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 P-700. The biophysical properties of P-700 are well un derstood, but the protein environment that gives it such unique proper ties is unknown. We have characterized site-directed mutants of the ph otosystem I reaction center protein PsaB and identified an amino acid, His-656, that interacts closely with one of the P-700 chlorophylls, M utation of His-656 to Asn or Ser increases the oxidation midpoint pote ntial of P-700/P-700(+.) by 40 mV, The P-700/P-700(+.) optical differe nce spectra show the appearance of a new bleaching band at 667 nm. Ele ctron nuclear double resonance spectroscopy indicates a significant in crease in the hyperfine coupling corresponding to methyl protons at po sition 12 of the spin carrying chlorophyll n of P-700(+.) The implicat ion of these results to current structural models of the photosystem I reaction center is discussed.