Studies on the Mechanism of the Water Oxidation in Photosynthesis

Abstract

The effect of a number of substances on the rate of the deactivation reactions of the oxidizing equivalents stored in the water-splitting enzyme system Y (these oxidizing equivalents are designated as trapped holes) has been investigated in spinach chloroplasts. The following conclusions were reached.1. A number of substances, the so-called accelerating agents (acceleration of the deactivation reactions of the water-splitting enzyme system Y), accelerate the discharge rate of the trapped holes.2. The accelerating effect is not influenced by the electrical field across the thylakoid membrane. It is only slightly dependent on glutaraldehyde fixation of the chloroplasts and is not caused by accelerating agents inducing irreversible conformational changes in the chloroplasts. Furthermore, for stoichiometrical reasons the accelerating effect is not explainable by the reduction of the trapped holes with accelerating agents as irreversible electron donors.3. The stability of the trapped holes is not influenced by uncouplers of the ammonium iontype, by proton buffers such as imidezole or by ionophores (e.g. nonactin).From these results it is suggested that the accelerating agents only act as catalysts for the natural discharge reactions of the trapped holes.A molecular mechanism for the photosynthetic water oxidation is proposed and the accelerating effect is explained on the basis of this mechanism. The open questions concerning the deactivation reactions of the trapped holes are discussed.The nature of the photosynthetic water-splitting enzyme system Y mediating the oxidation of H,O induced by chlorophyll a11 [1,2] is unknown (see Fig. 1). For the water oxidation to molecular oxygen either by electrolysis or by photolysis the cooperation of four holes (oxidizing equivalents) is required. I n this respect a number of fundamental questions arise. a) Which mechanism generates the holes responsible for the water oxidation in the photosynthetic apparatus ? b) What is the spatial arrangement of these trapped holes ? c) I n which way is the fundamental cooperativity of four holes realized in the oxygen-evolving system Y of photosynthesis?Taking into account the facts so far known about the water-splitting enzyme system Y the following conclusions can be made [7,8] (see Fig. 2).a) The holes produced by photosystem I1 and primarily localized at the primary donor D (these holes are designated as free holes) are not able to oxidize the water molecules directly. b) A secondary donor system, Mn, containing manganese in a definite valence state and probably chloride as functional elements is the site of the oxygen-evolving enzymatic activity.

Oxygen-Evolving Activity of Thylakoids from Barley Plants Cultivated on Different Concentrations of Jasmonic Acid

“…an ADRY reagent (15). If this assumption is correct, JA applied exogenously to isolated thylakoid preparations from barley must show the same effect, since it is well known that the ADRY agents react directly with the trapped holes (the oxi- dizing equivalents) of the water-splitting enzyme system and accelerate the deactivation of the S-states (16). Our attempts at establishing a direct effect of this growth substance on the S state deactivations and on the Hill reaction activity yielded no positive result and, regardless of the existing analogy in the effect of the ADRY reagents and in the in vivo applied JA, the latter may not be referred to the group of chemical substances accelerating the deactivation processes of the water-splitting enzyme system.…”

Photosystem II Peripheral Accessory Chlorophyll Mutants inChlamydomonas reinhardtii. Biochemical Characterization and Sensitivity to Photo-Inhibition,

“…To further investigate these effects, we assessed the stability of the charge accumulating, water-splitting apparatus during S-state advancement. We measured the stability of the S states and their tendency to decay to lower S states by measuring oxygen production during a series of single-turnover flashes with varying dark intervals between each flash (Renger, 1972). The flash-dependent yield of oxygen typically decreases as the dark interval between flashes increases.…”

Photoreduction of silicomolybdate in chloroplasts by agents accelerating the deactivation reactions of the water‐oxidizing system

“…The diffusion mobility and binding/release kinetics of the ADRY agent molecules seem to determine the occurrence of the residual 02 evolution when 'the enzyme' is saturated with substrate (with the ADRY agent molecules). Thus, ADRY agents at low concentrations are catalysts of the cyclic electron transfer around PSII [3,13,19,20], and at high concentrations they appear to serve as electron donors for non-cyclic electron transfer involving PSII.…”

“…$2 and $3 states are unstable and relax in the dark down to S~ in tens of seconds [2]. Deactivation of the S 2 and S 3 states that is followed by the inhibition of 02 evolution is accelerated by compounds known as ADRY agents [3]. These agents also induce a light-dependent oxidation of carotenoids [4~7] and chlorophyll a [4], and transform cytochrome b-559 from the reduced high-potential form to the oxidized low-potential form [8,9].…”