Studies on the structural and functional organization of water cleavage by visible light in photosynthesis

Abstract

Photosynthetics water cleavage by visible light into molecular oxygen and metabolically bound hydrogen is supplied by excition dissociation into strongly oxidizing defect electrons and moderately reducing electrons at a special chlorophyll- a -complex (Chl- a II ) associated with acceptor component(s). This device is referred to as the reaction center of system II (RC-II). Water oxidation by the defect electrons originally localized at Chl- a II + , is catalyzed by a manganoprotein, the so-called water-splitting enzyme system Y. Spectroscopic measurements of flash induced absorption changes in the range of 250–850 nm as well as polarographic detection of oxygen formation lead to the following conclusions about the functional and structural organization scheme of photosynthetic water cleavage: o 1. Exciton dissociation leading to water oxidation and electric field formation across the thylakoid membrane requires a special plastoquinone molecule (X-320) as electron acceptor. Under conditions of X-320 remaining functionally blocked in its reduced state, exciton dissociation under the formation of Chl- a II + is still possible, but this reaction is highly dissipative. 2. The electron transfer from system Y to Chl- a II + , which reflects the functional coupling of these units, occurs via a multiphasic kinetics in the micro- and submicrosecond time scale. These kinetics are dependent on the charge accumulation state of system Y and the pH of the inner thylakoid space. 3. A postulated model for the reaction sequence in system Y is discussed. 4. The life times of the intermediary stages of water oxidation in system Y can selectively be modified by special chemical (ADRY-reagents). 5. The reaction centers RC-II nearly totally span the impermeable core of the thylakoid membrane, with Chl- a II located towards the inner side and X-320 attached near the outer side. The redox component X a , which is assumed to be the primary electron acceptor, is inferred to be located rather close to Chl- a II . 6. X-320 is enwrapped by a proteinaceous component, which provides — via another special plastoquinone molecule (B) — the functional connection to the plastoquinone pool. This trypsin digestible component probably functions as an allosteric regulator for the electron flux from X-320 to the pool and its blockage by DCMU-type inhibitors. On the basis of the experimental results and theoretical considerations a preliminary model is proposed for the functional and structural organization of water oxidation by visible light in photosynthesizing organisms.