Rate constants protein conformation changes

In [31] kinetics of the surface tension decrease was described using the model accounting for diffusion-controlled adsorption of protein molecule and for conformational changes of adsorbed molecule. The model corresponds to one proposed by Serrien [32] and describes diffusion toward a/w surface and subsequent reorientation and other changes in adsorption layer, which usually one gives a sence of conformational changes the adsorbed protein. The model yields the diffusion relaxation time (t) and (kc) - the rate constant of conformational changes. 

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. ... 

There is currently much interest in electron transfer processes in metal complexes and biological material (1-16, 35). Experimental data for electron transfer rates over long distances in proteins are scarce, however, and the semi-metheme-rythrin disproportionation system appears to be a rare authentic example of slow electron transfer over distances of about 2.8 nm. Iron site and conformational changes may also attend this process and the tunneling distances from iron-coordinated histidine edges to similar positions in the adjacent irons may be reduced from the 3.0 nm value. The first-order rate constant is some 5-8 orders of magnitude smaller than those for electron transfer involving some heme proteins for which reaction distances of 1.5-2.0 nm appear established (35). 

Although conformational changes are essential features of proteins, the conformational basis of protein activity is not yet understood at the molecular and atomic levels. It is generally assumed that the mechanism of enzyme-catalyzed reactions would he defined if all the intermediates and transition states between the initial and final stages, as well as the rate constants, could be characterized. But in spite of constant progress in such characterization, most enzymatic mechanisms are not understood in terms of physical organic chemistry and enzyme activity is still regarded as a miracle as compared to classical catalysis. 

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