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

Perturbation of the catalytic inorganic core (Mn₄Ca ₁O_xCl_y) 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-O^2-)Ca²⁺, for IM₁*. We postulate an analogous inhibited intermediate with Cd²⁺ replacing Ca²⁺.