On the determination of redox midpoint potential of the primary quinone electron acceptor, QA, in Photosystem II

Bicarbonate-induced redox tuning in Photosystem II for regulation and protection

“…These titrations were done with a limited set of redox mediators that did not equilibrate with the Mn cluster. Subsequently, some evidence was published indicating that, when titrations were done with the same mediators, the lower potential values were obtained, whereas in the absence of mediators, the higher potential values similar to those by Krieger and coworkers (12)(13)(14) were obtained (23). However, low potential values were also recently reported without the use of mediators (24).…”

“…Recently, Shibamoto et al (21,22) used an optically transparent thin-layer electrochemical cell to obtain data that confirmed the 150-mV redox difference with and without the Mn cluster, but they also showed that both values shifted to lower potentials by ∼80 mV compared with the earlier data (12). These titrations were done with a limited set of redox mediators that did not equilibrate with the Mn cluster.…”

“…The redox potential of Q A has been the subject of research for decades (12). The current detailed thermodynamic picture of PSII redox chemistry is based on estimates of energy differences between the electron transfer components, but these estimates rely on measurements of redox potentials to fix absolute values (1)(2)(3) A , differing by ∼150 mV, depending on the nature of the PSII (12)(13)(14).…”

“…The fully active enzyme showed an E m value that was ∼150 mV lower than that in PSII lacking the Mn 4 CaO 5 cluster. This shift was, in fact, due to the binding (or absence thereof) of the Ca 2+ ion involved in water splitting, but because the Mn cluster provides the Ca binding site, the potential is indirectly determined by the presence of Mn cluster (12)(13)(14). Given the high valence state on the Mn cluster even in the lowest redox state of the water splitting cycle (15), it is not surprising that the use of chemical mediators, the role of which is to equilibrate the external redox potential with the cofactors, can lead to the reduction and consequent loss of the Mn cluster.…”

“…Given the high valence state on the Mn cluster even in the lowest redox state of the water splitting cycle (15), it is not surprising that the use of chemical mediators, the role of which is to equilibrate the external redox potential with the cofactors, can lead to the reduction and consequent loss of the Mn cluster. Because loss of the Mn leads to loss of Ca 2+ binding, then the high potential form of the Q A /Q A −• couple is easily generated during the course of redox titrations, and this technical problem explained some of the scatter in the E m values reported in the literature (12).…”