Glycine betaine may have opposite effects on protein stability at high and low pH values

The compatible osmolyte glycine betaine (GB) is the most efficient osmoprotectant and best excluder from the protein surface. It can reverse protein aggregation and correct mutant protein defects and counter the harmful effects of urea and salts in vivo and in vitro. In this study we have investigated the pH dependence of the stabilizing effect of GB on three different proteins, namely, α-lactalbumin (α-LA), lysozyme and ribonuclease-A (RNase-A). We show here that (a) GB stabilizes RNase-A at all pH values, and (b) GB has opposite effects on two proteins at high pH and low pH values, namely, α-LA and lysozyme. This conclusion was reached by determining Tm (midpoint of denaturation), ΔHm (denaturational enthalpy change at Tm), ΔCp (constant-pressure heat capacity change) and ΔGDo (denaturational Gibbs energy change at 25 °C) of proteins in the presence of different GB concentrations. Another conclusion of this study is that ΔHm and ΔCp are not significantly changed in the presence of GB. This study suggests that other methylated glycine osmolytes may also behave in the same manner.