Sedimentation coefficients of self-associating species

Sedimentation velocity analysis of heterogeneous protein-protein interactions: sedimentation coefficient distributions c(s) and asymptotic boundary profiles from Gilbert-Jenkins theory

Interacting proteins in rapid association equilibrium exhibit coupled migration under the influence of an external force. In sedimentation, two-component systems can exhibit bimodal boundaries, consisting of the undisturbed sedimentation of a fraction of the population of one component, and the coupled sedimentation of a mixture of both free and complex species in the reaction boundary. For the theoretical limit of diffusion-free sedimentation after infinite time, the shapes of the reaction boundaries and the sedimentation velocity gradients have been predicted by Gilbert and Jenkins. We compare these asymptotic gradients with sedimentation coefficient distributions, c(s), extracted from experimental sedimentation profiles by direct modeling with superpositions of Lamm equation solutions. The overall shapes are qualitatively consistent and the amplitudes and weight-average s-values of the different boundary components are quantitatively in good agreement. We propose that the concentration dependence of the area and weight-average s-value of the c(s) peaks can be modeled by isotherms based on Gilbert-Jenkins theory, providing a robust approach to exploit the bimodal structure of the reaction boundary for the analysis of experimental data. This can significantly improve the estimates for the determination of binding constants and hydrodynamic parameters of the complexes.

Sedimentation coefficients of self-associating species. II. Tests with a simulated example and with beta-lactoglobulin A

If sedimentation equilibrium and sedimentation velocity experiments are performed on a self-associating solute under the same solution conditions, it is possible to evaluate the sedimentation coefficients (si) of the self-associating species and the usual concentration dependence parameter (g or gs). We have tested some of these methods with simulated examples. A more critical test is to use real data. Sedimentation equilibrium experiments with beta-lactoglobulin A at 20 degrees C in 0.2 M glycine buffer (pH 2.46) indicated that a nonideal monomer-dimer association was present. Sedimentation velocity experiments were performed on beta-lactoglobulin A under the same conditions. Using data from both sets of experiments we were able to evaluate s1, s2, g and gs using two different models for swa, the apparent weight average sedimentation coefficient. The empirical model for swa developed by Weirich et al. [1] gave better variance than did the model for swa developed by Gilbert and his co-workers [2-5]. Using a simulated monomer-dimer association mimicking a system having higher sedimentation coefficients than beta-lactoglobulin A did, we were able to show that one could not obtain s2 from tangents to the plot of 1/swa vs. c in the high concentration region. The methods developed here for sedimentation coefficients can be applied to other experiments in which a weight average property (or its apparent value) of a self-associating solute is measured, provided the appropriate thermodynamic experiments are done under the same solution conditions.