Protolytic Reactions in the Thylakoid Interior

Abstract

The principles of electric potential generation and proton pumping in thylakoids are understood. There is one electrogenic reaction in each of the two photosystems and a third one associated with cyclic electron transfer through the cytochrome-b₆-segment of the electron transfer chain. Proton deposition into thylakoids occurs at the level of water oxidation and of plastohydroquinone oxidation. Proton uptake from outside is initiated when plastoquinone is reduced by photosystem II (or during cyclic electron transfer) and upon reduction of the terminal electron acceptor. Broadly speaking proton pumping in green plant photosynthesis seems to conform with the concept of alternating electron-hydrogen transport across the membrane. This was postulated by Mitchell (1961, 1966). We have reviewed these general features recently (Junge, Jackson (1982)). In this manuscript we focus on unorthodox aspects of proton transport: transient release and rebinding of protons by proteinaceous groups in response to electron transfer (membrane Bohr effect proton pump?), the question of localized or delocalized pathways for protons when they cycle between pumps and ATP-synthases and the entry of protons into CFO-CF1.