Effect of pH on the self-exchange reactivity of the plant plastocyanin from parsley

The self-exchange rate constant (25 degreesC) for parsley plastocyanin is 5 .0 x 10(4) M-1 s(-1) at pH* 7.5 (I = 0.10 M). This value is quite large for a higher plant plastocyanin and can be attributed to a diminished upper ac idic patch in this protein. The self-exchange rate constant is almost indep endent of pH* in the range 7.5-5.6, with a value (25 degreesC) of 5.6 x 10( 4) M-1 s(-1) at pH* 5.6 (I = 0.10 M). At this pH*, the ligand His87 is prot onated in similar to 50% of the reduced protein molecules (pK(a)* 5.6), and this would be expected to hinder electron transfer between the two oxidati on states. However, this effect is counterbalanced by the enhanced associat ion of two parsley plastocyanins at lower pH* due to the partial protonatio n of the acidic patch.