Affiliation:
1. Department of Chemistry, Princeton UniversityPrinceton, NJ 08544, USA
2. Environmental Institute, Princeton UniversityPrinceton, NJ 08544, USA
Abstract
Perturbation of the catalytic inorganic core (Mn
4
Ca
1
O
x
Cl
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. Cd
2+
inhibits the yield of reconstitution of O
2
-evolution activity, called photoactivation, starting from the free inorganic cofactors and the cofactor-depleted apo-WOC-PSII complex. Ca
2+
affinity increases following photooxidation of the first Mn
2+
to Mn
3+
bound to the ‘high-affinity’ site. Ca
2+
binding occurs in the dark and is the slowest overall step of photoactivation (IM
1
→IM
1
*
step). Cd
2+
competitively blocks the binding of Ca
2+
to its functional site with 10- to 30-fold higher affinity, but does not influence the binding of Mn
2+
to its high-affinity site. By contrast, even 10-fold higher concentrations of Cd
2+
have no effect on O
2
-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 Ca
2+
. 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 Cd
2+
binding following photooxidation of the first Mn
3+
, IM
1
→IM
1
*
,
causes three outcomes: (i) a longer intermediate lifetime that slows IM
1
decay to IM
0
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 Mn
2+
that form the functional cluster, and (iii) increased lability of Cd
2+
following Mn
4
cluster assembly results in (re)exchange of Cd
2+
by Ca
2+
which restores active O
2
-evolving centres. Prior EPR spectroscopic data provide evidence for an oxo-bridged assembly intermediate, Mn
3+
(μ-O
2−
)Ca
2+
, for IM
1
*
. We postulate an analogous inhibited intermediate with Cd
2+
replacing Ca
2+
.
Subject
General Agricultural and Biological Sciences,General Biochemistry, Genetics and Molecular Biology
Cited by
17 articles.
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