The reaction of H2S with the photosynthetic water-oxidizing complex and its lack of reaction with the primary electron acceptor in spinach

M. Sivaraja, D. Hunziker, G. C. Dismukes

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16 Citations (Scopus)


Inhibition of photosynthetic water oxidation by H2S, a substrate analog, has been investigated using equilibrium titrations and EPR spectroscopic detection of the electron donors in spinach Photosystem II (PS II) membranes and compared to inhibition by NH2OH. Like NH2OH, H2S inhibits formation of the S2 oxidation state of the water oxidizing complex by a two-step process in the dark. Initially, reversible inhibition of S2 occurs upon binding to a high affinity site in the dark (S1) state at a low concentration of inhibitor (50% inhibition: 0.07 μmol H2S/mg Chl, corresponding to about 15 H2S/PS II). This causes no loss of steady-state O2 evolution and can be reversed by illumination at room temperature, which causes multiple turnovers. At higher concentrations, irreversible inhibition occurs due to the cooperative release of 3 out of 4 Mn2+/PS II using mild washing conditions, with parallel loss of O2 evolution activity. This stoichiometry of Mn release is preserved throughout the entire concentration range of inhibition by both H2S and NH2OH, suggesting a common binding site for at least 3, and possibly all 4, of the Mn ions which are present in PS II. This consistent with independent earlier work showing the net release of 4 Mn/PS II using stronger washing conditions, and also with EPR spectroscopic evidence assigning the S2 multiline signal to a cluster of 3-4 Mn ions. The concentration of H2S which induces 50% irreversible inhibition is 17-fold greater than that required for NH2OH. Qualitatively, the weakerinhibition by H2S is consistent with its lower oxidation potential compared to N2OH. However, the quantitative agreement is poor, suggesting that other environmental factors must be involved in determining their relative inhibition strenghts. Unlike NH2OH, H2S does not affect the structure of the primary quinone electron acceptor, QA--His-Fe (structure by analogy to bacterial reaction centers), as seen by formation of the normal EPR signals at g = 1.8 and g = 1.9 for the photoreduced semiquinone, instead of shifted resonance at g = 2.1 observed with NH2OH. H2S is therfore unable to bind to the NH2OH site on the acceptor side of PS II.

Original languageEnglish
Pages (from-to)228-235
Number of pages8
JournalBBA - Bioenergetics
Issue number2
Publication statusPublished - Nov 16 1988


  • Metalloenzyme
  • Oxygen evolution
  • Photosynthesis
  • Photosystem II
  • Water oxidation

ASJC Scopus subject areas

  • Biophysics
  • Biochemistry
  • Cell Biology

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