Structure and X-ray amino acid sequence of a bacteriochlorophyll A protein from Prosthecochloris aestuarii refined at 1.9 A resolution

The structure of the water-soluble bacteriochlorophyll a protein (Bchl protein) from the green photosynthetic bacterium Prosthecochloris aestuarii has been refined at 1.9 A resolution to a crystallographic residual of 18.9%. The refinement was carried out without knowledge of the amino acid sequence and has led to an "X-ray sequence". The structure consists of seven Bchl molecules enclosed within a protein "bag" and the refinement supports the general conformation of the molecule described previously. The refinement also supports the previous suggestion that the ligands to the seven Bchl magnesiums are, respectively, five histidines, a carbonyl oxygen from the polypeptide backbone of the protein, and a bound water molecule. The conformations of the seven Bchl head-groups are described in detail. In two cases the magnesium atoms are approximately 0.48 A "below" the plane of the conjugated macrocycle while in the other five cases the atoms are, on average, 0.48 A "above" the plane. The acetyl ring substituents are more-or-less coplanar with the dihydrophorbin macrocycle, consistent with a previous resonance Raman study. The conjugated atoms in each of the seven macrocycles have significant departures from strict planarity. These deviations are similar for Bchls 1, 2 and 3 (class I) and are also similar for Bchls 4, 5, 6 and 7 (class II). Ethylchlorophillide also belongs to class II. The out-of-plane deformations for the class I and class II bacteriochlorophylls appear to correspond to two distinct modes of bending or curvature of the dihydrophorbin macrocycle.