The Influence of Aspartate 575PsaBon the Midpoint Potentials of Phylloquinones A1A/A1Band the Fx Iron-Sulfur Cluster in Photosystem I

Electrostatic calculations predict that the partial negative charge associated with D575PsaB plays a significant role in modulating the midpoint potentials of the A1A and A1B phylloquinones in PS I (Ishikita and Knapp 2003). To test this prediction, the side chain of residue 575PsaB was changed from negatively charged (D) to neutral (A) and to positively charged (K). In these variants, the rate of electron transfer from A1A to FX 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 A1A, A1B and FX 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 A1A, A1B and FX by different amounts, the difference in the shifts between A1A and FX or between A1B and FX is so small that the overall effect on the A1A to FX and A1B to FX electron transfer rate is almost negligible.