Structural Dynamics in the Plastocyanin−Photosystem 1 Electron-Transfer Complex as Revealed by Mutant Studies

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Abstract
A series of plastocyanin mutants have been constructed by site-directed mutagenesis and expressed in Escherichia coli to elucidate the interaction between plastocyanin and photosystem 1 in the photosynthetic electron-transfer chain. Leu-12 has been replaced with alanine, asparagine, glutamate, and lysine, while Tyr-83 has been exchanged for histidine, phenylalanine, and leucine. Phe-35, Asp-42, and Gln-88 have been mutated to tyrosine, asparagine, and glutamate, respectively. The mutations that have been introduced do not seem to place any strain on the tertiary structure according to optical absorption and electron paramagnetic resonance (EPR) spectroscopic studies. However, there are changes in the reduction potential for the Leu-12 mutants that cannot be accounted for by electrostatic interactions alone. For some of the mutants, the pI shifts, in accordance with the changes in the number of titratable groups. Only the Leu-12 mutants show any major change in their photosystem 1 kinetics, while the mutants in the acidic patch show minor changes, suggesting that both the hydrophobic and acidic patches make contact with photosystem 1 but that the electron transfer occurs at the hydrophobic interface, most probably via the His-87 residue. The kinetics are best described with a model in which a rate-limiting conformational change occurs in the plastocyanin−photosystem 1 complex [Bottin, H., & Mathis, P. (1985) Biochemistry 24, 6453−6460; Sigfridsson, K., Hansson, Ö., Karlsson, B. G., Baltzer, L., Nordling, M., & Lundberg, L. G. (1995) Biochim. Biophys. Acta1228, 28−36], where the changes observed are attributed to changes in the dynamics within the electron-transfer complex.