Lessons in stability from thermophilic proteins

Abstract

Studies that compare proteins from thermophilic and mesophilic organisms can provide insights into ability of thermophiles to function at their high habitat temperatures and may provide clues that enable us to better define the forces that stabilize all proteins. Most of the comparative studies have focused on thermal stability and show, as expected, that thermophilic proteins have higher Tm values than their mesophilic counterparts. Although these comparisons are useful, more detailed thermodynamic analyses are required to reach a more complete understanding of the mechanisms thermophilic protein employ to remain folded over a wider range of temperatures. This complete thermodynamic description allows one to generate a stability curve for a protein that defines how the conformational stability (DeltaG) varies with temperature. Here we compare stability curves for many pairs of homologous proteins from thermophilic and mesophilc organisms. Of the basic methods that can be employed to achieve enhanced thermostability, we find that most thermophilic proteins use the simple method that raises the DeltaG at all temperatures as the principal way to increase their Tm. We discuss and compare this thermodynamic method with the possible alternatives. In addition we propose ways that structural alterations and changes to the amino acid sequences might give rise to varied methods used to obtain thermostability.

Figure 1.

A stability curve for a hypothetical protein (Becktel and Schellman 1987). The stability of a protein is plotted as a function of temperature, and the data can be explained by a modified version of the Gibbs Helmholtz equation (Equation 1). Some key thermodynamic parameters are also marked on the plot. (For an explanation of the terms used, refer to the text.)

Figure 2.

Stability curves showing different methods to achieve a higher T_m. Starting with a stability curve for a hypothetical mesophilic protein (solid line), the protein may increase T_m by shifting the curve up (method I [diamonds]), by making the curve flatter (method II [circles]), or by shifting the curve to the right (method III [squares]). The thermodynamic bases and explanations for all these methods are discussed in the text.