T_(1)T_(2) for all of the species examined are accounted for by assuming that the Van Vleck Raman process dominates the electron spin-lattice relaxation. Over the entire temperature range examined, the relaxation of the type 1 coppers in six to seven times faster than that of type 2 copper, inorganic copper, and sulfur radicals, in spite of the similar g-anisotropies of these species. This result may indicate that the coupling of the phonon bath to the spin center is more effective in type 1 coppers than in the other complexes studied. The relaxation of Cu_A of cytochrome oxidase exhibits an unusual temperature dependence relative to the other copper complexes studied, suggesting that the protein environment of this center is different from that of the other copper centers studied and/or that Cu_A is influenced by a magnetic dipolar interaction with another, faster-relaxing paramagnetic site in the enzyme. A comparison of the saturation characteristics of the Cu_A EPR signal in native and partially reduced CO complexes of the enzyme also suggests the existence of such an interaction. The implications of these results with respect to the disposition of the metal centers in cytochrome oxidase are discusse

Toward understanding the S2-S3 transition in the Kok cycle of Photosystem II:Lessons from Sr-substituted structure

Understanding the water oxidation mechanism in Photosystem II (PSII) stimulates the design of biomimetic artificial systems that can convert solar energy into hydrogen fuel efficiently. The Sr2+-substituted PSII is active but slower than with the native Ca2+ containing PSII as an oxygen evolving catalyst. Here, we use Density Functional Theory (DFT) to compare the energetics of the S2 to S3 transition in the Mn4O5Ca2+ and Mn4O5Sr2+ clusters. The calculations show that deprotonation of the water bound to Ca2+ (W3), required for the S2 to S3 transition, is energetically more favorable in Mn4O5Ca2+ than Mn4O5Sr2+. In addition, we have calculated the pKa of the water that bridges Mn4 and the Ca2+/Sr2+ in the S2 state using continuum electrostatics. The calculations show that the pKa is higher by 4 pH units in the Mn4O5Sr2+cluster

The nature of CuA in cytochrome c oxidase

The isolation and purification of yeast cytochrome c oxidase is described. Characterization of the purified protein indicates that it is spectroscopically identical with cytochrome c oxidase isolated from beef heart. Preparations of isotopically substituted yeast cytochrome c oxidase are obtained incorporating [1,3-15N2]histidine or [beta,beta- 2H2]cysteine. Electron paramagnetic resonance and electron nuclear double resonance spectra of the isotopically substituted proteins identify unambiguously at least 1 cysteine and 1 histidine as ligands to CuA and suggest that substantial spin density is delocalized onto a cysteine sulfur in the oxidized protein to render the site Cu(I)-S