Redox potential of pheophytin a in photosystem II of two cyanobacteria having the different special pair chlorophylls

Abstract

Water oxidation by photosystem (PS) II in oxygenic photosynthetic organisms is a major source of energy on the earth, leading to production of a stable reductant. Mechanisms generating a high oxidation potential for water oxidation have been a major focus of photosynthesis research. This potential has not been estimated directly but has been measured by the redox potential of the primary electron acceptor, pheophytin (Phe) a. However, the reported values for Phe a are still controversial. Here, we measured the redox potential of Phe a under physiological conditions (pH 7.0, 25°C) in two cyanobacteria having different special pair chlorophylls (Chls): Synechocystis sp. PCC 6803, whose special pair for PS II consists of Chl a, and Acaryochloris marina MBIC 11017, whose special pair for PS II consists of Chl d. We obtained redox potentials of −536 ± 8 and −478 ± 24 mV on PS II complexes in the presence of 1.0 M betaine, for Synechocystis sp. PCC 6803 and A. marina, respectively. The difference in the redox potential of Phe a between the two species closely corresponded with the difference in the light energy absorbed by Chl a vs. Chl d. We estimated the potentials of the special pair of PS II to be 1.20 V and 1.18 V, for Synechocystis sp. PCC 6803 (P680) and A. marina (P713), respectively, clearly indicating conservation in the properties of water oxidation systems in oxygenic photosynthetic organisms irrespectively of the special pair chlorophylls.