The conformational equilibrium of human growth hormone

The structural stability of recombinant human growth hormone (rhGH) has been studied by differential scanning calorimetry, circular dichroism and by following the tyrosine and histidine chemical shifts in the 1H 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 high protein concentrations (10 mg/ml). At neutral pH reversibility is observed only at low protein concentrations (<0.5 mg/ml). The free energy of this intermediate conformation is only approximately 3 kcal/mol apart from the native state indicating that the conformational equilibrium can be effectively modulated by changes in solvent composition or physical conditions. According to the spectroscopic and thermodynamic results, the formation of the dimeric intermediate occurs without a major loss in helical content and is driven by the formation of substantial hydrophobic contacts between two partially folded molecules. A thermodynamic model that accounts quantitatively for the experimental data has been developed. These studies demonstrate that partially folded conformations of certain proteins are able to form stoichiometric complexes, and that the formation of these complexes provide a significant source of stabilizing Gibbs energy for conformational states that, otherwise, will be characterized by extremely unfavorable free energies.