Journal
BIOCHEMISTRY
Volume 46, Issue 11, Pages 3211-3223Publisher
AMER CHEMICAL SOC
DOI: 10.1021/bi062033i
Keywords
-
Categories
Funding
- NIGMS NIH HHS [GM32715] Funding Source: Medline
Ask authors/readers for more resources
Photosynthetic oxygen evolution in photosystem II (PSII) takes place in the oxygen-evolving complex (OEC) that is comprised of a tetranuclear manganese cluster (Mn-4), a redox-active tyrosine residue (Y-Z), and Ca2+ and Cl- cofactors. The OEC is successively oxidized by the absorption of 4 quanta of light that results in the oxidation of water and the release of O-2. Ca2+ is an essential cofactor in the water-oxidation reaction, as its depletion causes the loss of the oxygen-evolution activity in PSII. In recent X-ray crystal structures, Ca2+ has been revealed to be associated with the Mn-4 cluster of PSII. Although several mechanisms have been proposed for the water-oxidation reaction of PSII, the role of Ca2+ in oxygen evolution remains unclear. In this study, we probe the role of Ca2+ in oxygen evolution by monitoring the S-1 to S-2 state transition in PSII membranes and PSII core complexes upon inhibition of oxygen evolution by Dy3+, Cu2+, and Cd2+ ions. By using a cation-exchange procedure in which Ca2+ is not removed prior to addition of the studied cations, we achieve a high degree of reversible inhibition of PSII membranes and PSII core complexes by Dy3+, Cu2+, and Cd2+ ions. EPR spectroscopy is used to quantitate the number of bound Dy3+ and Cu2+ ions per PSII center and to determine the proximity of Dy3+ to other paramagnetic centers in PSII. We observe, for the first time, the S-2 state multiline electron paramagnetic resonance (EPR) signal in Dy3+- and Cd2+-inhibited PSII and conclude that the Ca2+ cofactor is not specifically required for the S-1 to S-2 state transition of PSII. This observation provides direct support for the proposal that Ca2+ plays a structural role in the early S-state transitions, which can be fulfilled by other cations of similar ionic radius, and that the functional role of Ca2+ to activate water in the O-O bond-forming reaction that occurs in the final step of the S state cycle can only be fulfilled by Ca2+ and Sr2+, which have similar Lewis acidities.
Authors
I am an author on this paper
Click your name to claim this paper and add it to your profile.
Reviews
Recommended
No Data Available