Protein Stability Depends Critically on the Surface Hydrogen-Bonding Network: A Case Study of Bid Protein.

Understanding how proteins retain structural stability is not only of fundamental importance in biophysics but also critical to industrial production of antibodies and vaccines. Protein stability is known to depend mainly on two effects: internal hydrophobicity and H-bonding between the protein surface and solvent. A challenging task is to identify their individual contributions to a protein. Here, we investigate the structural stability of the apoptotic Bid protein in solutions containing various concentrations of guanidinium hydrochloride and urea using a combination of recently developed methods including the QTY (glutamine, threonine, and tyrosine) code and electron spin resonance-based peak-height analysis. We show that when the internal hydrophobicity of Bid is broken down using the QTY code, the surface H-bonding alone is sufficient to retain the structural stability intact. When the surface H-bonding is disrupted, Bid becomes sensitive to the temperature-dependent internal hydrophobicity such that it exhibits a reversible cold unfolding above water's freezing point. Using the combined approach, we show that the free-energy contributions of the two effects can be more reliably obtained. The surface H bonds are more important than the other effect in determining the structural stability of Bid protein.