Elastic force hydrophobic association

Figure 5.8. Hydrophobic association to produce increased elastic force at fixed length, as experimentally shown in Figure 5.7. (Top) A series of clamshaped globular proteins, strung together by an elastic band attached near the mouth, are at equilibrium between open and closed states due to hydrophobic association. As the distribution shifts toward more closed states, the elastic force increases. (Bottom) A P-spiral with an equilibrium between...

A more complete technical description of the consilient mechanisms of concern to biological energy conversion encompasses two distinct but interlinked physical processes of hydrophobic association and of elastic force development. The latter results from a generally applicable... 

Contractility as the Coupling of Hydrophobic Association and Elastic Force Development... 

The Coupling of Hydrophobic Association with Development of Elastic Force... 

Complex III is an example of the consilient mechanism for elasticity that includes the coupling of hydrophobic association with development of an elastic force. In particular, the Rieske iron protein (RIP) of Complex III resides on the cytoplasmic side and contains a long hydrophobic a-helix that passes through the lipid bilayer from the cytoplasmic side to emerge on the matrix side with charged residues that combine to anchor the iron protein to the membrane. On the cytoplasmic side, a sequence of about 15 residues that is continuous with the transmembrane anchor... 

Thus, the challenge to the consilient mechanisms posed by the electron transport chain transforms into a showcase example that includes the two distinct but interlinked physical processes of the development of entropic elastic force during hydrophobic association to bring both consilient mechanisms to bear. 

The hydrophobic and elastic consilient mechanisms, two distinct but interlinked physical processes of hydrophobic association/dissocia-tion and elastic force development/relaxation, couple to achieve movement. In functional... 

The focus has been on both aspects of the consilient mechanism (hydrophobic association/ dissociation and elastic force development/ relaxation) involved in the unique domain movement for electron transfer within Complex III. In what follows, the same aspect of hydrophobic association/dissociation of the consilient mechanism is proposed for facilitating proton gating. 

Even so, crystal structures provide the best snapshots of forces in action. Crystal structures provide an unparalleled opportunity to assess relevance to the major protein-based machines of biology of the free energy transduction so dominantly displayed by elastic-contractile model proteins (as developed in Chapter 5). If the apolar-polar repulsive free energy of hydration, AG.p, the operative component of the Gibbs free energy of hydrophobic association, AGha> is active in ATP synthase, then it should become apparent in these snapshots. 

Hypothesis Efficient Production of Motion by Muscle Contraction Derives from the Hydrophobic and Elastic Consilient Mechanisms, Whereby Dephosphorylation Results in Hydrophobic Association Coupled to Near-ideal Elastic Force Development... 

S Evidence for Elastic Force Development Resulting from Hydrophobic Association... 

The energy conversions that produce motion in living organisms consist of two distinct but interlinked physical processes of hydrophobic association and elastic force development, collectively referred to as consilient mechanisms in that they each provide a common groundwork of explanation. The association of oil-like domains, hydrophobic association, has been characterized in terms of the comprehensive hydrophobic effect (CHE), and elastic force development has been described in terms of the damping of internal chain dynamics on deformation, whether deformation occurs by extension, compression or solvent-mediated repulsion (see section E.4.1.2 and Figures E.3 and E.4, below). 

Elastic forces come into play as hydrophobic associations stretch interconnecting chain segments. Only if the elastic deformation is ideal does all of the energy of deformation become recovered on relaxation. To the extent that hysteresis occurs in the elastic deformation/ relaxation, energy is lost and the protein-based machine loses efficiency. Thus, the elastic consilient mechanism, whereby the force-extension curve can be found to overlay the force-relaxation curve becomes the efficient mechanical coupler within the vital force. The objective now becomes one of understanding the age-old problem of a reluctance to discard past idols. 

Expectation 4 That the hydrophobic association of the powerstroke extends kinetically free chain segments to produce an elastic force. In order that there be a smooth and efficient conversion of energy from chemical to mechanical by the act of hydrophobic association, the energy must be temporarily stored in an elastic deformation with limited hysteresis. This occurs as hydrophobic asso-... 

As noted by Ra5mient and coworkers, a number of flexible loops occur in the crossbridge, and we suggest that these are potential sources for elastic force development that results from hydrophobic association of the powerstroke (see Chapter 8, section 8.5.4.5). 

Two basic components comprise the nanosensor of interest here (1) the atomic force microscope (AFM) designed to measme force as a function of extension or further modified to assess frequency dependence of loss shear modulus under isometric conditions and (2) the sensing element of elastic protein-based polymer containing a site or sites of interaction wherein the interaction changes the state of hydrophobic association by means of the apolar-polar repulsive free energy of hydration, AGap. 

Description of the mechanics of elastin requires the understanding of two interlinked but distinct physical processes the development of entropic elastic force and the occurrence of hydrophobic association. Elementary statistical-mechanical analysis of AFM single-chain force-extension data of elastin model molecules identifies damping of internal chain dynamics on extension as a fundamental source of entropic elastic force and eliminates the requirement of random chain networks. For elastin and its models, this simple analysis is substantiated experimentally by the observation of mechanical resonances in the dielectric relaxation and acoustic absorption spectra, and theoretically by the dependence of entropy on frequency of torsion-angle oscillations, and by classical molecular-mechanics and dynamics calculations of relaxed and extended states of the P-spiral description of the elastin repeat, (GVGVP) . The role of hydrophobic hydration in the mechanics of elastin becomes apparent under conditions of isometric contraction. 

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