Comparison of the structural requirements for bacteriochlorophyll binding in the core light-harvesting complexes of Rhodospirillum rubrum and Rhodospirillum sphaeroides using reconstitution methodology with bacteriochlorophyll analogs.

Bacteriochlorophyll (BChl) structural requirements for formation of the core light-harvesting complex (LH1) and its structural subunit complex were examined by reconstitution with BChl analogs and the alpha- and beta-polypeptides of Rhodospirillum rubrum and Rhodobacter sphaeroides. Comparable results were obtained with most of the BChl analogs and the polypeptides of each bacterium, indicating the conservation of BChl binding sites. These systems showed the following common requirements for formation of the subunit complex and LH1: (1) Mg or a metal of similar size and coordination chemistry (e.g., Zn, Cd, Ni), (2) a bacteriochlorin oxidation state of the macrocyclic ring, (3) a 13(2)-carbomethoxy group, and (4) an intact ring V. Some structural features were not as critically important. For example, the subunit complex and LH1 could be formed with both sets of polypeptides and BChl b, as well as with analogs containing either short (ethanol) or long (phytol) esterifying alcohols. Two derivatives were identified that behave differently with the two sets of polypeptides. The 3-acetyl group is required to form LH1 in both bacteria, although a subunit-type complex was readily formed with [3-vinyl]BChl a and the polypeptides of Rs. rubrum but formed only slightly under special conditions with polypeptides of Rb. sphaeroides. [13(2)-OH]BChl a(p) formed both subunit- and LH1-type complexes with the alpha- and beta-polypeptides of Rb. sphaeroides but not with those of Rs. rubrum. Thus, some subtle differences in the BChl binding sites exist in the LH1 complexes of these two bacteria.