A New Mechanism-Based Inhibitor of Photosynthetic Water Oxidation: Acetone Hydrazone. 1. Equilibrium Reactions

J. Tso, G. C. Dismukes, V. Petrouleas

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6 Scopus citations


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. AceH appears not to bind to the acceptor side of PSII as evidenced by normal EPR signals both for QAFe(II), the primary electron acceptor, and for the oxidized Fe(III) acceptor (Q400 species), in contrast to that observed with the NH2OH. AceH can be oxidized in solution by a variety of oxidants including Mn(III) to form a reactive diazo intermediate, (CH3)2CNN, which reacts with carbonyl compounds. Oxidation to this diazo intermediate is postulated to be responsible for inhibition of the WOC.

Original languageEnglish (US)
Pages (from-to)7759-7767
Number of pages9
Issue number33
StatePublished - Aug 1 1990
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Biochemistry


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