Hydrophobically associating. See

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-... 

At the level of the myofibril, the addition of calcium ion, the lowering of pH, and stretching have each been shown to activate muscle contraction as well as to drive contraction of suitably designed elastic contractile protein-based polymers by hydrophobic association (see more extensive discussion in Chapter 7). 

With alkali halide-TBA-W or alkali halide-PD-W systems, the parameters Bne are negative for volumes and heat capacities (see Figures 1-5 and 10). This sign seems to be the one usually observed for the interaction of a hydrophobic with a hydrophilic solute (6). At intermediate cosolvent concentration, AYe°(W — W + TBA) and AYe°(W — W + PD) deviate in the direction we would expect for hydrophobic association the volume increases sharply, and the heat capacity decreases further. Inorganic electrolytes lower the critical micelle concentration of surfactants by salting out the monomers, thus favoring micellization (25) in a similar way, in the co-sphere of a hydrophilic ion, the hydrophobic bonding between the cosolvent molecules may be enhanced. 

In spite of the frequent use of ammonium sulfate, other hydrophobic association promoting salts can be useful at neutral pH. In that way, it is possible to select the degree of lyotropic property adapted to the antibody to purify (see Table 6). 

The X-ray crystal-structure determination revealed a lack of any direct, helix-helix interaction between the a- and P-polypeptides within the hydrophobic core, as had been expected on the basis of earlier models [see Fig. 4 (B) and (C) above]. The association of the fMet-1 residue at the N-terminal of each a-polypeptide to a B800 BChl-a molecule prevents direct interaction between radially paired a-and P-helices. Interactions between the membrane-core helices are mediated only via pigment molecules or buried water molecules. The only polypeptide interaction occurs at the N- and C-terminal tails. In the C-terminal tail the large aromatic residues (a-Trp 40, a-Tyr 44, a-Trp 45 and P-Trp 39) contribute to binding between polypeptides via hydrogen bonds and hydrophobic interactions [see Fig. 5 (C)]. 

Semidilute Viscometrics. Solution viscometrics at concentrations above the overlap concentration (C ) indicated dramatic effects caused by the associative nature of the hydrophobic groups in the polymer. As shown by the reduced viscosity-concentration profiles of Figure 3, the introduction of only 1.0 mol % N-n-octylacrylamide to polyacrylamide can increase the viscosification efficiency dramatically. Increasing the hydrophobe level to 1.25 mol % further increased solution viscosity. At 2000 ppm, the presence of the hydrophobe caused a greater that 10-fold increase in viscosity. This result was in contrast to the behavior of these polymers in dilute solution see the box in Figure 3). The presence of hydrophobic functionality on the polymer resulted in a decrease in the reduced viscosity at concentrations below C. In dilute solution, intramolecular hydrophobic associations decreased the hydrodynamic radii of the polymer coils and thus reduced the... 

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... 

Phosphate attached to a model protein is three to four times more effective on a mole fraction basis than carboxylate in raising the T,-divide for hydrophobic association, which we have shown is due to a decrease in hydrophobic hydration (see Figures 5.25 and 5.27). Dephosphorylation, therefore, would re-establish hydrophobic hydration and dramatically lower the Trdivide, which is to lower the temperature range of the cusp of insolubility to below physiological temperature. The result would be an insolubilization of hydrophobic domains (a hydrophobic association) that we consider to be the power stroke of muscle contraction. 

Figure 7.8. Networks of ion pairs and hydrogen bonds of the switch region (A) and the joint-associated region (B) at the interface between the a P and the o p dimers that comprise the hemoglobin tetramer. These polar interactions result from the hydrophobic association between the dimers that is maximal in deoxyhemoglobin and partially relaxed on oxygenation.The key ion pairs that orient the a P and the a P dimers are indicated by the circled 1 and 2 and are identified as such on the interface in Figure 7.18. See text for discussion. (Adapted with permission from Voet and Voet. )...

Figure 7.9. Mean residue hydropho-bicity plots of the a-chain (A) and the P-chain (B) of hemoglobin and of myoglobin (C). The strong hydrophobidty peaks just above 100 residues are responsible for stable formation of the a P dimer. The strong hydrophobidty peaks near 40 residues may be considered part of the switch region. TTiese hydrophobic residues provide an arc for hydrophobic association at the a P -a P interface. By the consilient mechanism, decreased heme hydropho-bicity on oxygen binding relaxes this inter-dimer hydrophobic assodation and allows the T R transition, as shown in Figure 7.10, to occur. This is further visualized in Figure 7.18 as discussed in the text. See text for further discussion.

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