Redox potentials of the primary electron acceptor pheophytin a of photosystem II in Thermosynechococcus elongatus: Comparison of the potentials in D1:1 and D1:3

Abstract

The redox potential of pheophytin a, E_m(Ph/Ph^-), is one of the important physicochemical parameters for discussing the energetics within PSII; however, previous values are not consistent with energetic relationships estimated from kinetic analyses. This situation should be due to problems of titration employed traditionally for measuring E_m(Ph/Ph^-). In view of this, we have established spectroelectrochemical conditions overcoming the problems, and obtained a value of -505 mV for E_m(Ph/Ph^-) using PSII isolated from Thermosynechococcus elongatus (Kato et al. 2009). In this work, we investigated the influence of difference in the core proteins D1 of PSII on E_m(Ph/Ph^-), because T. elongatus has three D1 genes. The measurement for D1:1, which is expressed under normal cultivation conditions, yielded -522 mV; this result means the wild type maintains more negatively by 17 mV than that obtained by the previous measurement in D1:3, which is similar to the higher plant type.