STRUCTURE AND PROPERTIES OF THE BACTERIOCHLOROPHYLL BINDING-SITE IN PERIPHERAL LIGHT-HARVESTING COMPLEXES OF PURPLE BACTERIA

In this paper, we have examined, using FT resonance Raman spectroscopy , the bacteriochlorophyll (BChl) binding sites in the peripheral light -harvesting complexes extracted from a number of purple bacterial stra ins. A comparison of interactions of the BChl molecules with their bin ding sites in these LH2 complexes, together with the primary sequences of the alpha and beta polypeptides, allows three amino acids to be pr oposed to be involved in the hydrogen bonding of the 9-keto carbonyl o f one of the 850-nm-absorbing pair of BChl molecules. Specifically, we show that one keto carbonyl group, which is strongly hydrogen bonded in Rhodobacter sphaeroides LH2, is involved in much weaker interaction s in the LH2 complexes from all the other species studied (i.e., Rhodo bacter capsulatus, Rubrivivax gelatinosus, Rhodopseudomonas palustris, Rhodopseudomonas acidophila, and Rhodopseudomonas cryptolactis). This is correlated with the presence of three polar amino acids in the pri mary sequence of the alpha polypeptide of Rb. sphaeroides which are ab sent in the sequences from all the other bacteria and probably close t o a chromophore. These three residues are a serine at position -4, a t hreonine at position +6, and another serine at position +17 (numbering relative to the conserved histidine, considered as position 0), in th e alpha polypeptide of Rb. sphaeroides. Furthermore, the study of the interactions in natural B800-820 complexes shows that the two 2-acetyl groups of the 820-nm-absorbing BChl molecules are free from hydrogen- bonding interactions. In the light of previous site-selected mutagenes is studies, the lack of such hydrogen bonds seems to be a general phen omenon, associated with the 820-nm absorption of LH2 complexes, and su ggests that hydrogen-bonding interactions have a precise molecular rol e in finely tuning the functional properties of these complexes.