PROTON RELEASE DURING THE 4 STEPS OF PHOTOSYNTHETIC WATER OXIDATION - INDUCTION OF 1-1-1-1 PATTERN DUE TO LACK OF CHLOROPHYLL-A/B BINDING-PROTEINS

Abstract

In photosynthesis of green plants water is oxidized to dioxygen. This four-step process is accompanied by the release of four protons (per molecule of dioxygen) into the lumen of thylakoids. In dark-adapted thylakoids which are excited with a series of short flashes of light, the extent of proton release oscillates with period four as a function of flash number. Noninteger and pH-dependent proton/electron ratios (e.g., 1.1, 0.25, 1.0, and 1.65 at pH 7) have been attributed to a superposition of two reactions: chemical production of protons and transient electrostatic response of peripheral amino acid side chains. Aiming at the true pattern of proton production, we investigated the relative contribution of peripheral proteins. Thylakoids with and without chlorophyll a/b binding proteins were compared. Thylakoids lacking chlorophyll alb binding proteins were prepared from pea seedlings grown under intermittent light [Jahns, P., & Junge, W. (1992) Biochemistry (preceding paper in this issue)]. We found no oscillation of proton release in the pH range from 6 to 7.5. These and other results showed that chlorophyll a/b binding proteins, which primarily serve as light-harvesting antennas, modulate proton release by water oxidation. A non-oscillating pattern of proton release, with proton/electron ratios of 1:1:1:1 more closely represents the events in the catalytic center proper. This implies hydrogen abstraction rather than electron abstraction from water during the oxygen-evolving step S3 --> S0.