Evaluation of Structure−Function Relationships in the Core Light-Harvesting Complex of Photosynthetic Bacteria by Reconstitution with Mutant Polypeptides†

Seven mutant LH1 polypeptides of Rhodobactor sphaeroides have been isolated, and their behaviors in in vitro reconstitution of LH1 and its subunit complex have been characterized. Two mutants were selected to address the increased stability of the subunit complex of Rb. sphaeroides compared with that of Rhodobacter capsulatus. We found that this difference can be largely ascribed to the existence of Tyr at position +4 in the β-polypeptide (the numbering system used assigns position 0 to the His which provides the coordinating ligand to bacteriochlorophyll) of the former bacterium compared to Met in that position in the latter. The amount of energy involved in the increased interaction was 1.6 kcal/mol, which would be consistent with a hydrogen bond involving Tyr. Mutation of the His at position 0 to Asn allows an estimate of the binding energy for subunit formation contributed by coordination of the imidazole group of His to the Mg atom of bacteriochlorophyll of >4.5 kcal/mol per BChl. Finally, an evaluat...