Water-lysozyme systems

Fig. 3. Specific heat of the lysozyme-water system from 0 to 1.0 weight fraction of protein. Least-squares analysis of the linear portion of the heat capacity function from 0 to 0.73 weight fraction of water gives = 1.483 0.009 J K g ...

The process of protein hydration is the stepwise addition of water to dry protein, until the hydration end point is reached. Heat capacity measurements (Yang and Rupley, 1979) serve as a framework on which to develop a picture. Figure 38 gives the dependence on hydration level of various time-average properties of lysozyme, over the hydration range 0-0.4 h, from the dry protein to slightly beyond the end point of the process. Curve d shows the dependence of the apparent specific heat on hydration level. It is directly related to the extent to which the thermal response of the lysozyme-water system deviates from ideal behavior. The nonideality of the system shows three discontinuities at 0.07, 0.25, and 0.38 h. 

Figure 3. Properties of the lysozyme-water system as a function of water content The correspondence between the vertical bar in the figure and the units of measurement is given separately for each curve.

To further probe the role of hydration water in the high-T crossover, we measure the NMR proton spin-lattice relaxation time constant Ti of the lysozyme-water system with h = 0.3 in the interval 275K < T < 355K (Fig. 3b). Figure 3b also shows T for pure bulk water. Note that the hydration water Ti is characterized by two contributions, one coming from the hydration water protons (on the order of seconds, as in bulk water, Tih) and the other from the protein protons (on the order of 10 ms Tip). Figure 3b also shows that, as T increases, the bulk water Ti follows the VFT law across the entire temperature range, but the Tn, exhibits two... 

P.-H. Yang, J. A. Rupley, Protein-water imteractions. Heat capacity of the lysozyme-water system. Biochemistry 18 (1979) 2654-2661. 

Figure. 28 shows the DSC trace from the denaturation of RNase A which is a useful example of the thermodynamic relevance of calorimetric data [156-164] and can serve as a good control case. The same considerations hold for lysozyme which is reported to be 40% homologous with lactalbumin [165], a protein of great interest in foods. Heat capacity studies of the lysozyme - water system [166,167] are also relevant to hydrophilic/hydrophobic effects in food aqueous systems. 

A detailed examination of LN behavior is available [88] for the blocked alanine model, the proteins BPTI and lysozyme, and a large water system, compared to reference Langevin trajectories, in terms of energetic, geometric, and dynamic behavior. The middle timestep in LN can be considered an adjustable quantity (when force splitting is used), whose value does not significantly affect performance but does affect accuracy with respect to the reference trajectories. For example, we have used Atm = 3 fs for the proteins in vacuum, but 1 fs for the water system, where librational motions are rapid. 

Fig. 11. The Speedup of LN at increasing outer timesteps for BPTI (2712 variables), lysozyme (6090 variables), and a large water system (without nonbonded cutoffs 37179 variables). For lysozyme, the CPU distribution among the fast, medium, and slow forces is shown for LN 3, 24, and 48.

Figure 12. (a) The specific heat of the systems lysozyme-water (squares), and DNA-water (triangles), (b) The local tetrahedral order parameter derivative. d2/dT, for lysozyme (squares) and DNA hydration water (triangles), (c) Diffusion constant of lysozyme (squares), and DNA (triangles) hydration water [40]. 

Addition of the protein lysozyme to DC89PC dispersions resulted in the formation of conical tubules in ethanol-water solution.149 The scanning electron micrograph in Figure 5.37 shows that precipitate from this system is primarily composed of these cones, with a smaller number of cylinders. The cones exhibit more pronounced helical ridges on their exteriors than pure lipid cylinders, suggesting that protein selectively associates to the helical defects in a similar manner as the colloidal particles discussed above. 

These values are too short to be influenced significantly by Tr, the rotational correlation time of the enzyme-Gd3+ complex, or Tm, the mean residence time of water molecules in the first coordination sphere of the metal. Moreover, the minima in the plots of Tj p vs. Wj2 indicate that Tc must be dominated by Ts, the electron spin relaxation time. The Ts values for Gd + in this system are longer than most of those determined previously for Gd3+. The electron spin relaxation time for aqueous Gd3+ is (4-7) x 10 10s at 30 MHz (42), while values for Ts of (2-7) x 10 10s have been reported for complexes of Gd3+ with pyruvate kinase (37) and a value of 2.2 x 10- s has been found for a Gd 1"-lysozyme complex (36). Moreover, we have estimated a Tc of 6,8 x 10 10s for Gd + bound to parvalbumin.3 The long Gd3+ correlation times found in the present study are consistent with a poor accessibility of these Gd3+ sites to solvent water molecules. 

This effect of polyhydroxyl compounds may not be quite as simple as it has been described, as the structure of the polyhydroxyl compound may play some part in effective stabilization of enzymes in wet systems. Thus Fujita et al, (20) reported that inositol was more effective than sorbitol in stabilizing lysozyme in aqueous solutions. Both compounds contain six hydroxyl groups, but inositol is cyclic in structure whereas sorbitol is linear, Fig 10. The interaction of polyhydroxyl compounds with water promotes a change in the molecular structure of water. Inositol was reported to have a larger structure-making effect than sorbitol, which accounted for the greater stabilization effect of this compound. 

We have been more concerned with the nature of the water around proteins and peptides. To this end we have investigated the structure and energetics of the solvent, both ordered and disordered around the enzyme lysozyme, in the triclinic crystal[l7d]. In addition to lysozyme, we have characterized the water structure and fluctuations in the crystal of a cyclic hexapeptide, (L-Ala-L-Pro-D-Phe)9[20]. and studied the effect of solvent on the conformation of the dipeptide of alanine[2l] and on the equilibria between extended and helical alanine polypeptides such as those discussed in the previous section[22]. The latter systems simulate aqueous solution conditions rather than crystalline environment. 

As noted, for both the lysozyme—saccharide complex and the purple membrane, the critical point for protonic percoladon is at the onset of function. These observations may apply to other situations, in which a new property emerges suddenly at a critical water content, and may lead to understanding of function in terms of the building up of a statistical network of water-assisted pathways encompassing the system. Statistical... 

Clementi (1985) described ab initio computational chemistry as a global approach to simulations of complex chemical systems, derived directly from theory without recourse to empirical parametrizations. The intent is to break the computation into steps quantum mechanical computations for the elements of the system, construction of two-body potentials for the interactions between them, statistical mechanical simulations using the above potentials, and, finally, the treatment of higher levels of chemical complexity (e.g., dissipative behavior). This program has been followed for analysis of the hydration of DNA. Early work by Clementi et al. (1977) established intermolecular potentials for the interaction of lysozyme with water, given as maps of the energy of interaction of solvent water with the lysozyme surface. 

In an investigation of the role of water in enzymic catalysis. Brooks and Karplus (1989) chose lysozyme for their study. Stochastic boundary molecular dynamics methodology was applied, with which it was possible to focus on a small part of the overall system (i.e., the active site, substrate, and surrounding solvent). It was shown that both structure and dynamics are affected by solvent. These effects are mediated through solvation of polar residues, as well as stabilization of like-charged ion pairs. Conversely, the effects of the protein on solvent dynamics and... 

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