Effects of Sr2+-substitution on the reduction rates of Yz* in PSII membranes--evidence for concerted hydrogen-atom transfer in oxygen evolution.
Several groups have recently investigated the kinetic effects of biochemical treatments, site-directed mutagenesis, or substitution of essential cofactors on the stepwise, water-oxidizing chemistry catalyzed by Photosystem II. Consistently, these studies show evidence for a slowing of the final, oxygen-releasing step, S(3) --> S(0), of the catalytic cycle. To a degree, some of this work also shows a slowing of the earlier S-state transitions. To study these processes in more detail, we have investigated the effect of replacing Ca(2+) with Sr(2+)on the rates of the S-state transitions by using time-resolved electron paramagnetic resonance. The results show a slowdown of the last transition in the cycle, consistent with a report from Boussac et al. [Boussac, A., Sétif, P., and Rutherford, A. W. (1992) Biochemistry 31, 1224-1234], and of the earlier S-state transitions as well, which suggests that a common molecular mechanism is at work and that Sr(2+) is less effective than Ca(2+) in supporting it. While the oxidation of Y(z) by P(680)(+) has been extensively studied and can be understood within the context of nonadiabatic electron tunneling combined with rapid, non-rate-limiting proton transfer in the holo-system [Tommos, C., and Babcock, G. T. (2000) Biochim. Biophys. Acta 1458, 199], the reduction of Y(z*) by the Mn cluster cannot be described effectively by a nonadiabatic electron-transfer formalism. This indicates that this reaction is rate limited by processes other than electron tunneling. We discuss our results for Y(z*) reduction and those of others for the activation parameters (E(a), A, KIE, and rates) associated with this process, in terms of both sequential and concerted proton-coupled, electron transfer. Our analysis indicates that concerted hydrogen-atom transfer processes best explain the observed characteristics of the S-state advances.
Westphal
K L
KL
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA.
Lydakis-Simantiris
N
N
Cukier
R I
RI
Babcock
G T
GT
D029681:Arabidopsis Proteins; D004579:Electron Transport; D006859:Hydrogen / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D018724:Spinacia oleracea; D013324:Strontium / Q000737:chemistry*; D013816:Thermodynamics; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000737:chemistry* / Q000378:metabolism
D029681:Arabidopsis Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; C064388:oxygen-evolving complex, 33 kDa protein; C121497:psbQ protein, Arabidopsis; C114979:tyrosyl radical YZ; D014443:Tyrosine; D006859:Hydrogen; D010100:Oxygen; D013324:Strontium