A NEW MODEL FOR THE MOLECULAR MECHANISMS OF PHOTOSYNTHETIC WATER OXIDATION AND PHOTOPHOSPHORYLATION

It is postulated that trans-3-phosphatidyl glycerol, tightly bound to the inner side of the thylakoid membrane, catalyses-after its oxidatio n to the oxygen radical by P-680 in combination with the tyrosine radi cal Y-z(.) and after release of one proton-in cooperation with the Mn enzyme the first reaction in water splitting. In this way, four molecu les of water would be oxidized by four light flashes. In the last phas e the Mn enzyme would act as a ''catalase'' transforming four ''comple xed OH species'' (2H(2)O(2)) to 2H(2)O + O-2. The photophosphorylation is formulated analogously to the mitochondrial process, because the s tructure of the ATP synthase in chloroplasts on principle agrees with the mitochondrial enzyme complex. Cardiolipin or cardiolipin ketone, r espectively, may be exchanged by tightly-bound phosphatidyl glycerol o r glycerone, respectively. Accordingly, to enable ATP synthesis in pur ified in vitro systems (MF0F1 or CF0F1 ATP synthase), a redox reaction or light energy for formation of the ketyl radical and an H+/Na+ grad ient are necessary. SH compounds, valinomycin-K+, carbonyl cyanide m-c hlorophenylhydrazone (CCCP), organic acids, or cholesterol are suitabl e as electron donors. Moreover, it is postulated that MgATP, synthesiz ed by the catalytic centres of the F-1 part, is shifted to the alloste ric nucleotide-binding sites to elevate-before its release-the MgADP a ffinity of the respective following catalytic centre. In this way, the synthesis product MgATP is additionally used as an allosteric effecto r, before it is released for energy-yielding reactions. So the ATP syn thesis can proceed in an optimal rhythm. (C) 1998 Academic Press Limit ed.