A new mechanism-based inhibitor of photosynthetic water oxidation: Acetone hydrazone. I. Equilibrium reactions
- Princeton Univ., NJ (USA)
- Nuclear Research Center, Athens (Greece)
The process of photosynthetic water oxidation has been investigated by using a new type of water oxidation inhibitor, the alkyl hydrazones. Acetone hydrazone (AceH), (CH{sub 3}){sub 2}CNNH{sub 2}, inhibits water oxidation by a mechanism that is analogous to that of NH{sub 2}OH. This involves binding to the water-oxidizing complex (WOC), followed by photoreversible reduction of manganese (loss of the S{sub 1} {yields} S{sub 2} reaction). At higher AceH concentrations the S{sub 1} state is reduced in the dark and Mn is released, albeit to a lesser extent than with NH{sup 2}OH. Following extraction of Mn, AceH is able to donate electrons rapidly to the reaction center tyrosine radical Z{sup +} ({sup 161}Tyr-D{sub 1} protein), more slowly to a reaction center radical C{sup +}, and not at all to the dark-stable tyrosine radical D{sup +} ({sup 160}Tyr-D{sub 2} protein) which must be sequestered in an inaccessible site. Unexpectedly, Cl{sup {minus}} was found not to interfere or compete with AceH for binding to the WOC in the S{sub 1} state, in contrast to the reported rate of binding of N,N-dimethylhydroxylamine (CH{sub 3}){sub 2}NOH. The authors 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 Q{sub A}{sup {minus}}Fe(II), the primary electron acceptor, and for the oxidized Fe(III) acceptor (Q{sub 400} species), in contrast to that observed with NH{sub 2}OH. AceH can be oxidized in solution by a variety of oxidants including Mn(III) to form a reactive diazo intermediate, (CH{sub 3}){sub 2}CNN, which reacts with carbonyl compounds. Oxidation to this diazo intermediate is postulated to be responsible for inhibition of the WOC.
- DOE Contract Number:
- FG02-84CH10199
- OSTI ID:
- 5935763
- Journal Information:
- Biochemistry; (USA), Vol. 29:33; ISSN 0006-2960
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
HYDRAZONES
ELECTRON SPIN RESONANCE
PHOTOSYNTHESIS
INHIBITION
WATER
OXIDATION
MANGANESE COMPOUNDS
REACTION INTERMEDIATES
CHEMICAL REACTIONS
HYDROGEN COMPOUNDS
MAGNETIC RESONANCE
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXYGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
RESONANCE
SYNTHESIS
TRANSITION ELEMENT COMPOUNDS
550200* - Biochemistry