The Contribution of a Covalently Bound Cofactor to the Folding and Thermodynamic Stability of an Integral Membrane Protein

The factors controlling the stability, folding, and dynamics of integral membrane proteins are not fully understood. The high stability of the membrane protein bacteriorhodopsin (bR), an archetypal member of the rhodopsin photoreceptor family, has been ascribed to its covalently bound retinal cofactor. We investigate here the role of this cofactor in the thermodynamic stability and folding kinetics of bR. Multiple spectroscopic probes were used to determine the kinetics and energetics of protein folding in mixed lipid/detergent micelles in the presence and absence of retinal. The presence of retinal increases extrapolated values for the overall unfolding free energy from 6.3 ± 0.4 kcal mol^− 1 to 23.4 ± 1.5 kcal mol^− 1 at zero denaturant, suggesting that the cofactor contributes 17.1 kcal mol^− 1 towards the overall stability of bR. In addition, the cooperativity of equilibrium unfolding curves is markedly reduced in the absence of retinal with overall m-values decreasing from 31.0 ± 2.0 kcal mol^− 1 to 10.9 ± 1.0 kcal mol^− 1, indicating that the folded state of the apoprotein is less compact than the equivalent for the holoprotein. This change in the denaturant response means that the difference in the unfolding free energy at a denaturant concentration midway between the two unfolding curves is only ca 3-6 kcal mol^− 1. Kinetic data show that the decrease in stability upon removal of retinal is associated with an increase in the apparent intrinsic rate constant of unfolding, k_u^H2O, from ~1 × 10^− 16 s^− 1 to ~1 × 10^− 4 s^− 1 at 25 °C. This correlates with a decrease in the unfolding activation energy by 16.3 kcal mol^− 1 in the apoprotein, extrapolated to zero SDS. These results suggest that changes in bR stability induced by retinal binding are mediated solely by changes in the activation barrier for unfolding. The results are consistent with a model in which bR is kinetically stabilized via a very slow rate of unfolding arising from protein-retinal interactions that increase the rigidity and compactness of the polypeptide chain.