Elastic model proteins GVGVP

Z.2 The Phase Diagram of the Elastic Model Protein (GVGVP)2si... 

Figure 5.14. Sheet of elastic model protein of y-irradiation cross-linked (GVGVP)25i as demonstrated in Figure 5.13 directly usable in thermally driven contraction and in chemically driven contraction.

The family of dynamic elastic-contractile model proteins that form the basis for the assertions of the central message of this volume do not lend themselves to the precise spatial descriptions of proteins that form crystals. Nonetheless, important structural description is possible for the poly(GVGVP) family. Indeed, the experimental and computational elucidation of... 

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-...

For our model protein, poly(GVGVP), replacement of either of the Gly residues or the Pro residue markedly alters the structure and properties of the resulting protein-based polymer. For example, substitution of the Gly and Pro residues can destroy the favorable elasticity of the polymers. These positions will not be used for the comparisons of interest here. Substitution of the Val preceding the Pro is possible by most amino acid residues, but not all. Fortunately, any one of the 20 naturally occurring amino acid residues can replace the V of GVG without significant change to the basic structure and function. 

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... 

Raising the temperature to drive contraction by hydrophobic association is the fundamental property of the consilient mechanism as demonstrated in Chapter 5 by means of designed elastic-contractile model proteins. Thermal activation of muscle contraction also correlates with contraction by hydrophobic association, but assisted in this case by the thermal instability of phosphoanhydride bonds associated with ATP, which on breakdown most dramatically drive hydrophobic association. In particular, both muscle and cross-linked elastic protein-based polymer, (GVGVP) contract on raising... 

By way of example, consider the two different compositions of model elastic proteins, (GVGVP) and (GVGIP) . These two polypen-tapeptides differ only by a single CH2 moiety per pentamer. In particular, the R-group for V residue, —CH(CH3)2, differs from the R-group of the I residue, —CH(CH3)—CH2—CHj, by the addition of a single CH2 moiety. At the temperature for each of their respective phase transitions where Pha = Phd, we can write,... 

Figure 9.43. Soybean trypsin inhibitor (STI), a protein model for loading and release studies using positively charged elastic protein-based polymers of varied hydrophobicity. (A) Loading curve for the protein-based polymer K/3F (GVGVP GVGVP...

Figure 2. Schematic representation of phase diagrams for several model elastic proteins based on the elastin repeat, (GVGVP) (see text for discussion). Solid curves adapted with permission from Sciortino et al (1990) and (1993).

Figure 5. AFM single-chain force-extension curves of the model elastic proteins based on elastin, Cys-(GVGVP) ,a5i-Cys (A) and Cys-(GVGIP) ,<26o-Cys (B), showing the ideal elasticity due to perfect reversibility of traces 2 and 5 from bottom of A and marked hysteresis in the lower trace of B. See text for discussion. Reproduced with permission from Urry et al. (2002a).

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