Thermodynamics of membrane polypeptide oligomerization in light-harvesting complexes and associated structural changes.

Investigation of the equilibrium between the dissociated B777 form of the light-harvesting complex of Rhodospirillum rubrum and the oligomeric B820 form demonstrates that the B777 form consists of bacteriochlorophyll a (BChl) bound to the alpha or beta polypeptide chains; this binding appears to be reasonably stable at room temperature with little dissociation to free BChl and polypeptides. Analysis of the reaction order for the B777 association reaction to form B820 shows that this reaction requires four components, presumably two alpha-B777 units and two beta-B777 units, implying that the B820 subunit contains four BChl molecules. Estimations of the enthalpy and entropy changes associated with the tetramerization give values of, respectively, -175 kJ mol-1 and -0.46 kJ and mol-1 K-1. Soret resonance Raman and Fourier transform preresonance Raman spectra of BChl in detergent together with those of the B777, B820 and native B873 forms of the light harvesting complex illustrate significant changes occurring to the environments of the C-2 acetyl groups (Fischer numbering system) during dissociation to form B820 and a loss of order in the C-9 keto environments on formation of B777. Attenuated total reflectance Fourier transform infrared absorption spectra of the three antennae forms demonstrate little perturbation of the approximately 50% alpha-helical secondary structure during dissociation. These observations are discussed in terms of the energetics of membrane protein folding and the structure of the light harvesting complex.