Low-Temperature Electron Transfer from Cytochrome to the Special Pair in Rhodopseudomonas viridis: Role of the L162 Residue

Abstract

Electron transfer from the tetraheme cytochrome c to the special pair of bacteriochlorophylls (P) has been studied by flash absorption spectroscopy in reaction centers isolated from seven strains of the photosynthetic purple bacterium Rhodopseudomonas viridis, where the residue L162, 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 very 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 type), 200 ns (G), 390 ns (L), and 430 ns (T). These data show that electron transfer is fast whatever the nature of the amino acid at position 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 from 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, remaining 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 cases, a decrease in amplitudes of the fast phases is paralleled by an increase 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 cytochrome structural reorganization.

Reconstitution of Photosynthetic Reaction Centers and Core Antenna-Reaction Center Complexes in Liposomes and Their Thermal Stability

“…24) These facts perhaps imply that the tightly bound cytochrome contributes not only to the transport of electrons to the photo-oxidized special pair cation, but also to the stability of the special pair pigment from denaturing. 4,5,25,26) Figure 5 shows the absorption spectra of ICMs from T. tepidum at 40, 60, 70, 75, and 80 C. The three peaks observed are assigned to B915 of LH1 and B850 and B800 of LH2. 12) The magnitude of the LH1 peak decreased with rising temperature, and finally its absorbance bleached completely at 80 C, whereas LH2 absorbance remained almost unchanged under these conditions.…”

Protein Dynamics Control of Electron Transfer in Photosynthetic Reaction Centers from Rps. Sulfoviridis

“…At temperatures where the structural diffusion is much slower or much faster than the reaction, the survival probability, Q(t), for the initial reactant state is independent of τ; the experimental data for such conditions will be used for determining the static parameters (5). The theoretical expressions for survival probability in the limits of slow and fast diffusion are given by 25 7and (8) respectively, where (9) is the average reaction rate over all conformational states and 10is the initial equilibrium distribution of the conformational states. 33 As one can see, in this case, the ET kinetics does not depend on the conformational dynamic parameters, and one does not need to solve the diffusion eq 1.…”

Electron transfer in the c-type cytochrome subunit of the photosynthetic reaction center ofRhodopseudomonas viridis:ab initio theoretical study

“…Among others, the π‐ring of the tyrosine of TYR L162 is very important in the one‐bridge term and also in the two‐bridge term, together with the π rings of phenylalanines of PHE C253 and PHE C230. Experimentally, the time for ET from c559 to P + is observed to be 1.2–2.2×10 −7 s 8–11. The time for ET from excited P (P*) to bacteriopheophytin b in the L‐region (H L ) is estimated to be 3×10 −12 s. Therefore, the first process is about 10 −5 times slower than the second process.…”

“…The roles of these four hemes have been studied by various methods,3–7 since the structure of the reaction center was determined by X‐ray chrystallography 2. Despite the large distance (8.7 Å) between P and c559, which is the proximate chromophore to P in the cytochrome subunit, rather fast electron transfer (120–220 ns) has been observed 8–11. Several amino acid residues exist between c559 and P, and especially TYR L162 was considered to participate in this ET.…”