Effect of pH on the self-exchange reactivity of the plant plastocyanin from parsley.
The self-exchange rate constant (25 degrees C) 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 acidic patch in this protein. The self-exchange rate constant is almost independent of pH* in the range 7.5-5.6, with a value (25 degrees C) 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 protonated in approximately 50% of the reduced protein molecules (pKa* 5.6), and this would be expected to hinder electron transfer between the two oxidation states. However, this effect is counterbalanced by the enhanced association of two parsley plastocyanins at lower pH* due to the partial protonation of the acidic patch.
Hunter
D M
DM
Department of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K.
McFarlane
W
W
Sykes
A G
AG
Dennison
C
C
D019661:Apiaceae / Q000737:chemistry*; D003300:Copper / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D010970:Plastocyanin / Q000737:chemistry*
D003300:Copper; D010970:Plastocyanin