Azide as a competitor of chloride in oxygen evolution by photosystem II

Oxygen evolution by higher plants requires chloride, which binds to a site associated with the oxygen-evolving complex of photosystem II (PSII). In this study, the inhibitory effect of the anion azide was characterized using steady state measurements of oxygen evolution activity in PSII-enriched thylakoid membranes. N₃^- (7.8 mM) inhibited O₂ evolution activity by 50% when a standard buffer containing chloride was used. By considering Cl^- as the substrate in O₂ evolution assays, we found azide to be primarily competitive with Cl^- with an inhibitor dissociation constant K(i) of about 0.6 mM. An uncompetitive component with a K(i)' of 11 mM was also found. Removal of the 17 and 23 kDa polypeptides resulted in a decrease in each inhibition constant. A pH dependence study of O₂ evolution activity showed that the pH maximum became narrower and shifted to a higher pH in the presence of azide. Analysis of the data indicated that an acidic residue defined the low side of the pH maximum with an apparent pK(a) of 6.7 in the presence of azide compared with 5.5 for the control. A basic residue was also affected, exhibiting an apparent pK(a) of 7.1 compared with a value of 7.6 for the control. This result can be explained by a simple model in which azide binding to the chloride site moves negative charge of the anion away from the basic residue and toward the acidic residue relative to chloride. As a competitor of chloride, azide may provide an interesting probe of the oxygen-evolving complex in future studies.