Determination of the equilibrium constants of self-associating protein systems. XI. The application of C-(r) in graphical analysis and the enumeration of interacting species in the ultracentrifuge

Average quantities accessible to the analysis of sedimentation equilibrium data for self-association systems

This paper shows that analysis of sedimentation equilibrium data, searching for average molecular weights, gives quantities which are dependent on the total protein concentration of the samples, while knowledge of the molecular weight of the monomeric subunits allows a more meaningful search for the concentrations of the individual polymeric components of the system. From these the various average molecular weights can be construed, and the various dissociation equilibrium constants evaluated. Also, in this paper considerations are proposed on the meaning of nonideality terms in associating systems and possible ways for estimating them. As an example the proposed procedures have been applied to measurements of sedimentation equilibrium in carbonmonoxyhemoglobin.

Graphical analysis of nonideal monomer N-mer, isodesmic, and type II indefinite self-associating systems by equilibrium ultracentrifugation

A graphical procedure is described by which one can obtain in principle the monomer molecular weight, stoichiometry, equilibrium constant, and second virial coefficient of nonideal monomer N-mer, isodesmic, and type II indefinite self-associating systems. In addition, a method is presented for obtaining both the equilibrium constant and the second virial coefficient from the maximum in a plot of apparent molecular weight vs. concentration if the monomer molecular weight and stoichiometry are known. The usefulness and limitations of the methods are discussed, as well as the quality and range of data required for determination of the relevant parameters. The techniques described are applicable to analysis of self-associating systems by osmotic pressure and light scattering, as well as equilibrium ultracentrifugation measurements.

Self-association of beta-lactoglobulin c in acetate buffers

The self-association of beta-lactoglobulin C at pH 4.65 (23 degrees C) in acetate buffer has been studied at various temperatures, 10, 16, 20 and 25 degrees C, by a series of sedimentation equilibrium experiments. Two different buffers were used. Buffer I with an ionic strength of 0.1 consisted of 0.1M acetic acid and 0.1M sodium acetate; buffer II had 0.1M KCl in addiation so that its ionic strength was 0.2. The variation of the apparent weight average molecular weight, Mwa, with the total solute concentration, c, was characteristic of a self-association. In contrast to the behavior of beta-lactoglobulin A in acetate buffer, the association of beta-lactoglobulin C did not proceed beyond dimer. Furthermore, within the experimental error, the self-association of beta-lactoglobulin C was independent of temperature and ionic strength; all experimental data could be put on the same Mwa (or M1/Mwa) vs. c plot! Several models were used to test the self-association, and a monomer--dimer association with K2 = 2.10 X 10(3) dl/g and BM1 =- 1.2 X 10(-2) dl/g seemed to give a good description of the M1/Mwa vs. c curve.