The Influence of Aspartate 575_PsaBon the Midpoint Potentials of Phylloquinones A_1A/A_1Band the F_x Iron-Sulfur Cluster in Photosystem I

Abstract

Electrostatic calculations predict 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 PS I (Ishikita and Knapp 2003). 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). In these variants, the rate of electron transfer from A_1A to F_X was found to decrease slightly according to the sequence D/A/K. These results are consistent with the expectation that changing these residues will shift the midpoint potentials of A_1A, A_1B and F_X to more positive values and that the magnitude of this shift will depend on the distance between the cofactors and the residues being changed. This rationalization of the changes in kinetics is compared with the results of electrostaticcalculations. 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 so small that the overall effect on the A_1A to F_X and A_1B to F_X electron transfer rate is almost negligible.