Calcium controls the assembly of the photosynthetic water-oxidizing complex: A cadmium(II) inorganic mutant of the Mn4Ca core

John E. Bartlett, Sergei V. Baranov, Gennady M. Ananyev, G. Charles Dismukes

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19 Scopus citations


Perturbation of the catalytic inorganic core (Mn4Ca 1OxCly) of the photosystem II-water-oxidizing complex (PSII-WOC) isolated from spinach is examined by substitution of Ca 2+ with cadmium(II) during core assembly. Cd2+ inhibits the yield of reconstitution of O2-evolution activity, called photoactivation, starting from the free inorganic cofactors and the cofactor-depleted apo-WOC-PSII complex. Ca2+ affinity increases following photooxidation of the first Mn2+ to Mn3+ bound to the 'high-affinity' site. Ca2+ binding occurs in the dark and is the slowest overall step of photoactivation (IM1→IM 1* step). Cd2+ competitively blocks the binding of Ca2+ to its functional site with 10- to 30-fold higher affinity, but does not influence the binding of Mn2+ to its high-affinity site. By contrast, even 10-fold higher concentrations of Cd2+ have no effect on O2-evolution activity in intact PSII-WOC. Paradoxically, Cd 2+ both inhibits photoactivation yield, while accelerating the rate of photoassembly of active centres 10-fold relative to Ca2+. Cd 2+ increases the kinetic stability of the photooxidized Mn 3+ assembly intermediate(s) by twofold (mean lifetime for dark decay). The rate data provide evidence that Cd2+ binding following photooxidation of the first Mn3+, IM1→IM 1*, causes three outcomes: (i) a longer intermediate lifetime that slows IM1 decay to IM0 by charge recombination, (ii) 10-fold higher probability of attaining the degrees of freedom (either or both cofactor and protein d.f.) needed to bind and photooxidize the remaining 3 Mn2+ that form the functional cluster, and (iii) increased lability of Cd2+ following Mn4 cluster assembly results in (re)exchange of Cd2+ by Ca2+ which restores active O2-evolving centres. Prior EPR spectroscopic data provide evidence for an oxo-bridged assembly intermediate, Mn 3+(μ-O2-)Ca2+, for IM1*. We postulate an analogous inhibited intermediate with Cd2+ replacing Ca2+.

Original languageEnglish (US)
Pages (from-to)1253-1261
Number of pages9
JournalPhilosophical Transactions of the Royal Society B: Biological Sciences
Issue number1494
StatePublished - Mar 27 2008
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Biochemistry, Genetics and Molecular Biology(all)
  • Agricultural and Biological Sciences(all)


  • Calcium
  • Manganese
  • Oxygen evolution
  • Photosynthesis
  • Photosystem II
  • Water oxidation

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