A thermodynamic model for denaturation of granulocyte colony-stimulating factor: O-linked sugar chain suppresses not the triggering deprotonation but the succeeding denaturation.

We have previously reported that the O-linked sugar chain of human granulocyte colony-stimulating factor (G-CSF) protects it against denaturation (Oh-eda et al. (1990), J. Biol. Chem. 265, 11432-11435). Theoretically, the mechanism of denaturation can be argued by supposing an ionized intermediate. At first it was thought from the pH dependence of the thermostability that denaturation was triggered by a deprotonation with a different pK between intact and deglycosylated G-CSF. The theoretical model revealed, however, that intact G-CSF has almost the same pK of 7.4 for deprotonation as deglycosylated G-CSF has, but a 10-fold smaller rate constant for the succeeding denaturation of the ionized intermediate. A sugar chain of human G-CSF, by standing close by Cys-17, may prevent free radicals from attacking the deprotonated sulfhydryl group.