Genetically modified photosynthetic antenna complexes with blueshifted absorbance bands

Abstract

Light energy for photosynthesis is collected by the antenna system, creating an excited state which migrates energetically 'downhill'. To achieve efficient migration of energy the antenna is populated with a series of pigments absorbing at progressively redshifted wavelengths. This variety in absorbing species in vivo has been created in a biosynthetically economical fashion by modulating the absorbance behaviour of one kind of (bacterio)chlorophyll molecule. This modulation is poorly understood but has been ascribed to pigment-pigment and pigment-protein interactions. We have examined the relationship between aromatic residues in antenna polypeptides and pigment absorption, by studying the effects of site-directed mutagenesis on a bacterial antenna complex. A clear correlation was observed between the absorbance of bacteriochlorophyll a and the presence of two tyrosine residues, alpha Tyr44 and alpha Tyr45, in the alpha subunit of the peripheral light-harvesting complex of Rhodobacter sphaeroides, a purple photosynthetic bacterium that provides a well characterized system for site-specific mutagenesis. By constructing single (alpha Tyr44, alpha Tyr45----PheTyr) and then double (alpha Tyr44, alpha Tyr45----PheLeu) site-specific mutants, the absorbance of bacteriochlorophyll was blueshifted by 11 and 24 nm at 77 K, respectively. The results suggest that there is a close approach of tyrosine residues to bacteriochlorophyll, and that this proximity may promote redshifts in vivo.

α-Helices direct excitation energy flow in the Fenna–Matthews–Olson protein

“…Accordingly, the charged amino acids do not play the dominant role that was attributed to them in earlier studies (15)(16)(17)28), where large parts of the protein charge distribution, in particular, the backbone, were not modeled, and standard protonation states of titratable residues were assumed (15)(16)(17). In qualitative agreement with experimental results on other antenna systems (29), hydrogen bond donors to BChla cause red shifts of the site energies, but in the present system this shift never exceeds Ϸ130 cm Ϫ1 (SI Table 4). …”

Electronic and vibrational spectroscopy of the cytochrome c: Cytochrome c oxidase complexes from bovine and Paracoccus denitrificans

“…Alternatively, Fowler et al (1992) have recently shown that the red shifts associated with some photosynthetic pigments are due, in part, to T-?T interactions between the chromophore and nearby tyrosine residues. When these tyrosines are replaced by site-directed mutagenesis, the lowest energy T-T* transition of the chromophore is blue shifted by as much as 24 nm.…”

Bacteriochlorin‐protein interactions in native B800‐B850, B800 deficient and B800‐Bchlap‐reconstituted complexes from Rhodopseudomonas acidophila, strain 10050

“…The contribution of protein-Bchla interactions to the red shift of the Bchla Q y transition observed in the LH2 complexes has been extensively studied by resonance Raman spectroscopy [6]. In this regard, it has been especially useful in combination with the technique site-directed mutagenesis, which has been used to create LH complexes with altered spectral properties [7,8].…”