Influence of an amino-acid residue on the optical properties and electron transfer dynamics of a photosynthetic reaction centre complex

Bacteriopheophytin (Bphe) is the first kinetically and spectrally resolved electron carrier in the electron transfer pathway^1-2 of the bacterial photosynthetic reaction centre (RC), the only membrane protein whose structure has been determined to atomic resolution^3-5. The two Bphes in the RC are not equivalent spectrally, and only the long-wavelength-absorbing Bphe, which is associated with the L subunit (Bphe_L)⁶, is active in electron transfer. There has been considerable speculation as to whether the spectroscopic red shift of Bphe_L, and its exclusive involvement in electron transfer can be attributed to its interaction with a glutamic acid residue (L104) found within hydrogen-bonding distance to this Bphe (for examples see refs 7-10). Here we report the results of changing this glutamic acid residue to leucine, glutamine and lysine in RCs of Low-temperature absorption spectra of these genetically modified RCs indicate that the glutamic acid residue at L104 is largely responsible for the spectroscopic red shift of the photoactive Bphe. However, time-resolved optical measurements indicate that the Glu^L104 → Leu RC maintains the same high quantum yield of charge separation observed in wild-type by using only the electron transfer pathway associated with the L subunit. These results indicate that the GluBphenteraction is not the dominant contributor to the directionality of electron transfer in RCs.