KINETIC FACTORS IN THE BICYCLE MODEL OF OXYGEN EVOLUTION BY PHOTOSYSTEM-II

Flash-induced oxygen evolution and many related processes in thylakoid s of oxygenic organisms are modulated with period four by the S-state transitions associated with the oxygen evolving system of Photosystem II (PS II). To analyze these phenomena, we have interpreted the S-stat e model on the basis of the charge accumulating activities on both sid es of PS II - 4 charges on the donor side and 2 charges on the accepto r side. This results in the recognition of two parallel reaction cente r cycles V and W of PS II function (V.P. Shinkarev and C.A. Wraight (1 993) Proc Natl Acad Sci USA 90: 1834-1838). The description of damping of the period four oscillations is here extended to include kinetic s ources of misses in both cycles. Such misses arise in reaction centers (RCs) in which back reaction between P+ and Q(A)(-) occurs before the electron transfer equilibria on the donor and acceptor sides of the R C are reached. These are in addition to misses which are determined by reaction centers (RCs) that are inactive at the time of the flash due to the presence of either P+ or Q(A)(-) according to the electron tra nsfer equilibria on the donor and acceptor sides of the RC. Using know n or estimated values of the equilibrium and rate constants for donor and acceptor side reactions of the RC, this provides a natural and qua ntitatively reasonable description of the flash number dependence of o xygen evolution and other period four modulated processes of PS II. Th e estimated miss factors are different for both cycles V and W and are dependent on flash number and pH. Estimates based on existing data sh ow that miss factors of the first type (kinetic) are dominant at low p H, while those of the second type (equilibrium) are dominant at high p H.