FIGURE 1.

Lineweaver-Burk double-reciprocal plots, considering chloride as the substrate and showing the inhibitory effect of azide on PSII oxygen evolution activity. PSII was treated with sulfate to deplete chloride, and the PSII sample was then treated with 0 (red), 0.25 (orange), 0.38 (green), 0.50 (light blue), 0.75 (dark blue), or 1.0 (purple), 1.8 (black), and 2.5 (brown; pH 7.5 only) mM In A, the assay conditions were taken from reference (38) and employed 400 mM sucrose, 50 mM MES-NaOH, pH 6.3, and 2 mM recrystallized PPBQ. In B, the assay conditions were 400 mM sucrose, 50 mM HEPES-NaOH, pH 7.5, and 2 mM recrystallized PPBQ. The concentrations of chloride and azide were adjusted by addition from 1 M NaCl and 100 mM NaN₃ stock solutions, respectively, in the appropriate assay buffer. Error bars represent 1 SD. The color coded, superimposed lines were generated with V_max, K_m, K_i, and K_i′ values, derived from fits to the hyperbolic oxygen evolution plots (see Results section and the Supplementary Material, Data S1). The double reciprocal plots were fit with the equation