The pH Dependence of Saccharides' Influence on Thermal Denaturation of Two Model Proteins Supports an Excluded Volume Model for Stabilization Generalized to Allow for Intramolecular Electrostatic Interactions

Ilyas Beg,Allen P Minton,Asimul Islam,Md Imtaiyaz Hassan,Faizan Ahmad

doi:10.1074/jbc.m116.757302

Abstract

The reversible thermal denaturation of apo α-lactalbumin (α-LA) and lysozyme was measured in the absence and presence of multiple concentrations of each of seven saccharides (glucose, galactose, fructose, sucrose, trehalose, raffinose, and stachyose) at multiple pH values. It was observed that with increasing pH, the absolute stability of α-LA decreased, whereas the stabilizing effect per mole of all saccharides increased, and that the absolute stability of lysozyme increased, whereas the stabilizing effect per mole of all saccharides decreased. All of the data may be accounted for quantitatively by straightforward electrostatic generalization of a previously introduced coarse-grained model for stabilization of proteins by sugars.

Highlights

The reversible thermal denaturation of apo ␣-lactalbumin (␣-LA) and lysozyme was measured in the absence and presence of multiple concentrations of each of seven saccharides at multiple pH values
This preferential interaction alters the thermodynamic equilibrium between the native (N) and unfolded (U) states (i.e. N 7 U) of the protein by shifting it toward that state with the least exposed surface, i.e. the native state [13,14,15,16,17,18]
We hypothesized that conformational differences between the proteins due to differences in electrostatic interactions between charged residues in the native and unfolded states were the reason for the difference in sugar-mediated stabilization

Summary

Edited by Norma Allewell

The reversible thermal denaturation of apo ␣-lactalbumin (␣-LA) and lysozyme was measured in the absence and presence of multiple concentrations of each of seven saccharides (glucose, galactose, fructose, sucrose, trehalose, raffinose, and stachyose) at multiple pH values. We hypothesized that conformational differences between the proteins due to differences in electrostatic interactions between charged residues in the native and unfolded states were the reason for the difference in sugar-mediated stabilization To test this “electrostatic hypothesis,” we have assessed the pH dependence of reversible thermal denaturation of apo ␣-LA and lysozyme at multiple concentrations of each of the seven saccharides. It was found that the absolute thermal stability of each protein decreased as the pH of the solution moved away from the protein pI, the stabilizing effect of added sugar at that pH increased Both observations may be accounted for by a straightforward generalization of the previously proposed coarse-grained model for stabilization of proteins by sugars [19].

Results

Discussion

Experimental Procedures

Data Analysis