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  • Photosystem II  (2)
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  • 1
    Electronic Resource
    Electronic Resource
    Mn2+ reduces Yz + in manganese-depleted Photosystem II preparations (1989)
    Springer
    Photosynthesis research 22 (1989), S. 285-293 
    Details
    ISSN: 1573-5079
    Keywords: Photosystem II ; oxygen evolution ; manganese oxidation ; tyrosine free radical ; electron transfer kinetics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Manganese in the oxygen-evolving complex is a physiological electron donor to Photosystem II. PS II depleted of manganese may oxidize exogenous reductants including benzidine and Mn2+. Using flash photolysis with electron spin resonance detection, we examined the room-temperature reaction kinetics of these reductants with Yz +, the tyrosine radical formed in PS II membranes under illumination. Kinetics were measured with membranes that did or did not contain the 33 kDa extrinsic polypeptide of PS II, whose presence had no effect on the reaction kinetics with either reductant. The rate of Yz + reduction by benzidine was a linear function of benzidine concentration. The rate of Yz + reduction by Mn2+ at pH 6 increased linearly at low Mn2+ concentrations and reached a maximum at the Mn2+ concentrations equal to several times the reaction center concentration. The rate was inhibited by K+, Ca2+ and Mg2+. These data are described by a model in which negative charge on the membrane causes a local increase in the cation concentration. The rate of Yz + reduction at pH 7.5 was biphasic with a fast 400 μs phase that suggests binding of Mn2+ near Yz + at a site that may be one of the native manganese binding sites.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00048306
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  • 2
    Electronic Resource
    Electronic Resource
    A hydrogen-atom abstraction model for the function of YZ in photosynthetic oxygen evolution (1995)
    Springer
    Photosynthesis research 46 (1995), S. 177-184 
    Details
    ISSN: 1573-5079
    Keywords: electron paramagnetic resonance ; managanese cluster ; oxygen evolution ; oxygen evolving complex ; Photosystem II ; proton transfer ; tyrosine radical ; water oxidation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Recent magnetic-resonance work on YŻ suggests that this species exhibits considerable motional flexibility in its functional site and that its phenol oxygen is not involved in a well-ordered hydrogen-bond interaction (Tang et al., submitted; Tommos et al., in press). Both of these observations are inconsistent with a simple electron-transfer function for this radical in photosynthetic water oxidation. By considering the roles of catalytically active amino acid radicals in other enzymes and recent data on the water-oxidation process in Photosystem II, we rationalize these observations by suggesting that YŻ functions to abstract hydrogen atoms from aquo- and hydroxy-bound managanese ions in the (Mn)4 cluster on each S-state transition. The hydrogen-atom abstraction process may occur either by sequential or concerted kinetic pathways. Within this model, the (Mn)4/YZ center forms a single catalytic center that comprises the Oxygen Evolving Complex in Photosystem II.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00020428
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    Paper (German National Licenses)
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