Functions of Conserved Tryptophan Residues of the Core Light-Harvesting Complex of Rhodobacter sphaeroides

We have examined mutants in the core light-harvesting complex of Rhodobacter sphaeroides in which the tryptophan residues located at positions alpha+11, beta+6, and beta+9 have been mutated to each of the three other aromatic amino acids, namely tyrosine, phenylalanine, and histidine. We confirm that the alpha+11 residue and show that the beta+9 residue each form a hydrogen bond to a C2-acetyl group of a BChl molecule. Mutation of either of these residues to a phenylalanine results in a breakage of the normal hydrogen bond, whereas a histidine in either of these positions is able to form a hydrogen bond to the BChl. Comparison of the absorption spectra with the hydrogen bonding of the C2-acetyl groups for the various mutants demonstrates a role for this molecular interaction in the tuning of the absorption properties of the complex. We further demonstrate that there is a consistent linear relationship between the downshift in the C2-acetyl stretching mode and the red shift in the absorption maximum, in both core and peripheral antenna complexes. This linear relationship allows us to estimate the contribution of H bonding to the red shifts of these complexes. Though the residue beta+6 is found not to be directly involved in interactions with the pigment molecules, mutation of this residue is shown in some cases to result in both a destabilization of the complex and a decrease in the binding site homogeneity. Finally, a consideration of the amount of antenna complex present in the various mutants shows an important role for the reaction center and/or the pufX gene product in the assembly or stabilization of this membrane protein.