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

Abstract

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 p K between intact and deglycosylated G-CSF. The theoretical model revealed, however, that intact G-CSF has almost the same p K 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.