Scanning Calorimetry and Fourier-Transform Infrared Studies into the Thermal Stability of Cleaved Bacteriorhodopsin Systems

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Abstract
Differential scanning calorimetry and Fourier-transform infrared spectroscopy have been used to characterize the thermal stability of bacteriorhodopsin (BR) cleaved within different loops connecting the helical rods. The results are compared to those of the native protein. We show that the denaturation temperature and enthalpy of BR cleaved at peptide bond 71-72 or 155-156 are lower than those of the intact protein, and that these values become even lower for the BR cleaved at both peptide bonds. The effect of cleavage on the denaturation temperature and enthalpy values seems to be additive as has been previously suggested [Khan, T. W., Sturtevant, J. M., & Engelman, D. M. (1992) Biochemistry 31, 8829]. The thermal denaturation of all the samples was irreversible and scan-rate dependent. When cleaved at the 71-72 bond BR follows quantitatively the predictions of the two-state kinetic model at pH 9.5, with an activation energy of 374 kJ/mol, similar to that of native BR. Calorimetry experiments with different populations of intact and cleaved BR provide direct evidence for some intermolecular cooperativity upon denaturation. The denatured samples maintain a large proportion of alpha helices and beta structure, a fact which seems to be related to their low denaturation enthalpy as compared to that of water-soluble, globular proteins.