pH-CONTROLLED CHANGE OF TOBACCO PPO II ACTIVE SITE.

PPO II is a novel enzyme that plays a key role in the plant defense system. PPO II has the active site of a pair of antiferromagnetic coppers with phenolate oxygen bridged at neutral pH medium. At low pH, the coordination ability of phenolate oxygen becomes weak and two H2O molecular replaces the phenolate oxygen bridge in the active center and the absorption of PPO II is changed to a novel state as that of mushroom PPO. A new PPO II active site is formed between pH 8.5 and 9.0 suddenly. The phenolate oxygen bridge has been broken, hydroxide coordination with one of coppers is formed. The other copper is still coordinated with the phenolate oxygen. The CD data shows at the pH 3.0 and pH 9.0 the second structure keeps stabile as the resting PPO II and is broken at high pH 10.0. [ABSTRACT FROM AUTHOR]