Uni-site catalysis in thylakoids. The influence of membrane energization on ATP hydrolysis and ATP-Pi exchange.
ATP-hydrolysis was measured with thylakoid membranes during continuous illumination. The concentrations of free and enzyme-bound ATP, ADP and Pi were measured using either cold ATP, [gamma-32P]ATP or [14C]ATP. The concentration of free ATP was constant, free ADP and enzyme-bound ATP were below the detection limit. Nevertheless, [gamma-32P]ATP was bound, hydrolyzed and 32Pi was released. The ADP was not released from the enzyme but cold Pi was bound from the medium, cold ATP was resynthesized and released. A quantitative analysis gave the following rate constants: ATP-binding kATP = 2 . 10(5) M-1 s-1, ADP-release: kADP less than 10(-2)s-1, Pi-release: kPi = 0.1 s-1. These rate constants are considerably smaller than under deenergized conditions. The rate constant for the release of ATP can be estimated to be at least 0.2 s-1 under energized conditions. Obviously, energization of the membrane, i.e. protonation of the enzyme leads mainly to a decrease of the rate of ATP-binding, to an increase of the rate of ATP release and to a decrease of the rate of ADP-release.
Fromme
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P
Max Volmer Institut für Biophysikalische und Physikalische Chemie, Technische Universität Berlin, FRG.
Gräber
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P
D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism*; D002736:Chloroplasts / Q000502:physiology*; D004734:Energy Metabolism; D066298:In Vitro Techniques; D007425:Intracellular Membranes / Q000502:physiology*; D008027:Light; D010944:Plants; D006180:Proton-Translocating ATPases / Q000378:metabolism*
D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D006180:Proton-Translocating ATPases