Protein rate processes, hydration

Protein rate processes are strongly affected by hydration. The dry protein shows greatly reduced internal motions, measured by Moss-liauer spectroscopy, neutron scattering, fluorescence spectroscopy, and other methods. Surface motions, monitored by spin probes or spin or Mossbauer labels, are similarly frozen in the dry protein. The following paragraphs comment on the appearance of motion characteristic of the hydrated protein and on the coupling between protein and solvent motions. 

Comments. Dynamic properties show changes at hydration levels above 0.4 h, the point of completion of the changes in static properties. Because the later reflect a monolayer of water about the protein, the additional water seen in the dynamic measurements is "multi-layer" water. Furthermore, hydration affects the several rate properties differently. The more complex hydration dependence of dynamic compared with static properties is to be expected. Static properties, at least the thermodynamic, have a single molecular basis. In contrast, the various transition states governing the rate processes are necessarily different. 

Proteins, lipids and phosphorus are the minor non-carbohydrate components of barley starch. These components have been shown to influence hydration rates, thermal properties, retrogradation characteristics, rhelogical characteristics, susceptibility towards a-amylolysis and processing, and product qualities of starch hydrolysates. 

As noted already, the hydration level above which the protein heat capacity is constant defines completion of the hydration process. The value estimated for lysozyme is 0.38 g of water/g of protein, equivalent to 300 molecules of water/molecule of lysozyme. With regard to other thermodynamic measurements, the sorption Isotherm is not able to define completion of the hydration process, and there can be difficulty in Interpreting scanning calorimetric experiments in terms of completion of hydration, because different states of the system are being compared (frozen and solution, or native and denatured) and during a scanning calorimetric measurement the system is not at equilibrium, allowing reaction rates to influence the response. 

This is also known as photon correlation spectroscopy (PCS) or quasi-elastic light scattering (QELS). It uses scattered light to measure the rate of diffusion of protein particles in a sample. The data on molecular motion are digitally processed to yield a size distribution of particles in the sample, where the size is given by the mean Stokes radius or hydrodynamic radius of the protein particles this is the effective radius of a particle in its hydrated state. Clearly, the hydrodynamic radius depends on both mass and shape. 

The mechanism and the rate of hydrogen-bond breaking in the hydration layer surrounding an aqueous protein have been studied by using a time correlation function technique to understand these aspects in the hydration layer of lysozyme. Water molecules in Ihe layer are found to exhibit three distinct bond-breaking mechanisms compared to bulk reorientation [4]. The reorientation processes are associated wilh the hydrogen-bond breakin switching events (HBSE). Three important characteristics that are common for almost aU of the reorientation processes... 

Stretch activation of muscle is a well-described phenomenon it was the subject of The Croonian Lecture (1977) given by Pringle,and it has been extensively researched and reported in the literature over the ensuing decades. For example, the basic description becomes When active insect flight muscle is stretched, its ATPase rate increases.. . This we take as yet another demonstration of a fundamental process whereby a phosphate present in a protein can be activated, energized, as the result of an increase in hydrophobicity. It is an example of the competition for hydration between apolar and polar species, that is, an example of the apolar-polar repulsive free energy of hydration active in muscle contraction. 

The reduction of ferricytochrome c by at neutral pH appears to be a three-step process. In the first step (A =4.5x 10 lmol- s ) a transient complex is formed between the cytochrome and the hydrated electron, in the second (k= 5 X 10 s ) the haem iron is reduced, and in the third (/ = 1.3 x 10 s ) the protein conformation changes from that appropriate for Fe to that appropriate for Fe. The authors favour a specific pathway for the movement of the electron from the surface of the molecule to the haem iron (step 1). No intermediate complexes were observed in the reduction of ferricytochrome c by the superoxide radical ion. At 20 °C the rate constant for the reaction at pH 4.7—6.7 is 1.4 x 10 1 mol s and as the pH increases above 6.7 the rate constant steadily decreases (eventually reaching zero, indicating that the neutral and high-pH forms if ferricytochrome c are un-reactive). The activation enthalpy is 18 kJ mol and it seems that little protein rearrangement is required for the formation of the activated complex. The kinetics have been reported for the reduction by Cr + of 2-hydroxy-5-nitrobenzyltryptophyl cytochrome c and of iV-formyltryptophyl cytochrome c. ... 

The reduction of ferricytochrome c by hydrated electrons and by several free radicals has been studied by pulse radiolysis. The reduction of oxidized cytochrome c by [Fe(edta)] - follows first-order kinetics for both protein and reductant, with a rate constant of 2.57 x 10 1 mol" s" at pH 7 and activation enthalpy and entropy of 6.0 kcal mol" and —18 cal K" mol", respectively. These values are comparable to those for outer-sphere cytochrome c reductions and redox reactions involving simple iron complexes, and are compatible with outer-sphere attack of [Fe(edta)] " at the exposed haem edge, although the possibility of adjacent attack through the haem pocket is not ruled out. The rate data at pH 9 are consistent with [Fe(edta)] " reduction of two slowly interconverting forms of the protein, native kt = 2.05 X10 1 mol" S" ) and high-pH kt = 2.67 x 10 1 mol" s" ) isomers. A possible route for the transfer of the electron from Cr + to ferricytochrome c has been suggested as a result of the chemical analysis of the chromium(m) product. The reduction by Cr + of the native protein and of ferricytochrome c carboxy-methylated at the haem-linked methionine (residue 80) has been studied kinetically. At pH 6.5 the former process is simple and corresponds to a second-order rate constant of 1.21 x 10 1 mol" s". The latter, however, is complex - two chromium-... 

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