Tuning the thermodynamics of association of transmembrane helices.

Modular photosynthetic LH1 complex is applied as a model system to investigate the thermodynamics of a self-assembling membrane protein and the effects of cosolvents and cofactor (carotenoid) on the process. Native chromophores of LH1, bacteriochlorophyll, and carotenoid are excellent intrinsic spectroscopic reporter molecules. Their presence allows us to follow the association of transmembrane helices of LH1, without the use of any external markers, by electronic absorption/emission and circular dichroism. Furthermore, the assembly correctness can be monitored by the intracomplex energy transfer. Both the cosolvent and carotenoid markedly affect DeltaH degrees and DeltaS degrees associated with the complex formation in detergent, but the driving force of the process remains almost constant due to an efficient enthalpy-entropy compensation in the system. In the absence of cosolvent and cofactor, the energy of interactions between transmembrane helices in LH1 equals -580 kJ/mol. DeltaH degrees drastically increases upon the addition of acetone (-1160 kJ/mol) and carotenoid (-1900 kJ/mol), whereas DeltaS degrees lowers from +1.5 kJ/mol.K to -0.4 kJ/mol.K and to -2.6 kJ/mol.K, respectively. The stabilization of the ensemble by cofactor seems to be due to the pi-pi stacking of aromatic residues of LH1 polypeptides with the carotenoid pi-electron system. The cosolvent, lowering the medium permittivity and thus enhancing helix-helix interactions, has an ordering effect on the system (DeltaS degrees<0). This effect of cosolvent on DeltaH degrees and DeltaS degrees of association of transmembrane helices is relevant for crystallization of membrane proteins, as it explains in thermodynamic terms the action of amphiphiles used for crystallization of membrane proteins in the micellar phase.