Protein dynamics and reactions of photosystem II

The kinetics of charge recombination by electron transfer from Q(A)(*-) to P680(*+) on the reducing branch of PSII is likely to be strongly dependent on protein dynamics, in analogy with the kinetics of the corresponding reaction in the reaction center of purple bacteria [Biophys. J. 74 (1998) 2567]. On the oxidizing branch of PSII, the kinetics of electron hole transfer from P680(*+) to Y(Z) is known to be multiexponential. This transfer is in the Babcock model of the reactions of the water-oxidizing complex coupled with proton transfer from Y(Z). The proton is via switching hydrogen bonds in the protein transferred to the thylakoid lumen. The demand for successive proton transfers requires rearrangement of the hydrogen bonds, which in turn requires a flexible protein making fluctuating excursions among all its conformations. In the equilibrated protein, only a fractional part of the molecules is in a conformation that is able to support the proton transfer from Y(Z). The kinetics of the rearrangement to this active conformation will be multiexponential and dependent on the distribution among all conformations, which is likely to be sensitive to various influences, in particular from changes in the protein coordination to the (Mn)(4) cluster between the different S states.