Effects of Hydrogen Bonding to a Bacteriochlorophyll−Bacteriopheophytin Dimer in Reaction Centers from Rhodobacter sphaeroides

Abstract

The properties of the primary electron donor in reaction centers from Rhodobacter sphaeroides have been investigated in mutants containing a bacteriochlorophyll (BChl)--bacteriopheophytin (BPhe) dimer with and without hydrogen bonds to the conjugated carbonyl groups. The heterodimer mutation His M202 to Leu was combined with each of the following mutations: His L168 to Phe, which should remove an existing hydrogen bond to the BChl molecule; Leu L131 to His, which should add a hydrogen bond to the BChl molecule; and Leu M160 to His and Phe M197 to His, each of which should add a hydrogen bond to the BPhe molecule [Rautter, J., Lendzian, F., Schulz, C., Fetsch, A., Kuhn M., Lin, X., Williams, J. C., Allen J. P., & Lubitz, W. (1995) Biochemistry 34, 8130-8143]. Pigment extractions and Fourier transform Raman spectra confirm that all of the mutants contain a heterodimer. The bands in the resonance Raman spectra arising from the BPhe molecule, which is selectively enhanced, exhibit the shifts expected for the addition of a hydrogen bond to the 9-keto and 2-acetyl carbonyl groups. The oxidation--reduction midpoint potential of the donor is increased by approximately 85 mV by the addition of a hydrogen bond to the BChl molecule but is only increased by approximately 15 mV by the addition of a hydrogen bond to the BPhe molecule. An increase in the rate of charge recombination from the primary quinone is correlated with an increase in the midpoint potential. The yield of electron transfer to the primary quinone is 5-fold reduced for the mutants with a hydrogen bond to the BPhe molecule. Room- and low-temperature optical absorption spectra show small differences from the features that are typical for the heterodimer, except that a large increase in absorption is observed around 860-900 nm for the donor Qy band in the mutant that adds a hydrogen bond to the BChl molecule. The changes in the optical spectra and the yield of electron transfer are consistent with a model in which the addition of a hydrogen bond to the BChl molecule increases the energy of an internal charge transfer state while the addition to the BPhe molecule stabilizes this state. The results show that the properties of the heterodimer are different depending on which side is hydrogen-bonded and suggest that the hydrogen bonds alter the energy of the internal charge transfer state in a well-defined manner.

How photosynthetic reaction centers control oxidation power in chlorophyll pairs P680, P700, and P870

“…The corresponding pair in PbRC mutant His(M202)Leu (12) consists of BChla͞bacteriopheophy-tin a (Bpheoa) at P L/M positions and is assumed to be a suitable model system that lacks electronic coupling between P L and P M . To estimate the electronic coupling effect on E m (P870), we calculated E m (P L ) ϭ 639 mV in the PbRC mutant His(M202)Leu (12), in excellent agreement with the measured value of 640 mV (6). In the present study, we calculated the E m for both monomer BChla in P L/M , without considering possible couplings between them (20,21).…”

“…Hence, the effect on E m (P L ) should be small. Thus, as suggested previously (6), E m (P L ) in the PbRC mutant His(M202)Leu seems to refer to E m (P L ) of the WT PbRC, ignoring possible coupling between P L and P M .…”

“…Part of these E m differences were associated with electronic coupling, which is weak between Chla in P D1/D2 but strong between Bchla in P L/M because of mutual overlap of BChla rings I. Indeed, in the PbRC mutant His(M202)Leu, where His-202 that coordinates BChla P M is lost, P M is replaced by bacteriopheophytin a, yielding a larger measured value of E m (P L ) ϭ 640 mV (6). A significant part of this E m difference (140 mV) may be due to absence of strong electronic coupling.…”

Directing electron transfer within Photosystem I by breaking H-bonds in the cofactor branches

“…1). The importance of H-bonds to cofactors in the bacterial RC has been well studied (13). As shown previously in the case of Photosystem II (14), the loss of such a H-bond should shift the Chl's reduction potential to more negative values, thus altering the rate, and possibly the yield, of ET to the targeted ec3 Chl.…”

Spectral properties of LH2 exhibit very similar even when heterologously express LH2 with <i>β</i>-subunit fusion protein in <i>Rhodobacter</i> <i>sphaeroides</i>

“…Dissimilar to the purple non-sulfur bacteria, calcium ions were demonstrated to be involved in the unusual red shift of the LH1 Q y transition of the core complex in Thermochromatium tepidum [20]. Hydrogen bonds were intensively studied in the binding of BChl molecules to RC [21]. On the other hand, Crt molecules exerts an effect on the structure of the BChl binding site in LH2 [22].…”