Dissociation of the peptide/MHC class I complex: pH dependence and effect of endogenous peptides on the activation energy.

Dansylated peptides were used to characterize the dissociation of peptides from a recombinant class I major histocompatibility complex protein. Dissociation of endogenous, low-affinity peptides from the class I molecule Kd had an activation energy of 6.78 +/- 0.64 kcal/mol in the 14 to 26 degrees C temperature range, but there was a break in the Arrhenius plot between 12 and 14 degrees C. Dissociation of a dansylated, high-affinity peptide had an activation energy of 20.24 +/- 1.69 kcal/mol, and there was similarly a break in the plot. Both direct interactions between peptide and the class I heavy chain and indirect effects of the peptide affinity on the extent of light chain association with heavy chain may contribute to the difference in activation energies, while the break in the Arrhenius plots implies a temperature-dependent conformational change. Dissociation was also slowest at neutral pH, but the peptide/class I complex dissociated rapidly at pH greater than 9 and less than 5, suggesting that endocytosed class I proteins would most likely lose their bound peptides at the acidic pH of endosomes.