The Acidic Molten Globule State of α-Lactalbumin Probed by Sound Velocity

We have investigated the structure and the chemical and conformational relaxation in the acidic molten globule state of α-lactalbumin (A-state) by using ultrasonic velocimetry. The A-state of the α-lactalbumin at pH 2 exhibits an 11% lower sound-velocity increment than that of the native state at neutral pH and 2-MHz sound frequency. An excess of calcium decreases this effect. For lysozyme, the difference in sound velocity between pH 7 and 2 is significantly smaller. Lysozyme has a large sequence homology and structure similarity to the native α-lactalbumin, but no molten globule state in aqueous solution has been described. The destabilized apo-state of α-lactalbumin at neutral pH exhibits a small decrease of roughly 4% in its sound-velocity increment relative to the native state. The large sound-velocity decrease of the A-state relative to that of the native state of α-lactalbumin indicates significant conformational relaxation at a time scale faster than 500 ns in the acidic molten globule at pH 2.