Fine tuning of the spectral properties of LH2 by single amino acid residues

Abstract

The peripheral light-harvesting complex, LH2, of Rhodobacter sphaeroides consists of an assembly of membrane-spanning alpha and beta polypeptides which assemble the photoactive bacteriochlorophyll and carotenoid molecules. In this study we systematically investigated bacteriochlorophyll-protein interactions and their effect on functional bacteriochlorophyll assembly by site-directed mutations of the LH2 alpha-subunit. The amino acid residues, isoleucine at position -1 and serine at position -4 were replaced by 12 and 13 other residues, respectively. All residues replacing isoleucine at position -1 supported the functional assembly of LH2. The replacement of isoleucine by glycine, glutamine or asparagine, however, produced LH2 complex with significantly altered spectral properties in comparison to LH2 WT. As indicated by resonance Raman spectroscopy extensive rearrangement of the bacteriochlorophyll-B850 macrocycle(s) took place in LH2 in which isoleucine -1 was replaced by glycine. The replacement results in disruption of the H-bond between the C3 acetyl groups and the aromatic residues +13/+14 without affecting the H-bond involving the C13(1) keto group. In contrast, nearly all amino acid replacements of serine at position -4 resulted in shifting of the bacteriochlorophyll-B850 red most absorption maximum. Interestingly, the extent of shifting closely correlated with the volume of the residue at position -4. These results illustrate that fine tuning of the spectral properties of the bacteriochlorophyll-B850 molecules depend on their packing with single amino acid residues at distinct positions.

Design principles for chlorophyll‐binding sites in helical proteins

“…This implies contributions of alanines at these positions to the overall stability of the protein fold. Interestingly, although impossible to verify statistically because of sample size, the rare occurrence of glycine in all critical positions and its complete exclusion from position −1 is noteworthy and consistent with our previous experimental data 37, 38. Our experiments revealed that placing glycine at position −1 in natural and model (B)Chl protein complexes significantly destabilized the complex, while other residues showed little effect.…”

“…Interestingly, although impossible to verify statistically because of sample size, the rare occurrence of glycine in all critical positions and its complete exclusion from position −1 is noteworthy, and consistent with our previous experimental data. 37,38 Our experiments revealed that placing glycine at position −1 in natural and model (B)Chl protein complexes significantly destabilized the complex, while other residues showed little effect. One explanation for this glycine-specific effect at the (B)Chl-protein interface may be that the planar structure of the phorphyrin core is devoid of groups that could fill the void created by the absence of any side chain.…”

“…Generally, the occurrence of histidine (as well as other charged and polar residues) in TM proteins is restricted to specific functional motifs. In this context, the absence of histidine from position −4 is noteworthy, although our experimental data showed that histidine may, in principle, be placed at position i − 4 37. Apparently, it hints at a functional/structurally disfavored configuration of two closely spaced (B)Chls on the same helix interface.…”

“…In this context, the absence of histidine from position −4 is noteworthy, although our experimental data showed that histidine may, in principle, be placed at position i -4. 37 Apparently, it hints at a functional/structurally disfavoured configuration of two closely spaced (B)Chls on the same helix interface. Alanine is significantly overrepresented in positions −1 and +3, regardless of interactions with (B)Chls.…”

Ca2+-binding reduces conformational flexibility of RC–LH1 core complex from thermophile Thermochromatium tepidum

“…Ample factors related to the thermostability are discussed, amongst those are (i) optimizing and increasing the packing density by filling cavities in the protein structure (Silber et al 2008;Ishikawa et al 1993) and (ii) increased polar surface in comparison with non-polar surfaces. Comparisons between thermophile and mesophile proteins indicate that the thermophile proteins generally contain more charged residues and an increased number of salt bridges (e.g.…”