The protonic shortcircuit by DCCD in photosystem II A common feature of all redox transitions of water oxidation

Abstract

Photosystem II of green plants oxidizes water at an Mn centre and reduces bound plastoquinone to supply electrons to photosystem I. Owing to the vectorial properties of the electron transfer (water-)protons are released into the lumen of thylakoids, while (quinole-)protons are taken up from the medium. The proton-pumping activity can be shortcircuited by covalent modification of two polypeptides with N,N′dicyclohexylcarbodiimide (DCCD). Then (water-)protons are no longer released but, within milliseconds, they are funnelled through the protein to the reduced quinone [(1988) EMBO J. 7,589-594]. When dark-adapted thylakoids are excited by a series of light flashes water oxidation to dioxygen is synchronised to follow a stepped reaction pattern with period four, which is apparent in the pattern of proton release. We asked whether or not the above 'protonic shortcircuit' was restricted to certain reactions out of the four-stepped cycle. This was not observed. Instead, each step was equally affected and the complement of the period four pattern was observed for proton uptake at the site of quinone reduction. This proved that seemingly non-released protons were transferred to the quinone and, for unmodified membranes, it suggested that protons produced during the partial reactions of water oxidation used one and the same DCCD-sensitive outlet to the lumen.