Hypothesis for a Mechanism of Energy Transduction Sigmoidal Kinetics of Chloroplast’s Heat-Activated Atpase

Abstract

Chloroplasts have very low endogenous ATPase activity (AVRON & JAGENDORF 1959). However, conditions have been described under which ATPase activity in chloroplasts can be induced by light (PETRACK & LIPMANN 1961, AVRON 1962). The kinetics of these ATPase reactions indicate light-triggered and 1ight-dependent mechanisms for the release of ATPase activity. This has been interpreted as indicating that activation of the ATPases has requirements for respectively, long-lived and short-lived light-formed compounds, or conformational changes (BENNUN & AVRON 1964). Photophosphorylation is modified under the condition required for light-dependent and light-triggered ATPases (BENNUN & AVRON 1965). The affinity for ADP and GDP as substrates (Km) of the residual and modified photophosphorylation is identical to their affinity as competitive inhibitors (Ki) of light-requiring ATPases (BENNUN & AVRON 1965). The participation of an enzyme with a single active center in both ATPases and photophosphorylation is apparently contradictory because under optimal conditions photophosphorylation appears as a unidirectional catalytic process (AVRON & JAGENDORF 1959, AVRON 1962). However, it was found that a single coupling factor is required for the simultaneous reconstitution of ch1oroplast1s light-requiring ATPases and for photophosphorylation (BENNUN & AVRON 1965).