Model proteins inverse temperature

Quite the inverse occurs for water-dissolved protein of interest here that is, by the consilient mechanism, heating from below to above the folding transition increases the order of the model protein. Because heating increases protein order, the transition is called an inverse temperature transition. 

Figure 2.7. These crystals of cyclo(GVGVAPGVG-VAP) form when the temperature of aqueous solutions is raised and dissolve when the temperature is lowered. This finding represents an unambiguous demonstration that the model protein component of the aqueous solution becomes more ordered on higher temperature and is one of the reasons that the transition is called an inverse temperature transition. (Adapted with permission from Urry et al. )...

The above equations for photosynthesis and respiration, exactly balanced with respect to CO2, H2O, [C(H20)]6, and O2, mask an extraordinarily intricate set of reactions where balance tends to be masked by blurring detail. The objective here, however, is to dissect out sufficient detail to expose the primary energyconverting steps common to both processes and to demonstrate that model proteins, utilizing inverse temperature transitions, emulate key elements of those energy-converting steps. ... 

What can be demonstrated with model proteins functioning as contractile molecular machines is that two of the most effective means of lowering the temperature of an inverse temperatine transition to drive contraction are positively charged calcium ions (Ca ) binding at paired negatively charged carboxylates (COO ) to decrease net charge... 

The essential aspect of the capacity of the inverse temperature transition to achieve diverse energy conversions resides within large chain molecules, which were just becoming known when the first edition of Schrodinger s book appeared. As we have sketched above, the functional properties of the model protein-... 

Figure 2.18. Energies are shown that can be inter-converted by means of elastic-contractile model proteins capable of exhibiting inverse temperature transitions functioning by means of the competition for hydration between oil-like and charged groups called an apolar-polar repulsive free energy of hydration. See Chapter 5 for a more complete development of the phenomenology and physical basis and Chapter 8 for details of the molecular process.

A smart plastic would harmlessly disintegrate once its useful Life were completed. Plastics made of plastic-contractile model proteins with controllable inverse temperature transitions can be designed as smart plastics. A smart protein-based plastic, having fulfilled its role, would swell and become a fragile, edible gelatin-like substance. Rather than foretell death for the fishes, a smart protein-based plastic could provide food for the fishes, once its useful Life as a plastic were complete. 

D-amino acid residue on the right (an optical isomer that does occur in biology, but in those peptides not encoded for by the genetic code). C The effect of insertion of a D-amino acid residue in an otherwise L-amino acid residue protein in the P-spiral structure of the elastic-contractile model protein of our focus would be to disrupt the regular structure. This is difficult to avoid completely in chemical synthesis, and it increases the temperature at which occurs the inverse temperature transition and decreases the heat of the transition due to less optimal association of oil-like groupings. 

Figure 5.2. The four phase transitions of the model protein (GVGVP)2si in water over the temperature range from -20 to 120°C. The familiar transition of the melting of ice and the vaporization of water are shown with the relative magnitudes of the heats of these transitions to those of protein heat denaturation and to the innocuous looking inverse temperature transition near 30°C that we believe to be the basis of the function of protein-based machines of Life. See text for discussion.

Despite the absorption of heat for the transition and the overall increase in entropy of -(-4.0 EU for the water plus protein, the protein component actually increases in order on raising the temperature. As unambiguously demonstrated by crystallization of a cyclic analog (see Figure 2.7), in this case the protein component of the water plus protein system becomes more ordered as the temperature is raised. For this and additional reasons, noted below in section 5.1.3, we call this transition exhibited by our model protein, poly (GVGVP), an inverse temperature transition. 

As demonstrated in Figure 5.3 for several model proteins, essentially unlimited solubility occurs at low temperature, and phase separation (insolubility) occurs as the temperature is raised. Also, for our model protein composi-tions, - the curvature of the coexistence line is inverted, having the shape of a valley instead of a smooth mountain peak. Because of this we call the phase transition, exhibited by elastic-contractile model proteins, an inverse temperature transition. Even more compelling reasons exist for the inverse temperature transition label. 

A vital property of these model proteins is that they are more ordered above the transition temperature defined by the binodal or coexistence line in Figure 5.3. The polymer component of this water-polypeptide system becomes more ordered or structured on increased temperature from below to above the transition. This behavior is the inverse of that observed for most systems, as discussed above. In particular, we developed the term inverse temperature transition when the precursor protein and chemical fragmentation products of the mammalian elastic fiber changed from a dissolved state, and therefore when molecules were randomly dispersed in solution, to a state of parallel-aligned twisted filaments as the temperature was raised from below to above the phase transition. - ... 

