The interactions between cellular retinol-binding protein (CRBP-I) and retinal: A vibrational spectroscopic study

Preresonance Raman difference spectra have been obtained for all-trans retinal in dilute CCl₄ solution, complexed with cellular retinol-binding protein (CRBP-I) and retinol-binding protein (RBP). These spectra indicate that retinal is of a slightly more planar conformation within the binding pocket of CRBP-I than in solution or hydrophobically complexed with RBP. Compared to retinal in solution or bound to RBP, the conformation of the polyene tail of the retinal chromophore is perturbed from C8 through C11. This perturbation is probably due to the close proximity of the Lys40 in the CRBP-I binding pocket to the above-mentioned carbons. The C=O stretching vibration of bound retinal carbonyl has been found to shift from 1664 cm^-1 solubilized in CCl₄ to 1650 and 1645 cm^-1 in RBP and CRBP-I, respectively, and significantly broadened in both cases. The frequency shift and broadening have been attributed to hydrogen bonding. These have been compared to calibrations of frequency shift (ΔνC=O) vs. ΔH and ΔG of all-trans retinal complexed with a series of phenol derivatives of incremental proton-donating ability as obtained by the relationship of van't Hoff. By this relationship, the binding enthalpy of the all-trans retinal carbonyl moiety bound to CRBP-I and RBP is -28.1 kJ/mol (-6.7 kcal/mol) and -23.5 kJ/mol (-5.6 kcal/ mol), respectively. The free energy of binding of the retinal carbonyl bound to CRBP-I and RBP has been determined to be -10.5 kJ/mol (-2.5 kcal/mol) and -7.2 kJ/mol (-1.7 kcal/mol), respectively. The hydrogen-bonded C=O moiety of retinal complexed with CRBP-I accounts for a substantial (25%) but not overriding amount of the binding energy of CRBP-I for retinal, and it also accounts for the protein's preference for binding retinol.