Magnesium-induced self-association of calf brain tubulin. II. Thermodynamics.
The thermodynamic parameters of the magnesium ion induced self-association of calf brain tubulin in pH 7.0, 0.01 M phosphate buffer containing 10(-4) M GTP, were determined from sedimentation velocity experiments. This reaction proceeds by an isodesmic mechanism terminated by the highly favored formation of a closed ring shaped polymer of degree of association 26 +/- 4. Analysis of the variation of the apparent dimerization constant in the isodesmic mechanism s,ows that this self-association is characterized by positive enthalpy, entropy, heat capacity, and molar volume changes, as well as the binding of one additional magnesium ion, which is probably not involved as a bridge between the protein molecules. The addition of the last monomeric subunit has a free energy which is about three times that of dimer formation. Under the conditions of these experiments, tubulin binds 48 +/- 5 magnesium ions with a free energy of --2.8 kcal/mol.
D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000378:metabolism*; D002417:Cattle; D006023:Glycoproteins / Q000378:metabolism*; D046911:Macromolecular Substances; D008274:Magnesium / Q000494:pharmacology*; D008956:Models, Chemical; D011485:Protein Binding; D011487:Protein Conformation; D013816:Thermodynamics; D014404:Tubulin / Q000378:metabolism*
D006023:Glycoproteins; D046911:Macromolecular Substances; D014404:Tubulin; D008274:Magnesium