Figure 5.3. Phase diagram for several elastic-contractile model proteins, showing an inverted curvature to the binodal or coexistence line (when compared with petroleum-based polymers) that is equivalent to the T,-divide, with the value of T, determined as noted in Figure 5.IB. Solubility is also inverted with insolubility above and solubility below the binodal line, that is, solubility is lost on raising the temperature whereas solubility is achieved by raising the temperature of most petroleum-based polymers in their solvents. Note that addition of a CHj group lowers the T,-divide and removal of the CH2 group raises the T,-divide. For these and the additional reason of increased ordering on increasing the temperature, the phase transitions of elastic-contractile model proteins are called inverse temperature transitions. (The curve for poly[GVGVP] is adapted with permission from Manno et al. and Sciortino et al. ).

Figure 5.5. Transitions, plotted as independent variable versus dependent variable, showing a response limited to a partieular range of independent variable. (A) Representation of the thermally driven contraction for an elastic-contractile model protein, such as the cross-linked poly(GVGVP), plotted as the percent contraction (dependent variable) versus temperature (independent variable). The plot shows a poorly responsive range below the onset of the transition, the temperature interval of the inverse temperature transition for hydrophobic association, and another poorly responsive region above the tem-...

As mentioned above in reference to Figure 5.5A, as the temperature is raised, contraction of a band composed of elastic-contractile model protein occurs. Contraction occurs as the temperature is raised through a temperature interval. Crossing over the T,-divide, defined in Figure 5.3, is to pass through the temperature interval over which contraction occurs it is the result of the phase separation, specifically of the inverse temperature transition. Furthermore, the temperature interval for contraction occurs at a lower temperature when the model protein is more hydrophobic and at a higher temperature when the model protein is less hydrophobic. 

More oil-like R-groups in our model protein studies resulted in lower temperatures for the onset of the inverse temperature transition of hydrophobic folding and assembly (see section 5.3.2). We argued that more oil-like R-groups in... 

Several points require consideration on identification of AH,(CH2) - T,(GVGIP)AS,(CH2) as -AGha(CH2). The points include the separability assumption of Equations (5.3) and (5.4), the relevance of the model protein to such identification, and the choice of reference state in order that the nonlinearity of hydrophobic-induced pKa shifts be included. From the data of Butler, the separability is reasonable for a simple CH group, but examination of the calculated result is required to be satisfied whether or not extension to more complex substituents is warranted. As the inverse temperature transition of (GVGVP) has been experimentally shown to involve no Raman detectable changes in secondary structure, the elastic-contractile model proteins of focus here reasonably represent the best known model available for such an effort. It should be noted, however, that NMR studies on the temperature and solvent dependence of peptide NH and... 

The hydrophobically associated state represents the insolubility side of the T,-(solubility/insolubility)divide. As discussed in Chapter 5, contraction of the model elastic-contractile model proteins capable of inverse temperature transitions arises due to hydrophobic association. Hydrophobic association occurs, most fundamentally, on raising the temperature, on adding acid (H" ) to protonate and neutralize carboxylates (-COO ), and on adding calcium ion to bind to and neutralize carboxylates. Most dramatically, hydrophobic association occurs on dephosphorylation of (i.e., phosphate release from) protein, and it commonly occurs with formation of ion pairs or salt bridges between associated hydrophobic domains. 

The prion protein, therefore, exhibits a phase transition from a monomeric protein to an aggregated protein that bears analogy in fundamental respects to the phase transition exhibited by the model proteins discussed in Chapter 5. For the phase transition of our model proteins, association is relatively fast and dissociation or dissolution is quite slow. The difference with the prion phase transition is only qualitative in that the association step of the phase transition of prion protein is extremely slow, and dissociation is so slow as to be irreversible. The slow relentless growth of insoluble prion protein fibers continues until it destroys the cell or tissue with which it may be associated. This calls to mind sickle cell anemia due to an inverse temperature transition of hemoglobin S wherein intracellular fiber growth distorts and sickles red blood cells and leads to their destruction (see Figure 7.22). 

Differential scanning calorimetry data of the inverse temperature transition for hydrophobic association of a series of model proteins, reported in Chapter 5, allowed calculation of... 

Early in our studies it was expected that the post-translational modification of proline hydroxylation, so important to proper collagen structure and function, would raise the value of the temperature, T, for the onset of the inverse temperature transition for models of elastin. Accordingly, hydroxyproline (Hyp) was incorporated by chemical synthesis into the basic repeating sequence to give the protein-based polymers poly[fvs,i(Val-Pro-Gly-Val-Gly), fHyp( al-Hyp-Gly-Val-Gly)], where f sl -i- fnyp = 1 and values of fnyp were 0, 0.01, and 0.1. The effect of prolyl hydroxylation is shown in Figure 7.49. Replacement of proline by hydroxyproline markedly raises the temperature for hydrophobic association. Prolyl hydroxylation moves the movable cusp of... 

Jones 6-12 potential or the Buckingham potential functions. It will be interesting, in future work, to determine the relative magnitude of the endothermic and exothermic components for each of the amino acid residues and for other biologically relevant chemical modifications, as they contribute as guest residues to the inverse temperature transition of (GVGVP) and of other informative host model proteins. 

---