Hydrophobic dissociation

RPLC can be used for determining various physiochemical properties such as hydrophobicity, dissociation constant, and complex formation constants... 

Phosphorylation, the covalent attachment of a phosphate to an OH group, has been to date the most effective way to raise the temperature of the T,-divide. Thus, dephosphorylation, removal of phosphate, has been the most effective way to lower the temperature of the T,-divide and thereby the most effective way to drive hydrophobic association and its equivalent of contraction. This is similar to a primary event in muscle contraction (see Chapters 7 and 8). The shift in the T,-divide on binding of ATP can be as great as or greater than simple phosphorylation, depending on the interactions of ATP at the binding site. As discussed in Chapter 8, section 8.5, ATP binding drives hydrophobic dissociation, whereas loss of phosphate drives hydrophobic association both for the attachment to actin and for the power stroke to... 

Basis for the pKa Shift Remaining After Complete Hydrophobic Dissociation... 

Accordingly, there results a residualpKa shift, after complete hydrophobic dissociation, as the result of the sequence-imposed proximity of the charged and hydrophobic side chains. 

Hydrophobically Associated and Hydrophobically Dissociated Two States for Diverse Allostery... 

V in Table 5.5 with 0,2,3,4, and 5 F residues per 30-mer exhibits a systematic nonlinear increase in steepness, that is, in positive cooperativity, and an associated nonlinear increased pKa shift, as plotted in Figure 5.34. The energy required to convert from the COOH state to the COO" state systematically in a supralinear way becomes less and less, as more Phe residues replace Val residues. The energy required to convert from the hydrophobically dissociated state of COO" to the hydrophobically associated (contracted) state of COOH becomes less, as the model protein becomes more hydro-phobic. The elastic-contractile protein-based machine becomes more efficient as it becomes more hydrophobic. The cooperativity of Model Protein iv with a Hill coefficient of 2.6 is similar... 

Relationship of the Consilient Mechanism to Cold Denaturation and to Hydrophobic Dissociation on Being Made More Polar... 

Hydrophobic association on raising the temperature is the most fundamental aspect of the consilient mechanism, arising as it does from the inverse temperature transition. An equivalent statement would be that hydrophobic dissociation on lowering the temperature is fundamental to the consilient mechanism. Historically, this has been called cold denaturation of enzymes. In our view, those protein systems that associate on heating to physiological temperatures in order to achieve a functional state should be considered in terms of the consilient mechanism. 

When concerned with the physiological operating temperature of 37°C, a model protein with its T,(b)-value at 37°C will not have appreciably hydrophobically associated. Any variable that lowers the Tt(b)-value to 25°C, which is the width of the transition zone for (GVGVP)25i, will have essentially completed the transition to the hydrophobically associated state, as depicted in Figure 5.33. The variable will have moved the cusp of insolubility across the transition zone for biology. In particular, the interest is in the variable of the chemical energy per mole required for a AT, just sufficient to traverse the transition zone from hydrophobically dissociated at 37°C to hydrophobically associated at 25°C. 

Figure 7.1. The movable cusp of insolubility represented as the calorimetry curve for the inverse temperature transition of (GVGVP)25, due to hydrophobic association. Hydrophobic association occurs either by raising the temperature from below to above the temperature interval of the transition or by introducing an energy that lowers the cusp of insolubility, that is, lowers the temperature at which the transition occurs. Hydrophobic dissociation occurs either by lowering the temperature from above to below physiological temperature or by moving the cusp to higher temperatures by the introduction of an energy that increases the temperature at which the transition occurs from below to above physiological temperature.

Bulone et report the average number of water molecules that attends the T to R state transition in hemoglobin is 75. Colombo and coworkers independently report the uptake of a similar number of water molecules, 60, on conversion from the hydrophobically associated to the partially dissociated state. At normal concentrations, this addition of water can be identified with the partial hydrophobic dissociation at the interfaces between the paired aP dimers (a p and a P ). As noted above, at low concentrations (less than 3pM heme), the dissociation of the functional units, the paired aP dimers (a P and a P ), is complete on the binding of oxygen. ... 

Whenever an opening fluctuation toward hydrophobic dissociation occurs, progression to dissociation continues only as long as too much hydrophobic hydration does not result. Should... 

Applying what is now the consilient mechanism for hydrophobic association to the elastic fiber, the prediction became that oxidation of the elastic fiber by a natural enzyme with the biological role of producing superoxide and hydrogen peroxide would cause hydrophobic dissociation evidenced by a swelling and a loss of elastic recoil. A xanthine oxidase superoxide... 

Figure 7.50. Effect of enzymatic superoxide generating system on the elastic properties of ligamen-tum nuchae elastin. With time of exposure of purified fibrous elastin to Oa" and H2O2, the elastic modulus decreases, and larger and larger extensions are required before development of an elastic force occurs. This is the result of swelling due to hydrophobic dissociation. Once the fiber is hydrophobically dissociated, it becomes susceptible to proteolytic degradation. (Reproduced with permission from Urry et al. )...

By the hydrophobic consilient mechanism for the myosin II motor and specifically by means of AG p, ATP binding effects both hydrophobic dissociation from the actin binding site and release of the hydrophobic association at the head of the lever arm, allowing the cross-bridge to move forward toward the next attachment site. Of course, loss of phosphate would reconstitute the hydrophobic associations, that is, would effect hydrophobic re-attachment to the actin binding site in concert with re-association of the head of the lever arm with the amino-termincil domain to result in the powerstroke. Section 8.5.4 presents crystal structure stereo views from which the above-noted perspective derives. 

Binding ATP Energizes a Protein-based Machine by the Mechanical Result of Hydrophobic Dissociation Due to AG,... 

Release of the y-phosphate from protein-bound ATP, leaving bound ADP, restores much hydrophobic association that existed in the protein before ATP binding. Release of the y-phosphate permits reconstitution of sufficient hydrophobic hydration to drive hydrophobic association. In the broad view of the consilient mechanism, as regards ATPases, ATP binding causes hydrophobic dissociation, and P and ADP release re-establishes maximal hydro-phobic association. In terms of the movable cusp of insolubility, binding of the polar ATP molecule raises the temperature of the movable cusp of insolubility to give solubility, and the decrease in polaritys on phosphate release lowers the movable cusp of insolubility to re-establish the insolubility of hydrophobic association... 

Before electron transfer, as fluctuations toward dissociation occur with occupancy by the hydrophobic QH2 molecule, sufficient hydrophobic hydration would form to result in water insolubility such that reassociation results. If, on the other hand, fluctuations toward dissociation occur for QH occupancy, as hydrophobic hydration formed it would be recruited to align for hydration of the positively charged QHi. Thus, increased hydrophobic hydration that ensured hydrophobic reassociation would not occur and hydrophobic dissociation would be the result. Without the favorable decrease in Gibbs free energy for association, the deforming force that caused extension of the tether would be gone, and elastic retraction would result. Additional mechanical details of the resulting domain movement follow below. 

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