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 oxidation to the oxygen radical by P680 in combination with the tyrosine radical YZ. and after release of one proton-in cooperation with the Mn enzyme the first reaction in water splitting. In this way, four molecules of water would be oxidized by four light flashes. In the last phase the Mn enzyme would act as a "catalase" transforming four "complexed OH species" (2H2O2) to 2H2O+O2. The photophosphorylation is formulated analogously to the mitochondrial process, because the structure of the ATP synthase in chloroplasts on principle agrees with the mitochondrial enzyme complex. Cardiolipin or cardiolipin ketone, respectively, may be exchanged by tightly-bound phosphatidyl glycerol or glycerone, respectively. Accordingly, to enable ATP synthesis in purified in vitro systems (MF0F1 or CF0F1 ATP synthase), a redox reaction or light energy for formation of the ketyl radical and an H+/Na+ gradient are necessary. SH compounds, valinomycin-K+, carbonyl cyanide m-chlorophenylhydrazone (CCCP), organic acids, or cholesterol are suitable as electron donors. Moreover, it is postulated that MgATP, synthesized by the catalytic centres of the F1 part, is shifted to the allosteric nucleotide-binding sites to elevate-before its release-the MgADP affinity of the respective following catalytic centre. In this way, the synthesis product MgATP is additionally used as an allosteric effector, before it is released for energy-yielding reactions. So the ATP synthesis can proceed in an optimal rhythm.Copyright 1998 Academic Press Limited
Schole
J
Institut für Physiologische Chemie der Tierärztlichen Hochschule Hannover, Bünteweg 17, D-30559 Hannover, Germany
Schole
C