Intactness of the oxygen-evolving system in thylakoids and Photosystem II particles

Abstract

Photosystem II activity of oxygen-evolving membranes can be quantified by their capacity to do charge separation or their capacity to transport electrons. In this study using flash excitation of saturating intensity, charge separation is measured by absorption changes in the ultraviolet region of the spectra associated with primary-quinone reduction, and electron transport is measured by oxygen flash yield. These methods are applied to thylakoids and three different types of Photosystem II particles. In thylakoids electron-transport activity is 75–85% of charge separation activity. In Photosystem II particles this percentage is 60–70%, except for the BBY type (Berthold, D.A., Babcock, G.T. and Yocum, C.F. (1981) FEBS Lett. 135, 231–234), in which it is only 29%. These estimates of non-functional oxygen-evolving centers agree within experimental error, except for the BBY particle, with the quantum requirement for oxygen evolution measured under light-limited conditions. These reaction centers that are non-functional in oxygen evolution occur during sample preparation and are not a result of inhibition by ferricyanide or quinone acceptor systems. In thylakoids on the first flash, absorption changes at 325 nm do not show significant contributions from oxygen evolution S-state transitions. In the presence of ferricyanide the absorption change at 325 nm does have a significant contribution from Q 400 in thylakoids, but considerably less in Photosystem II particles.