Effect of the B ring and the C-7 substituent on the kinetics of colchicinoid-tubulin associations
| Autor: | Erica A. Pyles, Susan Bane Hastie |
|---|---|
| Rok vydání: | 1993 |
| Předmět: |
Steric effects
biology Stereochemistry Chemistry Enthalpy Substituent macromolecular substances Biochemistry Dissociation (chemistry) Transition state chemistry.chemical_compound Kinetics Tubulin Reaction rate constant biology.protein Electrochemistry Animals Thermodynamics Cattle Colchicine Equilibrium constant Protein Binding |
| Zdroj: | Biochemistry. 32(9) |
| ISSN: | 0006-2960 |
| Popis: | The kinetics of four B-ring derivatives of colchicine binding to tubulin has been examined quantitatively. The bindings of deacetamidocolchicine, deacetylcolchicine, demecolcine, and N-methyl- demecolcine to tubulin were biphasic processes. The association rate constants were determined as a function of temperature, and the thermodynamic parameters for the transition states of the fast phase were calculated. The kinetic parameters for the formation of the deacetylcolchicine-tubulin, demecolcine-tubulin, and N-methyldemecolcine-tubulin complexes were very similar to each other, but different from the parameters for the colchicine-tubulin association. In particular, the global activation enthalpies for the formation of the three aminocolchicinoid-tubulin complexes were 3-5 kcal/mol greater than the global activation enthalpy of colchicine binding to tubulin. These results indicate that electronic rather than steric properties of the B-ring substituent are of greater importance in the activation enthalpy of colchicinoids binding to tubulin. In contrast, the global activation enthalpy for deacetamidocolchicine, which lacks a substituent on the C-7 carbon, binding to tubulin was virtually identical to the global activation enthalpy previously found for the colchicine analog that lacks the B ring, 2-methoxy-5-(2,3,4-trimethoxyphenyl)tropone, binding to tubulin (Bane, S., Puett, D., Macdonald, T. L., & Williams, R. C., Jr. (1984) J. Biol. Chem. 259, 7391-7398). This result demonstrates that the carbons of the B ring are not involved in the transition state for the formation of colchicinoid-tubulin complexes. The first-order dissociation rate constants of the colchicinoid- tubulin complexes were determined at 37 O C. The dissociation profiles of the colchicinoid-tubulin complexes also consisted of two phases. The equilibrium constants for the two kinetic phases were determined from the rate constants for each ligand at 37 OC. The equilibrium constants for each kinetic phase of the aminocolchicinoid-tubulin complexes were indistinguishable within experimental error. The equilibrium constants for the two kinetic phases of the deacetamidocolchicine-tubulin complex differed by up to a factor of 4 and were in reasonably good agreement with the equilibrium constant previously found for the association by equilibrium methods (Choudhury, G. G., Banerjee, A,, Bhattacharyya, B., & Biswas, B. B. (1983) FEBS Lett. 161, 55-59). It is concluded that tubulin heterogeneity with respect to colchicinoid binding may be detected by kinetic means but is unlikely to be observed in unfractionated tubulin using equilibrium methods. |
| Databáze: | OpenAIRE |
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