Contributions of the protein environment to the midpoint Potentials of the A1 phylloquinones and the Fx iron-sulfur cluster in photosystem I

Electrostatic calculations have predicted that the partial negative charge associated with D575(PsaB) plays a significant role in modulating the midpoint potentials of the A(1A) and A(1B) phylloquinones in photosystem I. To test this prediction, the side chain of residue 575(PsaB) was changed from negatively charged (D) to neutral (A) and to positively charged (K). D566(PsaB), which is located at a considerable distance from either A(1A) or A(1B), and should affect primarily the midpoint potential of F-X, was similarly changed. In the 575(PsaB) variants, the rate of electron transfer from A(1A) to F-X is observed to decrease slightly according to the sequence D/A/K. In the 566(PsaB) variants, the opposite effect of a slight increase in the rate is observed according to the same sequence D/A/K. These results are consistent with the expectation that changing these residues will shift the midpoint potentials of nearby cofactors to more positive values and that the magnitude of this shift will depend on the distance between the cofactors and the residues being changed. Thus, the midpoint potentials of A(1A) and A(1B) should experience a larger shift than will F-X in the 575(PsaB) variants, while F-X should experience a larger shift than will either A(1A) or A(1B) in the 566(PsaB) variants. As a result, the driving energy for electron transfer from A(1A) and A(1B) to F-X will be decreased in the former and increased in the latter. This rationalization of the changes in kinetics is compared with the results of electrostatic calculations. While the altered amino acids shift the midpoint potentials of A(1A), A(1B), and F-X by different amounts, the difference in the shifts between A(1A) and F-X or between A(1B) and F-X is small so that the overall effect on the electron transfer rate between A(1A) and F-X or between A(1B) and F-X is predicted to be small. These conclusions are borne out by experiment.