The thermal denaturation of bovine cardiac G-actin has been studied by ultraviolet difference spectroscopy and circular dichroism between pH 7.5 and 10.5. As with proteins previously studied, thermal unfolding is incomplete compared with unfolding by urea or GuHCl. However, the same conformational change is observed over the pH range studied, and the available evidence indicates it is a two-state transition. Thermodynamic analysis of the data shows that deltaHo and deltaSo are strongly dependent on the temperature, that deltaCp is 1300 cal deg-1 mol-1, and that G-actin has a temperature of maximum stability near -5 degrees C.

No abstract

1. The effect of glycerol on the association-dissociation behaviour of beta-galactosidases from Escherichia coli is described. Two strains, K12 and ML308, were used as sources of enzyme. The conditions used, involving glycerol at a concentration of 90%, result in dissociation of the active 540000-dalton form to inactive structural subunits of 135000 daltons. 2. A pH-dependent process, assumed to be cyclic in mechanism, allows reassociation to an active form indistinguishable from the initial protein. 3. The apparently identical structural subunits, if produced in the presence of EDTA, were found to give rise to two electrophoretically distinguishable species. 4. Enzymes from both strains of E. coli can be distinguished electrophoretically but exhibit the same behaviour in glycerol. 5. A scheme of the association-dissociation is presented that is consistent with the behaviour observed and that has some predictive value.

Elimination of the polymerization of glucagon has been achieved in the presence of glycols. The monomeric form of glucagon, found to be exclusively present under these conditions, is studied for its structural response to changes in the hydrophobicity of the environment. A reversible folding of the α-helix from about 0 to 80% is found to occur and it is suggested to proceed in two sequentially formed helical segments. Changes in the other spectroscopic properties are interpreted as changes in the tertiary structure. Implications for the biological activity are discussed.

Previous work has shown that native alpha-lactalbumin (N) is completely denatured by the addition of guanidine hydrochloride (conformation D) but that partially denatured conformations appear in other denaturants. In particular, conformation I appears when the pH is lowered from 5.5 to 2.2 (I2.2) or when LiClO4 is added at pH 5.5 (I5.5). We have now determined the free energy changes for the processes N leads to I5.5, N leads to D5.5, and I2.2 leads to D2.2. We have also estimated the maximum value of the free energy change for the process N leads to I.22, and this allows us to estimate the changes for all conformational changes between any two of these five conformations.