Investigation of the functional role of Ca2+ in the oxygen-evolving complex of photosystem II

A pH-dependence study of the substitution of Ca2+ by Sr2+

Cheng I. Lee, Gary W Brudvig

Research output: Contribution to journalArticle

14 Citations (Scopus)

Abstract

The oxygen-evolving complex (OEC) of photosystem II (PSII) is the catalytic site for the oxidation of water to O2 in photosynthesis. The OEC consists of a tetranuclear manganese (Mn4) cluster with calcium and chloride ions functioning as essential cofactors. Previous studies have shown that substitution of Ca2+ in the OEC by various divalent and trivalent metal ions results in the loss of oxygen-evolution activity of PSII. Sr2+ is the only ion that has been found to restore the oxygen-evolving activity in PSII. Although several models have been proposed for the mechanism of water oxidation in PSII, the functional role of Ca 2+ remains unclear. In order to test the proposal that Ca 2+ functions as a Lewis acid in water oxidation chemistry, the pH dependence of the oxygen-evolution activity of intact Ca2+-containing PSII and Sr2+-substituted PSII has been investigated. In both samples, the pH dependence exhibits a bell-shaped curve. Sr2+ substitution shifts the peak of the curve to higher pH in comparison to Ca 2+-containing PSII. The pH-dependent O2-evolving rates of these two samples are fit to a diprotic model in which protonation of an essential basic group at acidic pH inhibits activity (pKa1) and deprotonation of an essential acidic group at higher pH inhibits activity (pKa2). Sr2+ substitution has no effect on pKa 2. This result suggests that the high pH inhibition may be on the electron-acceptor side of PSII. On the other hand, Sr2+ substitution causes pKa1 to be shifted to higher pH by 1.0 pH unit. This pH-shift indicates a direct role of Ca2+/Sr2+ in the OEC. The functional role of Ca2+ in oxygen evolution in PSII is discussed based on these results.

Original languageEnglish
Pages (from-to)1221-1228
Number of pages8
JournalJournal of the Chinese Chemical Society
Volume51
Issue number5 B
Publication statusPublished - 2004

Fingerprint

Photosystem II Protein Complex
Substitution reactions
Oxygen
Oxidation
Water
Ions
Lewis Acids
Calcium Chloride
Deprotonation
Photosynthesis
Protonation
Manganese
Metal ions
Electrons

Keywords

  • Calcium
  • Oxygen-evolving complex
  • pH-Dependence
  • Photosystem II
  • Strontium
  • Water oxidation

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

@article{80be54d87b3a40e2bd17b31f8f378ba3,
title = "Investigation of the functional role of Ca2+ in the oxygen-evolving complex of photosystem II: A pH-dependence study of the substitution of Ca2+ by Sr2+",
abstract = "The oxygen-evolving complex (OEC) of photosystem II (PSII) is the catalytic site for the oxidation of water to O2 in photosynthesis. The OEC consists of a tetranuclear manganese (Mn4) cluster with calcium and chloride ions functioning as essential cofactors. Previous studies have shown that substitution of Ca2+ in the OEC by various divalent and trivalent metal ions results in the loss of oxygen-evolution activity of PSII. Sr2+ is the only ion that has been found to restore the oxygen-evolving activity in PSII. Although several models have been proposed for the mechanism of water oxidation in PSII, the functional role of Ca 2+ remains unclear. In order to test the proposal that Ca 2+ functions as a Lewis acid in water oxidation chemistry, the pH dependence of the oxygen-evolution activity of intact Ca2+-containing PSII and Sr2+-substituted PSII has been investigated. In both samples, the pH dependence exhibits a bell-shaped curve. Sr2+ substitution shifts the peak of the curve to higher pH in comparison to Ca 2+-containing PSII. The pH-dependent O2-evolving rates of these two samples are fit to a diprotic model in which protonation of an essential basic group at acidic pH inhibits activity (pKa1) and deprotonation of an essential acidic group at higher pH inhibits activity (pKa2). Sr2+ substitution has no effect on pKa 2. This result suggests that the high pH inhibition may be on the electron-acceptor side of PSII. On the other hand, Sr2+ substitution causes pKa1 to be shifted to higher pH by 1.0 pH unit. This pH-shift indicates a direct role of Ca2+/Sr2+ in the OEC. The functional role of Ca2+ in oxygen evolution in PSII is discussed based on these results.",
keywords = "Calcium, Oxygen-evolving complex, pH-Dependence, Photosystem II, Strontium, Water oxidation",
author = "Lee, {Cheng I.} and Brudvig, {Gary W}",
year = "2004",
language = "English",
volume = "51",
pages = "1221--1228",
journal = "Journal of the Chinese Chemical Society",
issn = "0009-4536",
publisher = "Chinese Chemical Society",
number = "5 B",

}

TY - JOUR

T1 - Investigation of the functional role of Ca2+ in the oxygen-evolving complex of photosystem II

T2 - A pH-dependence study of the substitution of Ca2+ by Sr2+

AU - Lee, Cheng I.

