A new mechanism-based inhibitor of photosynthetic water oxidation: acetone hydrazone. 1. Equilibrium reactions.
The process of photosynthetic water oxidation has been investigated by using a new type of water oxidation inhibitor, the alkyl hydrazones. Acetone hydrazone (AceH), (CH3)2CNNH2, inhibits water oxidation by a mechanism that is analogous to that of NH2OH. This involves binding to the water-oxidizing complex (WOC), followed by photoreversible reduction of manganese (loss of the S1----S2 reaction). At higher AceH concentrations the S1 state is reduced in the dark and Mn is released, albeit to a lesser extent than with NH2OH. Following extraction of Mn, AceH is able to donate electrons rapidly to the reaction center tyrosine radical Z+ (161Tyr-D1 protein), more slowly to a reaction center radical C+, and not at all to the dark-stable tyrosine radical D+ (160Tyr-D2 protein) which must be sequestered in an inaccessible site. Manganese, Z+, and C+ thus appear to be located in a common protein domain, with Mn being the first accessible donor, followed by Z+ and then C+. Photooxidation of Cyt b-559 is suppressed by AceH, indicating either reduction or competition for donation to P680+. Unexpectedly, Cl- was found not to interfere or compete with AceH for binding to the WOC in the S1 state, in contrast to the reported rate of binding of N,N-dimethylhydroxylamine, (CH3)2NOH [Beck, W., & Brudvig, G. (1988) J. Am. Chem. Soc. 110, 1517-1523]. We interpret the latter behavior as due to ionic screening of the thylakoid membrane, rather than a specific Cl- site involved in water oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)
Tso
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Department of Chemistry, Princeton University, New Jersey 08544.
Petrouleas
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Dismukes
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D000096:Acetone / Q000031:analogs & derivatives* / Q000494:pharmacology; D003573:Cytochrome b Group / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D006835:Hydrazones / Q000494:pharmacology*; D008345:Manganese / Q000378:metabolism; D010084:Oxidation-Reduction / Q000187:drug effects; D045332:Photosystem II Protein Complex; D010944:Plants / Q000187:drug effects; D014443:Tyrosine / Q000378:metabolism; D014867:Water / Q000378:metabolism*
D003573:Cytochrome b Group; D006835:Hydrazones; D045332:Photosystem II Protein Complex; D014867:Water; D000096:Acetone; D014443:Tyrosine; D008345:Manganese; C066837:acetone hydrazone; C019313:cytochrome b559