Effects of pH on the S3 State of the Oxygen Evolving Complex in Photosystem II Probed by EPR Split Signal Induction
2010 (English)In: Biochemistry, ISSN 0006-2960, E-ISSN 1520-4995, Vol. 49, no 45, 9800-9808 p.Article in journal (Refereed) Published
The electrons extracted from the CaMn4 cluster during water oxidation in photosystem II are transferred to P-680(+) via the redox-active tyrosine D1-Tyr161 (Y-z). Upon Y-z oxidation a proton moves in a hydrogen bond toward D1-His190 (His(z)). The deprotonation and reprotonation mechanism of Y-z-OH/Y-z-O is of key importance for the catalytic turnover of photosystem II. By light illumination at liquid helium temperatures (similar to 5 K) Y-z can be oxidized to its neutral radical, Y-z(center dot). This can be followed by the induction of a split EPR signal from Y-z(center dot) in a magnetic interaction with the CaMn4 cluster, offering a way to probe for Y-z oxidation in active photosystem II. In the S-3 state, light in the near-infrared region induces the split S-3 EPR signal, S-2'Y-z(center dot). Here we report on the pH dependence for the induction of S-2'Y-z(center dot) between pH 4.0 and pH 8.7. At acidic pH the split S-3 EPR signal decreases with the apparent pK(a) (pK(app)) similar to 4.1. This can be correlated to a titration event that disrupts the essential H-bond in the Y-z-His(z) motif. At alkaline pH, the split S-3 EPR signal decreases with the pK(app) similar to 7.5. The analysis of this pH dependence is complicated by the presence of an alkaline-induced split EPR signal (pK(app) similar to 8.3) promoted by a change in the redox potential of Y-z. Our results allow dissection of the proton-coupled electron transfer reactions in the S-3 state and provide further evidence that the radical involved in the split EPR signals is indeed Y-z(center dot).
Place, publisher, year, edition, pages
Easton: American Chemical Society (ACS), 2010. Vol. 49, no 45, 9800-9808 p.
IdentifiersURN: urn:nbn:se:uu:diva-135370DOI: 10.1021/bi101364tISI: 000283833800016OAI: oai:DiVA.org:uu-135370DiVA: diva2:374928