Dependence of the deactivation reactions of photosystem II on the redox state of plastoquinone pool A varied under anaerobic conditions; Equilibria on the acceptor side of photosystem II.

Dark adapted spinach chloroplasts and Chlorella with variably reduced plastoquinone pools were given 1 or 2 saturating flashes. Under these conditions the rate of deactivation of state S2 of the oxygen evolving site of Photosystem II (B, Kok, B, Forbush, M. McGloin (1970) Photochem. Photobiol. 11, 457-475) is highly dependent on the pool redox state, undergoing a nearly 10-fold acceleration upon transforming the plastoquinone pool (A) from 100% oxidized to 90% reduced. Deactivation of state S3 is unaffected by the same variation of the pool redox state. These observations are attributed to a high concentration of Photosystem II reduced primary electron acceptor, Q-, coincident with the formation of S2 and a low concentration coincident with the formation of S3, under the conditions of highly reduced plastoquinone pool. Simultaneous determination of Q- and A2- result in an estimated equilibrium constant of 15-20 for reaction Q-B in equilibrium QB- and a value greater than 50 for equilibrium Q-B- in equilibrium QB2-, where B is the secondary electron acceptor described by B. Bouges-Bocquet ((1973) Biochim. Biophys. Acta. 314, 250-256) and B.R. Velthuys and J. Amesz ((1974) Biochim. Biophys. Acta. 333, 85-94). It is proposed that doubly reduced B becomes protonated in the last reaction.