Incorporation of α-chymotrypsin into the 3D channels of bicontinuous cubic lipid mesophases

The effects of protein entrapment on the structure and phase behavior of periodically curved lipid mesostructures have been examined by synchrotron small-angle X-ray diffraction and FT-IR spectroscopy. The study was directed towards a better understanding of the effect of confinement in a lipid environment on the stability and unfolding behavior of α-chymotrypsin, and, vice versa, the effect of the entrapped protein on the lipid's mesophase structure and temperature- and pressure-dependent phase behavior. We compare the interaction of protein molecules of two different sizes (cytochrome c, 12.4 kDa, and α-chymotrypsin, 25.8 kDa) with the cubic Ia3d phase of monoolein (MO), which forms spontaneously in water. The cubic structure changes significantly when cyt c is incorporated: above a protein concentration of 0.2 wt.%, the interaction between the positively charged protein and the lipid headgroups leads to an increase in interfacial curvature which promotes the formation of a new micellar cubic phase, presumably of crystallographic space group P4332, which the lipid system does not form on its own. The larger α-chymotrypsin leads to a different scenario. On the basis of an examination of the calculated geometric parameters and water volume fractions, it is concluded that the α-chymotrypsin molecules cannot be located exclusively in the water channels of the cubic Ia3d or P4332 phases, but rather form new, less ordered (presumably cubic Pn3m) structures. The new structure disappears above the unfolding temperature of chymotrypsin and exhibits a pressure stability, which – in contrast to cyt c in MO – decreases with increasing chymotrypsin concentration in the system. While the secondary structure of cyt c remains unaffected in the confining lipid environment, the structure of α-chymotrypsin gets destabilized slightly, and the protein tends to aggregate even at relatively low concentrations.