On the Accessibility of Essential Tyrosines in Isolated and Activated Chloroplast H +-ATPase

The addition o f 7-chloro-4-nitrobenzofurazan to isolated and activated CF, creates a com­ pletely changed binding stoichiometry and subunit-distribution o f bound modifier in contrast to the binding pattern in not-activated CF,. The activation o f CF, by dithiothreitol and heat results in the accessibility o f three additional tyrosines in ß-subunits and one additional tyro­ sine in a-subunits. Binding o f pyridoxalphosphate to lysine in the active state suppresses the accessibility o f the additional tyrosines, suggesting that PLP inactivates the ATPase by induc­ ing a conformational change. Furthermore, two NBD-m olecules are bound to y-subunits when CF, is activated. These two molecules may be bound to sulfhydryl groups o f cysteines which become accessible to N BD after activation.