Uni‐site catalysis in thylakoids

Abstract

ATP-hydrolysis was measured with thylakoid membranes during continuous illumination. The concentrations of free and enzyme-bound ATP, ADP and P, were measured using either cold ATP, [Y-~~P]ATP or P*c]ATP. The concentration of free ATP was constant, free ADP and enxymebound ATP were below the detection limit. Nevertheless, [y-s2P]ATP was bound, hydrolyzed and '*Pi 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. 101 M-is-', ADP-release: k.,m < IO-*s-i, Pi-release: kn=O.l s-i. 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-i under energized conditions. Obviously, energixation 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.