Study of protein-bacteriochlorophyll and protein-lipid interactions of natural and model light-harvesting complex 2 in purple bacterium Rhodobacter sphaeroides [thesis]

Lee Gyan Kwa
The natural design of the photosystems of plants and photosynthetic bacteria using chlorophylls (Chls) or bacteriochlorophylls (BChls) as photoreceptors are robust. The basic principles of the biological system of light-harvesting complex 2 (LH2) are studied with the use of natural and model sequences expressed in vivo in modified Rhodobacter (Rb) sphaeroides strains. Three aspects have been explored in the thesis: (1) BChl's macrocycle-protein interactions, (2) BChl's phytol-protein
more » ... -protein interactions underlying the structural and functional assembly of the pigment-protein complexes, and (3) LH2-lipid interactions and the role of these interactions in photosynthetic membrane morphogenesis. BChls' macrocycle-protein interactions: Residues at the immediate BChl-B850/protein interface are found to have little effect on specifying the BChl-B850 array, and their light-harvesting activity in LH2. Nevertheless, these residues are important for the structural thermal stability. With the use of 'rescue' mutagenesis of the model BChl binding site, the hydrogen-bond between αSer -4 and the C131 keto carbonyl group of βBChl-B850 is shown to be a crucial motif for driving the assembly of model LH2 complex. Possibilities for residue modifications are limited in the β-subunits as compared to the α-subunits, which suggests that the two polypeptides have distinct roles in complex assembly. In the β-subunits, there are residues detected adjacent to the BChl-B850 site which are critical for the assembly of LH2. BChls' phytol-protein interactions: Mutagenesis of residues closely interacting with the BChl-B850 phytol moiety result in the pronounced loss of BChl-B800 from LH2. Dephytylation of bound BChls within assembled LH2 to BChlides also resulted in the loss of BChl-B800 and destabilisation of LH2 structural assembly. Thus, the phytol chains were shown to be important for optimal pigment binding, especially for BChl-B800; which appears to be highly sensitive to the proper packing of the phytols. The pattern of phytol interactions with [...]
doi:10.5282/edoc.6969 fatcat:uhwbxrohynd45o3jcqg5dlbmcy