Maximal stabilities of reversible two-state proteins

The hydrophobic effect is the major force driving protein folding. Around room temperature, small organic solutes and hydrophobic amino acids have low solubilities in water and the hydrophobic effect is the strongest. These facts suggest that globular proteins should be maximally stable around room temperature. While this fundamental paradigm has been expected, it has not actually been shown to hold. Toward this goal, we have collected and analyzed experimental thermodynamic data for 31 proteins that show reversible two-state folding ⇌ unfolding transitions at or near neutral pH. Twenty-six of these are unique, and 20 of the 26 are maximally stable around room temperature irrespective of their structural properties, the melting temperature, or the living temperatures of their source organisms. Their average temperature of maximal stability is 293 ± 8 K (20 ± 8 °C). These proteins differ in size, fold, and number of domains, hydrophobic folding units, and oligomeric states. They derive from the cold-loving psychrophiles, from mesophiles, and from thermophiles. Analysis of the single-domain proteins present in this set shows that the variations in their thermodynamic parameters are correlated in a way which may explain the adaptation of the proteins to the living temperatures of the organisms from which they derive. The average energetic contribution of the individual amino acids toward protein stability decreases with an increase in protein size, suggesting that there may be an upper limit for protein maximal thermodynamic stability. For the remaining proteins, deviation of the maximal stability temperatures from room temperature may be due to greater uncertainties in their heat capacity change (ΔC_p) values, a weaker hydrophobic effect, and/or a stronger electrostatic contribution.