Kinetic models of oxygen evolution in photosynthesis

Abstract

Oxygen production in photosynthesis occurs by a dark reaction of water molecules with a powerful oxidant produced by light absorbed in pigment system II. The production of the oxidant that reacts directly with water requires several steps. Recently, P. Joliot and co-workers (1969) and B. Kok and co-workers (1970, 1971) have proposed models that involve the accumulation of two or four positive charges before oxygen is evolved. In order to explain their experimental data, Joliot et al. additionally suggested a “flip flop” mechanism in the operation of the reaction center, and Kok et al. added “double hits” and “misses” in the same reaction center. Alternative models have been proposed here that equally well explain the data. In the first one, oxygen can be evolved from an accumulation of two positive charges as in Joliot's model; in contrast to Joliot et al.'s two charge “flip flop” model, this particular two charge hypothesis explains the existing experimental data. In the second one, an accumulation of four charges are needed for oxygen evolution as in Kok's model; this model, however, differs from the latter model because it does not require a “double hit” on the same reaction center, but in it, each oxygen evolving site has two bound reaction centers 11. This alternate four charge hypothesis also explains the existing experimental data. From detailed mathematical analyses of all the models, we conclude that the earlier models are not unique, and the two models presented here should be considered as valid alternate models for oxygen evolution in green plants. The pitfall of considering a model unique, because it fits all data, is, therefore, emphasized here.