| Photosystem II is a photosynthetic enzyme that catalyzes the light-driven oxidation of water to molecular oxygen. A total of four sequential light-induced turnovers of Photosystem II are required to form molecular oxygen. Each stable intermediate produced by a flash of light is called an Sn-state, n = 0--4. A general consensus has not yet been reached concerning the mechanism of this reaction or the structure of Photosystem II, although much progress has been made over the last few years. It is generally agreed upon that the oxygen-evolving complex of Photosystem II consists of a manganese cluster made up of four manganese atoms, a redox-active tyrosine called Y z, histidine 190, and Ca2+ and Cl-- ions. Recently, several models have been proposed that directly involve Y z in the water oxidation chemistry as a hydrogen atom abstractor. Perhaps the most elusive subject is the role of Ca2+ in PSII. Two main proposals have been set forth: (1) Ca2+ plays a structural role and a binding site for Cl-- and, (2) Ca 2+ is directly involved in the oxygen-oxygen bond forming reaction. One way to acquire more information on the role of Ca2+ is to perform biochemical treatments that perturb this cofactor and then monitor the kinetic efficiency of the oxygen-evolving complex.; Several groups have recently investigated the effects of biochemical treatments, site-directed mutagenesis, or substitution of essential cofactors on the kinetics of the stepwise, water-oxidizing chemistry catalyzed by Photosystem II. Consistently, these studies showed evidence for a slowing of the final, oxygen-releasing step, S3→S0, of the catalytic cycle. To test the roles that have been proposed for Ca2+, PSII membranes were depleted of Ca2+ and subsequently reconstitution with Sr2+. The rates of the S-state transitions in these samples were monitored by using time-resolved electron paramagnetic resonance spectroscopy. Both the lower and higher S-state transition rates decrease and suggest that a common molecular mechanism is at work and that Sr2+ is less effective than Ca2+ in supporting it.; The time-resolved electron paramagnetic resonance spectroscopy results were also analyzed in the context of the hydrogen atom abstraction mechanism. The retarding effect caused by Sr2+ substitution is additional evidence to support this mechanism. Furthermore, the hydrogen atom abstraction mechanism is best able to explain the unusual characteristics of the S-state transitions: slow rates, low activation energies, low pre-exponential factors, and small kinetic isotope effects. |
