Electron transfer between the spinach plastocyanin mutant Leu12His and Photosystem 1

A spinach plastocyanin (Pc) mutant, Pc(Leu12His), has been constructed by site-directed mutagenesis and expressed in Escherichia coli to probe the importance of the hydrophobic patch in the interaction with Photosystem 1. The mutant has been characterized by optical absorption, EPR spectroscopy and redox titration. The electron transfer to Photosystem 1 was investigated by flash-induced time-resolved absorption measurements at 830 nm. The Pc(Leu12His) mutant showed a major change in the Photosystem 1 kinetics compared to wild-type Pc. In contrast to the biphasic Photosystem 1 reduction observed for the physiological reaction partner, only the slow phase was discerned when Pc(Leu12His) replaced wild-type Pc as the electron donor. The reaction showed a hyperbolic dependence with increasing Pc concentration, saturating at a rate constant value of 2000 s^-1, which is about 10 times slower than the corresponding rate constant for wild-type Pc. Obviously, this position i s critical for a proper reaction. Moreover, the reaction showed a titrating behavior with a pK_a of 6.7. Thus, it appears that both shape and charge of the residue in this position are important. A plausible reaction mechanism for electron transfer between wild-type Pc and Photosystem 1 is discussed. The role of electrostatic interactions may be that of long-range guidance and initial recognition that allow the two proteins to seek a pre-docking configuration(s). Then a short-range rearrangement(s), involving also hydrophobic interactions, forms an optimum docking configuration prior to electron transfer.