LOW-TEMPERATURE ELECTRON-TRANSFER FROM CYTOCHROME TO THE SPECIAL PAIRIN RHODOPSEUDOMONAS-VIRIDIS - ROLE OF THE L162 RESIDUE

Electron transfer from the tetraheme cytochrome c to the special pair of bacteriochlorophylls (P) has been studied by flash absorption spect roscopy in reaction centers isolated from seven strains of the photosy nthetic purple bacterium Rhodopseudomonas viridis, where the residue L 162, located between the proximal heme c-559 and P, is Y (wild type), F, W, G, M, T, or L. Measurements were performed between 294 K and 8 K , under redox conditions in which the two high-potential hemes of the cytochrome were chemically reduced. At room temperature, the kinetics of P+ reduction include two phases in all of the strains: a dominant v ery fast phase (VF), and a minor fast phase (F). The VF phase has the following t(1/2): 90 ns (M), 130 ns (W), 135 ns (F), 189 ns (Y; wild t ype), 200 ns (G), 390 ns (L), and 430 ns (T). These data show that ele ctron transfer is fast whatever the nature of the amino acid at positi on L162. The amplitudes of both phases decrease suddenly around 200 K in Y, F, and W. The effect of temperature on the extent of fast phases is different in mutants G, M, L, and T, in which electron transfer fr om c-559 to P+ takes place at cryogenic temperatures in a substantial fraction of the reaction centers (T, 48%; G, 38%; L, 23%, at 40 K; and M, 28%, at 60 K), producing a stable charge separated state. In these nonaromatic mutants the rate of VF electron transfer from cytochrome to P+ is nearly temperature-independent between 294 K and 8 K, remaini ng very fast at very low temperatures (123 ns at 60 K for M; 251 ns at 40 K for L; 190 ns at 8 K for G, and 458 ns at 8 K for T). In all cas es, a decrease in amplitudes of the fast phases is paralleled by an in crease in very slow reduction of P+, presumably by back-reaction with Q(A)(-). The significance of these results is discussed in relation to electron transfer theories and to freezing at low temperatures of cyt ochrome structural reorganization.