Abstract
Purpose :
To model the rapidity with which surface charge and consequent local near-surface voltage patterns change on a phase-separating globular protein, and to compare mean times for pattern change with characteristic times for translational diffusion to a neighboring protein, and for rotational diffusion to substantially different orientations in solution.
Methods :
Our published, calibrated grand-canonical partition function model for the equilibrium charge regulation of bovine gammaB-crystallin indicates that to account for 95% of the coexisting charging patterns, approximately 400 distinct surface charge patterns are needed. Because of detailed balancing at equilibrium, such a model also gives ratios of rate constants describing protonation and deprotonation of each of the charged sites. We combine these ratios with existing literature data on typical rate constants for each type of titratable amino acid side chain and chain termini. By doing so we estimate individual rate constants for the rapidity of transitions between charging patterns connected by single protonation or deprotonation steps.
Results :
The typical transition times between charging patterns range in general from microseconds to milliseconds, and in concentrated solutions can be orders of magnitude longer than typical times for translational diffusion to a neighboring protein (a few nanoseconds to microseconds), or for rotational diffusion through an angle of one radian, which is typically about 10 nanoseconds for this protein.
Conclusions :
The implication of this kinetic model is that individual protein molecules that have a given charging pattern encounter many other protein molecules before their surface voltage patterns have a high likelihood of substantially changing. Because of this relative slowness of surface charge pattern changes, accurate modeling of the degree to which individual types of pairs are in close proximity, as well as how both direct energetic and hydrodynamic interactions affect their close-range mutual motion, can thus be accomplished, to a first approximation, by considering each such surface charge pattern pair independently.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.