Pressure effect on inverse temperature transitions: biological implications

Hierarchical and Modulable Hydrophobic Folding and Self-assembly in Elastic Protein-based Polymers: Implications for Signal Transduction

When the hydrophobic (apolar) and polar moieties of elastomeric polypeptides are properly balanced, the polypeptides are soluble in water at lower temperatures but undergo folding and assembly transitions to increased order on raising the temperature. The temperatures, Tt, and heats, ΔHt, of these inverse temperature transitions are determined by differential scanning calorimetry for a series of elastomeric polypentapeptides: poly(VPAVG), poly(IPAVG), poly(VPGVG), poly(IPGVG), poly[0.5(VPGVG),0.5(IPGVG)] and poly[0.82(IPGVG),0.18(IPGEG)] where V = Val, P = Pro, A = Ala, G = Gly, I = lle and E = Glu.

On increasing the hydrophobicity as when replacing V(Val) by I(lle) which is the addition of one CH2 moiety per pentamer, the temperature of the transition is lowered by 15 to 20°C and the heat of the transition is increased by more than one kcal/mole, for the above examples, by more than a factor of two.

When differential scanning calorimetry thermograms are obtained on mixtures of poly(VPAVG) plus poly(IPAVG) or of poly(VPGVG) plus poly(IPGVG), it is found that the polypentapeptides self-separate, i.e., they de-mix, even though in the latter case the conformations have been shown to be essentially identical before and after their respective transitions.

When the polymer, poly[0.82(IPGVG),0.18(IPGEG)], is studied as a function of pH, increasing the degree of ionization is found to increase the temperature and to decrease the heat of the transition such that, with the correct balance of I with the variable E(GluCOO−), the values of Tt and ΔHt can be made to approach those of poly(VPGVG). Acid-base titration studies indicate that less than one Glu(COO−) in 200 residues can raise the value of Tt by 25°C and decrease ΔHt by 90%.

These and additional data are interpreted to mean that there exists an hierarchical hydrophobic folding, that the hierarchical hydrophobic folding can be modulated by changing the degree of ionization or by changes in a number of intensive variables, that changes in these intensive variables can be used to drive folding/unfolding-assembly/disassembly transitions under isothermal conditions, and that these unfolding/folding and disassembly/assembly transitions can be used to achieve signal transduction. This is called the ΔTt mechanism of free energy (signal) transduction.

Reduction-driven polypeptide folding by the delta Tt mechanism

Poly(Gly-Val-Gly-Val-Pro), i.e., poly(GVGVP), exhibits composition and solute dependence of Tt, the temperature of the inverse temperature transition at which hydrophobic folding and assembly occur on raising the temperature. Importantly, a means whereby the value of Tt is lowered from above to below the working temperature becomes an isothermal means of driving folding and assembly, i.e., of achieving free energy transduction. Using poly[0.73(GVGVP),0.27(GK[NMeN]GVP)] where [NMeN] indicates N-methyl nicotinamide attached to the epsilon-NH2 of the Lys(K) residue, chemical and electrochemical reductions are found to remarkably lower the value of Tt; reduction can drive hydrophobic folding and assembly as effectively as decreasing ionization. Changing the redox state of a protein becomes yet another means of achieving free energy transduction by the delta Tt mechanism.

Hydrophobicity scale for proteins based on inverse temperature transitions

In general, proteins fold with hydrophobic residues buried, away from water. Reversible protein folding due to hydrophobic interactions results from inverse temperature transitions where folding occurs on raising the temperature. Because homoiothermic animals constitute an infinite heat reservoir, it is the transition temperature, Tt, not the endothermic heat of the transition, that determines the hydrophobically folded state of polypeptides at body temperature. Reported here is a new hydrophobicity scale based on the values of Tt for each amino acid residue as a guest in a natural repeating peptide sequence, the high polymers of which exhibit reversible inverse temperature transitions. Significantly, a number of ways have been demonstrated for changing Tt such that reversibly lowering Tt from above to below physiological temperature becomes a means of isothermally and reversibly driving hydrophobic folding. Accordingly, controlling Tt becomes a mechanism whereby proteins can be induced to carry out isothermal free energy transduction.

Hydrophobicity-induced pK shifts in elastin protein-based polymers

Three polypentapeptides--poly[0.8(GVGVP), 0.2(GEGVP)], poly[0.8(GVGIP), 0.2(GEGIP)], and poly[0.75(GFGVP), 0.25(GEGVP)]--all analogues of the polypentapeptide of elastin--(Val1-Pro2-Gly3-Val4-Gly5)n or poly(VPGVG)--have been prepared to determine the effect of changing the hydrophobicity, i.e., Val1----Ile1 (I) and Val4----Phe4 (F), on the pKa and the temperature dependence of pKa of the Glu (E) residue. Shifts in pKa as large as 1.7 units are observed and the temperature dependence is much steeper for the structure-dependent proximity of the more hydrophobic Ile1 residues to the Glu4 residue. Even though this system is dominated by the inverse temperature transition of hydrophobically driven folding on raising the temperature, the effect of adding 0.15 N NaCl is to suppress the hydrophobicity-induced pKa shift.