Effects of formate binding on the quinone–iron electron acceptor complex of photosystem II

EPR was used to study the influence of formate on the electron acceptor side of photosystem II (PSII) from Thermosynechococcus elongatus. Two new EPR signals were found and characterized. The first is assigned to the semiquinone form of QB interacting magnetically with a high spin, non-heme-iron (Fe2+, S = 2) when the native bicarbonate/carbonate ligand is replaced by formate. This assignment is based on several experimental observations, the most important of which were: (i) its presence in the dark in a significant fraction of centers, and (ii) the period-of-two variations in the concentration expected for QB•− when PSII underwent a series of single-electron turnovers. This signal is similar but not identical to the well-know formate-modified EPR signal observed for the QA•−Fe2+ complex (W.F.J. Vermaas and A.W. Rutherford, FEBS Lett. 175 (1984) 243–248). The formate-modified signals from QA•−Fe2+ and QB•−Fe2+ are also similar to native semiquinone–iron signals (QA•−Fe2+/QB•−Fe2+) seen in purple bacterial reaction centers where a glutamate provides the carboxylate ligand to the iron. The second new signal was formed when QA•− was generated in formate-inhibited PSII when the secondary acceptor was reduced by two electrons. While the signal is reminiscent of the formate-modified semiquinone–iron signals, it is broader and its main turning point has a major sub-peak at higher field. This new signal is attributed to the QA•−Fe2+ with formate bound but which is perturbed when QB is fully reduced, most likely as QBH2 (or possibly QBH•− or QB2•−). Flash experiments on formate-inhibited PSII monitoring these new EPR signals indicate that the outcome of charge separation on the first two flashes is not greatly modified by formate. However on the third flash and subsequent flashes, the modified QA•−Fe2+QBH2 signal is trapped in the EPR experiment and there is a marked decrease in the quantum yield of formation of stable charge pairs. The main effect of formate then appears to be on QBH2 exchange and this agrees with earlier studies using different methods.

Research Highlights► A new EPR signal from QB•−Fe is reported when formate is bound to PSII non-heme iron ► A new EPR signal from formate-modified QA•−Fe is reported when QBH2 is also present. ► Flash experiments using EPR indicate that formate blocks PSII at the QA•−FeQBH2 state.