Site-directed mutations affecting the spectroscopic characteristics and midpoint potential of the primary donor in photosystem I

Photosystem I is a member of the iron-sulfur center or type I reaction centers. The primary electron donor in photosystem I is a chlorophyll a dimer termed P₇₀₀. The biophysical properties of P₇₀₀ are well understood, but the protein environment that gives it such unique properties is unknown. We have characterized site-directed mutants of the photosystem I reaction center protein PsaB and identified an amino acid, His-656, that interacts closely with one of the P₇₀₀ chlorophylls. Mutation of His-656 to Asn or Ser increases the oxidation midpoint potential of P₇₀₀/P₇₀₀⁺· by 40 mV. The P₇₀₀/P₇₀₀⁺· optical difference spectra show the appearance of a new bleaching band at 667 nm. Electron nuclear double resonance spectroscopy indicates a significant increase in the hyperfine coupling corresponding to methyl protons at position 12 of the spin carrying chlorophyll a of P₇₀₀⁺·. The implication of these results to current structural models of the photosystem I reaction center is discussed.