Contraction mechanical work performed

Sadi Carnot in the 1820s used caloric theory in developing theories for the heat engine to explain the engine already developed by Watt. Heat engines perform mechanical work by expanding and contracting a piston at two different temperatures. 

Living organisms require a continual input of free energy for three major purposes (1) the performance of mechanical work in muscle contraction and other cellular movements, (2) the active transport of molecules and ions, and (3) the synthesis of macromolecules and other biomolecules from simple precursors. The free energy used in these processes, which maintain an organism in a state that is far from equilibrium, is derived from the environment. 

Skeletal muscle is specialized to perform intermittent mechanical work. As described previously, the energy sources that provide ATP for muscle contraction depend on the degree of muscular activity and the physical status of the individual. During fasting and prolonged starvation, some skeletal muscle protein is degraded to provide amino acids (e.g., alanine) to the liver for gluconeogenesis. 

In a differentiated organism, each tissue must be provided with fuels that it can utilize for its own energy needs to perform its function. For example, muscles need to generate adenosine triphosphate (ATP) for their mechanical work of contraction, and the liver needs ATP for the synthesis of plasma proteins and fatty acids, gluconeogenesis, or for the production of urea for the excretion of nitrogenous compounds. 

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. 

The same process of hydrophobic association, observed in solution and characterized by the phase diagram in Figure 5.3, occurs in the elastic matrix where hydrophobic association displays as a visible contraction. The parallel process of cloudy formation of polymers in solution transforms in the elastic matrix into a contraction capable of performing mechanical work. 

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. 

Accordingly, the values for An are likely to differ by significantly less than a factor of two for the two model proteins. Thus, simply by changing the composition from that of Model Protein ii to that of Model Protein I, the contraction with which to perform mechanical work could possibly occur using one-tenth the amount of the chemical energy and certainly with no more than one-fifth the amount of the chemical energy. (See section 5.9.5.1 for further discussion of the relative efficiencies of Model Proteins I and ii in Table 5.5.)... 

Axiom S At constant temperature, an energy input that changes the temperature interval for thermally driven hydrophobic association in a model protein can drive contraction, that is, oillike folding and assembly, with the performance of mechanical work in other words, the energy input moves the system through the transition zone for contraction due to hydrophobic association. 

Protein-based molecular machines of the first kind directly use the change in free energy of hydrophobic association, AGha, as a contraction for the performance of mechanical work. 

As shown in the hexagonal array in Figure 5.22, five different energy inputs can perform mechanical work by the consilient mechanism. The set of elastic-contractile model proteins capable of direct utilization of hydrophobic association for contraction are called protein-based molecular machines of the first kind. These are enumerated below with brief consideration of the reversibility of these machines. 

Polymers III through VI represent a systematic increase in the number of Val (V) residues replaced by more oil-like Phe (F) residues. Each step increase in oil-like character of the model protein, on going from 0 to 2 to 3 to 4 and to 5 Phe residues for every 30 residues, stepwise increases the affinity of Na for -COO . Each step increase in oil-like character means that less of an increase in salt is required to drive contraction. Polymer VI, when cross-linked into elastic sheets, provides the most efficient molecular machine of the set. This molecular machine requires less chemical energy to produce a given amount of motion, that is, to perform a given amount of mechanical work. 

As was discussed in Chapter 5, for Figure 5.17, addition of electrons to a positively charged redox group increases oil-like character and drives model protein folding, which result in contraction and the performance of mechanical work. The increase in affinity for electrons of the vitamin-like molecule that occurs on replacement of Val by Phe (see Figure 5.20C) makes for a more efficient electron-driven contraction. Thus, a genetic code that would allow easy mutational steps to become more oil-like would, here again, provide for evolution of more efficient protein-based machines. 

The development of force under conditions of fixed length, as in an isometric contraction, involves the elastic deformation of a chain or chains within the protein-based machine. On relaxation, ideal elastic elements return the total energy of deformation to the protein-based machine for the performance of mechanical work. Thus, the approach toward high efficiency for the function of a protein-based linear motor, or even for the RIP domain movement in Complex III, depends on how nearly the extension of an elastomeric chain segment approaches ideal elasticity. 

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