THE H+-ATPASE FROM CHLOROPLASTS - ENERGETICS OF THE CATALYTIC CYCLE

The H+-ATPase from chloroplasts couples a transmembrane proton transport with ATP synthesis and ATP hydrolysis. The ability of the enzyme to catalyze this reaction is regulated by a redox-reaction involving HS-groups and by an activation involving protonation/deprotonation reactions. When the enzyme is in the active reduced state the catalytic reaction cycle can be studied. A simple enzyme kinetic model is proposed. In the state E(1) proton binding sites in the CF, part are directed to the inside. After protonation ADP and P-i bind in a random sequence to the CF0 part. The enzyme changes into conformation E(2) where proton binding sites are directed to the outside and enzyme bound ATP is formed. The protons dissociate to the outside, ATP is released and finally a change back to the E(1) state closes the reaction cycle. Literature data are used to construct a Gibbs free enthalpy diagram for the reaction cycle.