Role of tyrosine M210 in the initial charge separation of reaction centers of Rhodobacter sphaeroides

Abstract

ABSTRACT:Femtosecond spectroscopy was used in combination with site-directed mutagenesis to study the influence of tyrosine M210 (YM210) on the primary electron transfer in the reaction center of Rhodobacter sphaeroides. The exchange of YM210 to phenylalanine caused the time constant of primary electron transfer to increase from 3.5 f 0.4 ps to 16 f 6 ps while the exchange to leucine increased the time constant even more to 22 f 8 ps. The results suggest that tyrosine M210 is important for the fast rate of the primary electron transfer.x e primary photochemical event during photosynthesis of bacteriochlorophyll-(Bchl-) containing organisms is a lightinduced charge separation within a transmembrane protein complex called the reaction center (RC). The crystal structures of RC's from Rhodopseudomonas (Rps.) viridis and Rhodobacter (Rb.) sphaeroides have been solved to high resolution [reviewed in Deisenhofer and Michel (1989), Chang et al. (1986), Tiede et al. (1988), andRees et al. (1989)l. The RC from Rb. sphaeroides contains three protein subunits referred to as L, M, and H, according to their respective mobilities in SDS-polyacrylamide gels. Associated with the L and M subunits are the cofactors, consisting of four Bchl a, two bacteriopheophytin (Bph) a, one atom of non-heme ferrous iron, two quinones (QA and Qe), and in some species one carotenoid [reviewed in Parson (1987) and Feher et al.' Financial support was from the Deutsche Forschungsgemeinschaft,

Photochemical trapping of a bacteriopheophytin anion in site‐specific reaction‐center mutants from the photosynthetic bacterium Rhodobacter sphaeroides

“…We have reported previously (Gray et al, 1990) that the mutant FF grows photoheterotrophically with no apparent difference from the wild-type. The new single-site mutation, YY, has no effect upon the ability of the cells to grow under photoheterotrophic conditions.…”

“…Reaction centers were isolated from semi-aerobically grown cells as previously described (Ogrodnik et al, 1988;Gray et al, 1990). Reaction center samples prepared at 5°C and at room temperature resulted in comparably stable samples.…”

“…Substitutions of the histidine hgdnds to the bacteriochlorophylls comprising the dimer have resulted in so-called heterodimers (containing one bacteriochlorophyll and one bacteriopheophytin) and studies of these reaction centers have been instrumental in more fully understanding the processes that occur within P after excitation Kirmaier et al, 1988Kirmaier et al, , 1989Breton et al, 1989;DiMagno et al, 1990;McDowell et al, 1990McDowell et al, , 1991. We (Gray et al, 1990 andFinkele et al, 1990) and Nagarajan et al (1990) have shown that Tyr210 on the M polypeptide (YM210) in the reaction center of R. sphaeroides influences the rate of initial electron transfer. YM210 is a member of one of several symmetry-breaking pairs of amino acid residues in the reaction center (Tiede et al, 1988).…”

“…viridis and R. sphaeroides (Becker et al, 1989). The substitution Tyr+Phe at M210 in the reaction center of R. sphaeroides (YM210) results in a decreased rate of decay of the excited state of P as well as the rise time of Ha-(both are approximately 3-5 times slower than in wild type; Finkele et al, 1990 andNagarajan et al, 1990). The comparison of the rates of the various substitutions of Tyr at M210 (Phe, Leu, Ile) has indicated that YM210 does not act as a direct electron carrier but more likely acts to optimize the energy levels of the intermediates.…”

Primary electron transfer kinetics in bacterial reaction centers with modified bacteriochlorophylls at the monomeric sites BA,B.

“…The molecular interpretation given above is related to the one used to explain the slowing down of the primary ET rate in genetically mutated RCs where the polar tyrosine M210 was replaced by nonpolar amino acids. Here the slowing down of the reaction speed was also attributed to the changed energetics (34)(35)(36)(37). The somewhat weaker slowing down of the decay of P* in mutated RCs (compared to the [3-vinyl]RCs) suggests that the [3-vinyl] modification yields a stronger increase in the energy of the intermediate P+B-A ttThis amplitude ratio deviates slightly from the average exchange rate mentioned above, indicating that the exchange yields of the BChl-a molecules are different on the two pigment branches.…”