Studies on the Mechanism of the Water Oxidation in Photosynthesis

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 imidazole 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.