THE CONFORMATIONAL EQUILIBRIUM OF HUMAN GROWTH-HORMONE

The structural stability of recombinant human growth hormone (rhGH) ha s been studied by differential scanning calorimetry, circular dichrois m and by following the tyrosine and histidine chemical shifts in the H -1 NMR spectrum. These studies demonstrate that the folding/unfolding equilibrium of rhGH involves a partially folded dimeric intermediate. The formation of this dimeric intermediate is a reversible process. At acid pH (pH 3) the conformational equilibrium is reversible even at h igh protein concentrations (10 mg/ml). At neutral pH reversibility is observed only at low protein concentrations (<0.5 mg/ml). The free ene rgy of this intermediate conformation is only similar to 3 kcal/mol ap art from the native state indicating that the conformational equilibri um can be effectively modulated by changes in solvent composition or p hysical conditions. According to the spectroscopic and thermodynamic r esults, the formation of the dimeric intermediate occurs without a maj or loss in helical content and is driven by the formation of substanti al hydrophobic contacts between two partially folded molecules. A ther modynamic model that accounts quantitatively for the experimental data has been developed. These studies demonstrate that partially folded c onformations of certain proteins are able to form stoichiometric compl exes, and that the formation of these complexes provide a significant source of stabilizing Gibbs energy for conformational states that, oth erwise, will be characterized by extremely unfavorable free energies. (C) 1998 Academic Press Limited.