Insights into substrate binding to the oxygen-evolving complex of photosystem II from ammonia inhibition studies

David J. Vinyard, Gary W Brudvig

Research output: Contribution to journalArticle

15 Citations (Scopus)

Abstract

Water oxidation in Photosystem II occurs at the oxygen-evolving complex (OEC), which cycles through distinct intermediates, S0-S4. The inhibitor ammonia selectively binds to the S2 state at an unresolved site that is not competitive with substrate water. By monitoring the yields of flash-induced oxygen production, we show that ammonia decreases the net efficiency of OEC turnover and slows the decay kinetics of S2 to S1. The temperature dependence of biphasic S2 decay kinetics provides activation energies that do not vary in control and ammonia conditions. We interpret our data in the broader context of previous studies by introducing a kinetic model for both the formation and decay of ammonia-bound S2. The model predicts ammonia binds to S2 rapidly (t1/2 = 1 ms) with a large equilibrium constant. This finding implies that ammonia decreases the reduction potential of S2 by at least 2.7 kcal mol-1 (>120 mV), which is not consistent with ammonia substitution of a terminal water ligand of Mn(IV). Instead, these data support the proposal that ammonia binds as a bridging ligand between two Mn atoms. Implications for the mechanism of O-O bond formation are discussed.

Original languageEnglish
Pages (from-to)622-628
Number of pages7
JournalBiochemistry
Volume54
Issue number2
DOIs
Publication statusPublished - Jan 20 2015

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Photosystem II Protein Complex
Ammonia
Oxygen
Substrates
Kinetics
Water
Ligands
Equilibrium constants
Substitution reactions
Activation energy
Atoms
Oxidation
Temperature
Monitoring

ASJC Scopus subject areas

  • Biochemistry
  • Medicine(all)

Cite this

Insights into substrate binding to the oxygen-evolving complex of photosystem II from ammonia inhibition studies. / Vinyard, David J.; Brudvig, Gary W.

In: Biochemistry, Vol. 54, No. 2, 20.01.2015, p. 622-628.

Research output: Contribution to journalArticle

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AB - Water oxidation in Photosystem II occurs at the oxygen-evolving complex (OEC), which cycles through distinct intermediates, S0-S4. The inhibitor ammonia selectively binds to the S2 state at an unresolved site that is not competitive with substrate water. By monitoring the yields of flash-induced oxygen production, we show that ammonia decreases the net efficiency of OEC turnover and slows the decay kinetics of S2 to S1. The temperature dependence of biphasic S2 decay kinetics provides activation energies that do not vary in control and ammonia conditions. We interpret our data in the broader context of previous studies by introducing a kinetic model for both the formation and decay of ammonia-bound S2. The model predicts ammonia binds to S2 rapidly (t1/2 = 1 ms) with a large equilibrium constant. This finding implies that ammonia decreases the reduction potential of S2 by at least 2.7 kcal mol-1 (>120 mV), which is not consistent with ammonia substitution of a terminal water ligand of Mn(IV). Instead, these data support the proposal that ammonia binds as a bridging ligand between two Mn atoms. Implications for the mechanism of O-O bond formation are discussed.

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