Contraction model protein

Stepwise increases in oil-like character, as when the mildly oil-like Val (V) residue is replaced by the very oil-like Phe (F) residue, cause the add-base titration curves to be shifted to higher pH values and to be steeper. The energy required to drive the model protein from the phase separated, contracted state to the swollen, relaxed state is proportional to the width of the curve, that is, inversely proportional to the steepness of the curve. Accordingly, the model protein with the steepest curve exhibits the most efficient function for performing the work of lifting a weight. 

Figure 1.7. Shown are the first reported data of the conversion by an elastic-contractile model protein of chemical energy due to an increase in concentration of acid into the mechanical work of contraction. A Length changes at constant force (isotonic contraction) in phosphate-buffered saline. B Force changes at constant length (isometric contraction) in phosphate-buffered saline. (Reproduced from Urry et al. )...

Figure 2.6. In general, the conversion from the extended state to the contracted state shown in Figure 2.5 is graphed here as a systematic family of sigmoid-shaped curves with a common dependence of oil-like character of the elastic-contractile model protein whether the energy input is thermal, chemi-...

The sigmoid-shaped curves of Figure 2.6A represent the shortening of contraction that occurs on raising the temperature through the relevant temperature interval for the particular extent of oil-like character of the model protein. Elastic-contractile model proteins of more oillike composition contract at lower temperatures and over narrower temperature intervals. 

As represented in Figure 2.6C, reduction of a more polar oxidized group attached to the model protein drives contraction. For more oillike model proteins, the affinity for electrons is greater, as in curve a, and reduction requires less electrical energy due to the steeper curve. More oil-like model proteins require less electrical energy to drive contraction. Furthermore, reduction of a group attached to a model protein increases the oil-like character of the model protein, because reduction lowers the temperature at which contraction occurs, as shown in Figure 2.6A. 

The above analysis of the processes of Figure 2.6B,C allows for the simplistic representation shown in Figure 2.9. As depicted in reaction (/) of Figure 2.9, addition of proton, H , to a carboxylate, -COO", makes the model protein more oil-like and allows formation of more structured water around the model protein, and the more oil-like model protein becomes insoluble. The result is contraction due to association of oil-like groups, because the... 

Figure 2.9. A set of reactions is shown, each of which causes the model protein to become more oil-like with the result of a contraction due to association of oil-like groups. See text for further discussion. (Reproduced with permission from Urry." )...

As depicted in reaction ( ) of Figure 2.9, the reaction of OH" with -NH3+ groups of a model protein results in formation of H2O and -NH2 to give a more oil-like model protein. The effect is to form so much oil-like hydration, as shown in the central structure of Figure 2.9, that oil-like solubility is lost with the result of contraction. 

As depicted in reaction (/v) of Figure 2.9, either the neutralization of a negative group (e.g., -COO") of the model protein by a positive ion (e.g., Na or Ca ) from solution or the neutralization of a positive group (e.g., -NHs ) of the model protein by a negative ion (e.g., CT) from solution can cause the model protein to become too oil-like with the result of contraction due to too much oil-like hydration. Contraction occurs as the result of insolubilization of the oil-like model protein and is an example of the lowering of the cusp of insolubility as represented in Figure 1.1. 

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 very most dramatic way to increase the oU-like nature of a model protein is the removal of an attached phosphate. This is demonstrated in Figure 2.12A. Calcium ion binding to a pair of carboxylates is second only to protonation of a carboxylate in driving hydrophobic association (See Figvnes 5.27 and 5.34). Tims, the combination of calcium ion binding to a pair of carboxylates followed by phosphate removal, as occurs in muscle contraction, provides perhaps the most potent means of bringing about hydrophobic association and the associated contraction. It is our view that hydropho-... 

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. 

Now, cross-linking the elastic model protein in the phase-separated state results in elastic bands. Similarly warming the band, swollen at room temperature (just below T,), to body temperature (some 15 degrees above T,) causes the band to contract with the performance of mechanical work. The band pumps iron on raising the temperature from below to above T,. As scientific accounts go, the T, perspective exemplifies simplicity. 

Contraction of elastic matrices occurs as the temperature is raised through the temperature interval of Figure 5.5A this constitutes the input of thermal energy (TE), measurable as the heat of the transition seen in Figure 5.1C. Visible contraction also occurs as any of the independent variables move through the transition zone in Figure 5.5B. Depending on the composition of the model protein that consti-... 

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.

Axiom 2 Heating to raise the temperature from below to above the temperature interval for hydrophobic association of cross-linked elastic model protein chains drives contraction with the performance of mechanical work. 

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