Electron Transfer from the Tetraheme Cytochrome to the Special Pair in the Rhodopseudomonas viridis Reaction Center: Effect of Mutations of Tyrosine L162

Abstract

The structure of the photosynthetic reaction center (RC) from Rhodopseudomonas viridis is known to high resolution. It contains a firmly bound tetraheme cytochrome from which electrons are donated to a special pair (P) of bacteriochlorophylls, which is photooxidized upon absorption of light. Tyrosine at position 162 of the L-subunit of the reaction center (L 162 Y) is a highly conserved residue positioned halfway between P and the proximal heme group (c-559) of the cytochrome. By specific mutagenesis this residue was exchanged against the amino acids phenylalanine (F), glycine (G), methionine (M), leucine (L), tryptophan (W), threonine (T), and histidine (H). All mutants were expressed in Rps. viridis using a recently established transformation system [Laussermair & Oesterhelt (1992) EMBO J. 11, 777-783]. They were shown biochemically to synthesize all four subunits of the RC (cytochrome, subunits L, M, and H) and to assemble them correctly into the membrane. The structures of two mutants (L 162 F and L 162 T) were determined and found not to differ significantly from the wild-type structure. All mutants grew photosynthetically. The absorption spectrum of all the mutants is the same as in WT, but the redox potential of P and of c-559 was changed by the mutations. The kinetics of electron transfer from the heme group to the special pair were measured in chromatophores by flash absorption. As found earlier in the wild type (Y) several exponential components were needed to fit the data.(ABSTRACT TRUNCATED AT 250 WORDS)

Interprotein electron transfer from cytochromec2to photosynthetic reaction center: Tunneling across an aqueous interface

“…Therefore, the large effect of the mutation of Tyr-L162 to other residues (13,49,50) on the first-order ET rate in RCs of Rhodobacter sphaeroides is probably a consequence of structural changes in the cyt:RC complex, leading to a change in the distance between cofactors. Supporting this hypothesis, this change in rate is not observed upon mutation of Tyr-L162 in Blastochoris viridis, where the cyt is tightly bound and a change in distance between redox centers is not likely to occur (51). The interprotein ET reaction, even with an intervening water layer in the interface, is reasonably fast over the relatively long distances between redox centers.…”

Very fast electron transfer from cytochrome to the bacterial photosynthetic reaction center at low temperature

“…We may wonder why electron transfer becomes blocked at low temperature in the L162Y protein and not (or at least only partly) in L162T. We have shown previously that the reaction center structure is nearly the same for the WT and the L162T mutant, with very limited structural differences restricted to the mutation site [11]. We hypothesize that water molecules involved in redox-associated structural changes are differently located in the mutant compared to the WT.…”

“…We have recently investigated the rate of electron transfer from the proximal heme c-559 to P + not only in WT RC of Rps. viridis [3,4], but also in proteins where tyrosine (Y) 162 of the L subunit of the RC is replaced by other amino acids (T, L, G, W, M or F) [11]. These mutated proteins were prepared in order to investigate whether the L162Y residue is required for rapid electron transfer to P + .…”

Electron Transfer in Natural Proteins Theory and Design

“…viridis reaction centers. Using crystallographic distances, reported redox midpoint potentials and assuming a typical reaction center reorganization energy of 0.7eV for heme to heme electron transfer, the estimates for the individual electron transfer rates from heme 1 to BChl 2 + is 4.1 10 6 s ñ1 compares with 5.4 10 6 s ñ1 measured (Shopes et al, 1987;Dohse et al, 1995). The estimated rate for the multistep, mixed endergonic transfers from the heme 3 to heme 1 rate of 1.8 10 5 s ñ1 compares with 2.8 10 5 s ñ1 (Shopes et al, 1987), while the overall chain rate from heme c 2 to BChl2 of 1.1 10 4 s ñ1 compares with 1.4 10 4 s ñ1 (Ortega et al, 1999, Meyer et al, 1993.…”

“…Nevertheless, there is often a belief that the identified pathway has been naturally selected to guide the electron and that changes to the pathway, for example through mutagenesis, will spoil the electron tunneling rate. In fact, a careful mutagenic crystallographic, electrochemical and kinetic series in the reaction centers shows this is not the case (Dohse et al, 1995). Mutations often have minor effects on tunneling rates that can be understood in terms of small changes in R, ∆G and λ (Page et al, 1999).…”