Structure of a bacteriochlorophyll a-protein from the green photosynthetic bacterium Prosthecochloris aestuarii

Abstract

The three-dimensional structure of a water-soluble bacteriochlorophyll a-containing protein from the green photosynthetic bacterium Prosthecochloris aestuarii has been determined by X-ray crystallography from a 2.8 Å resolution electron density map based on four isomorphous derivatives. Details of the crystallographic procedures used to obtain the map are presented. The bacteriochlorophyll a-protein is shown to consist of three identical subunits, tightly packed around a 3-fold symmetry axis. Each subunit consists of a core of seven bacteriochlorophyll a molecules enclosed within a “bag” of protein. The polypeptide chain forms an extensive 15-strand β-sheet, which is almost planar in its central region, and twisted at its extremities, and wraps around the chlorophyll core to form an efficient amphipathic layer between the chlorophylls and the aqueous environment. There are extensive contacts between the phytyl chains of the seven bacteriochlorophylls within each subunit. These hydrocarbon chains constitute an inner hydrophobic core of the molecule which may be important in forming the complex. There are also extensive contacts between the protein and both the bacteriochlorophyll head groups and tails, but relatively few contacts between the respective head groups. The seven magnesiums all appear to be five co-ordinated. In five cases the presumed ligand is a histidine side-chain, in one case a polypeptide carbonyl oxygen, and in the other case a water molecule. At low temperature, both the absorption and circular dichroism spectra of the bacteriochlorophyll a-protein show splitting which can be interpreted in general terms as due to exciton interactions between the seven chromophores, but calculations of the expected splitting based on the bacteriochlorophyll co-ordinates determined crystallographically are in poor agreement with the observed spectra. Furthermore, the observed red shift of the Q y absorption band of bacteriochlorophyll a, from about 770 nm in organic solvents to 809 nm in the bacteriochlorophyll a-protein, is not explained by the exciton calculations. It seems likely that the red shift is due to perturbations of the spectra of the individual bacteriochlorophylls by the protein environment, but, pending the determination of the amino acid sequence, it is not possible at this time to define in detail all the proteinchlorophyll interactions. It is suggested that the bacteriochlorophyll a-protein serves as a good model for the organization of chlorophyll in vivo, and that the types of interaction seen here between chlorophyll and protein are likely to be found in other chlorophyll proteins.