The Native-like Tertiary Fold in Molten Globule α-Lactalbumin Appears to be Controlled by a Continuous Phase Transition

Abstract

On account of its ability to discriminate between secondary, loop and sidegroup structure and its special sensitivity to conformational mobility, vibrational Raman optical activity (ROA) has provided new insights into the complexity of order within the molten globule state from measurements on α-lactalbumin at pH 2.0 over the temperature range 2 to 45°C. Thus while much of the secondary structure present in the native protein persists with only a small gradual decrease with increasing temperature, the tertiary backbone fold changes dramatically, being almost complete and native-like at 2°C and almost completely disordered at 35°C. The change of the tertiary fold with temperature is cooperative but has no latent heat, and so has the approximate characteristics of a continuous phase transition, being of the order-disorder type since it involves the interconversion of rigid, locally-ordered loop structure with disordered mobile backbone structure. This has implications for protein folding because the long-range correlations that exist in the critical region of a continuous (but not in a first-order) phase transition could resolve, in principle, the problem of how the protein finds its native-like folding pattern at the molten globule stage.