AU - Brudvig, Gary W

PY - 2004

Y1 - 2004

N2 - The oxygen-evolving complex (OEC) of photosystem II (PSII) is the catalytic site for the oxidation of water to O2 in photosynthesis. The OEC consists of a tetranuclear manganese (Mn4) cluster with calcium and chloride ions functioning as essential cofactors. Previous studies have shown that substitution of Ca2+ in the OEC by various divalent and trivalent metal ions results in the loss of oxygen-evolution activity of PSII. Sr2+ is the only ion that has been found to restore the oxygen-evolving activity in PSII. Although several models have been proposed for the mechanism of water oxidation in PSII, the functional role of Ca 2+ remains unclear. In order to test the proposal that Ca 2+ functions as a Lewis acid in water oxidation chemistry, the pH dependence of the oxygen-evolution activity of intact Ca2+-containing PSII and Sr2+-substituted PSII has been investigated. In both samples, the pH dependence exhibits a bell-shaped curve. Sr2+ substitution shifts the peak of the curve to higher pH in comparison to Ca 2+-containing PSII. The pH-dependent O2-evolving rates of these two samples are fit to a diprotic model in which protonation of an essential basic group at acidic pH inhibits activity (pKa1) and deprotonation of an essential acidic group at higher pH inhibits activity (pKa2). Sr2+ substitution has no effect on pKa 2. This result suggests that the high pH inhibition may be on the electron-acceptor side of PSII. On the other hand, Sr2+ substitution causes pKa1 to be shifted to higher pH by 1.0 pH unit. This pH-shift indicates a direct role of Ca2+/Sr2+ in the OEC. The functional role of Ca2+ in oxygen evolution in PSII is discussed based on these results.

AB - The oxygen-evolving complex (OEC) of photosystem II (PSII) is the catalytic site for the oxidation of water to O2 in photosynthesis. The OEC consists of a tetranuclear manganese (Mn4) cluster with calcium and chloride ions functioning as essential cofactors. Previous studies have shown that substitution of Ca2+ in the OEC by various divalent and trivalent metal ions results in the loss of oxygen-evolution activity of PSII. Sr2+ is the only ion that has been found to restore the oxygen-evolving activity in PSII. Although several models have been proposed for the mechanism of water oxidation in PSII, the functional role of Ca 2+ remains unclear. In order to test the proposal that Ca 2+ functions as a Lewis acid in water oxidation chemistry, the pH dependence of the oxygen-evolution activity of intact Ca2+-containing PSII and Sr2+-substituted PSII has been investigated. In both samples, the pH dependence exhibits a bell-shaped curve. Sr2+ substitution shifts the peak of the curve to higher pH in comparison to Ca 2+-containing PSII. The pH-dependent O2-evolving rates of these two samples are fit to a diprotic model in which protonation of an essential basic group at acidic pH inhibits activity (pKa1) and deprotonation of an essential acidic group at higher pH inhibits activity (pKa2). Sr2+ substitution has no effect on pKa 2. This result suggests that the high pH inhibition may be on the electron-acceptor side of PSII. On the other hand, Sr2+ substitution causes pKa1 to be shifted to higher pH by 1.0 pH unit. This pH-shift indicates a direct role of Ca2+/Sr2+ in the OEC. The functional role of Ca2+ in oxygen evolution in PSII is discussed based on these results.

KW - Calcium

KW - Oxygen-evolving complex

KW - pH-Dependence

KW - Photosystem II

KW - Strontium

KW - Water oxidation

UR - http://www.scopus.com/inward/record.url?scp=29244477131&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=29244477131&partnerID=8YFLogxK

M3 - Article

VL - 51

SP - 1221

EP - 1228

JO - Journal of the Chinese Chemical Society

JF - Journal of the Chinese Chemical Society

SN - 0009-4536

IS - 5 B

ER -