Info

to a dry powder, and then subjecting to different levels of rehydration to demonstrate that the onset of enzyme catalysis occurs at a hydration level of «0.2 g H2O/g dry lysozyme. This level of hydration was said to coincide with the onset of intermolecular motions, a greater conformational freedom of water arrangements at the protein surface, and a transition onset of more weakly interactive regions at the surface of the enzyme. The SH model prediction hB = 0.217 g/g agrees with these observations and, furthermore, predicts water in the zone II between h = 0.217 and 0.677 will form dielectric water clusters (Rahal and Fullerton, 2007) around the bridges, as shown in Fig. 1.2. These DWC water molecules bridge to remaining hydrophilic sites on the protein side chains. As shown in Fig. 1.1 there is small energy difference between polar sites in direct contact (nonhydrated) and those separated by a distance d when hydrated (Rahal and Fullerton, 2007). Because side chains have mobility, they are free to move and touch when dehydrated. When the protein is hydrated above h = 0.217 g/g the charge pairs on the side chains immobilized by strong electrostatic attraction are liberated, as the dielectric coefficient of water is so high that the electrostatic energy of the water-separated pair is nearly equivalent to the dry intimate-contact position. Thus the protein tertiary structure can now oscillate back and forth between two energetically equivalent configurations.

pH

NaCl salt concentration (mM)

Figure 1 .12 Plots of the solute—solvent interaction parameter I value (total encapsulated hydration henc) for bovine serum albumin as a function of pH and salt concentration show that the osmotically unresponsive water can greatly exceed monolayer hydration for a labile protein-like BSA under nonnative conditions. It is worth noting that the henc ~4 in the physiological range near pH 7 and salt concentration 100/200 mM, which implies water content ~80%. Reproduced with permission from Zimmerman et a/.(1995).

NaCl salt concentration (mM)

Figure 1 .12 Plots of the solute—solvent interaction parameter I value (total encapsulated hydration henc) for bovine serum albumin as a function of pH and salt concentration show that the osmotically unresponsive water can greatly exceed monolayer hydration for a labile protein-like BSA under nonnative conditions. It is worth noting that the henc ~4 in the physiological range near pH 7 and salt concentration 100/200 mM, which implies water content ~80%. Reproduced with permission from Zimmerman et a/.(1995).

With less than h = 0.217 g/g, polar amino acid side chains are locked in a dry charge-touching position. Only when water is added can the side chains move away to more distant positions, thereby allowing an internal vibration.

Was this article helpful?

0 0

Post a comment