Structural basis for difference in heat capacity increments for Ca(2+) binding to two alpha-lactalbumins

Area of Science:

• Biochemistry
• Protein thermodynamics
• Structural biology

Background:

• Alpha-lactalbumin is a key protein in milk.
• Calcium ions (Ca2+) play a crucial role in protein structure and function.
• Understanding protein conformational changes is vital for molecular biology.

Purpose of the Study:

• To investigate the thermodynamic parameters of unfolding and Ca(2+) binding in goat alpha-lactalbumin (GLA) and bovine alpha-lactalbumin (BLA).
• To compare the conformational changes associated with these processes in GLA and BLA.
• To elucidate the role of charge distribution in Ca(2+)-dependent conformational changes.

Main Methods:

• Isothermal titration calorimetry (ITC) was used to determine thermodynamic parameters.
• Analysis of tertiary protein structure was performed.
• Heat capacity increments (ΔCp) were analyzed to assess conformational changes.

Main Results:

• Both GLA and BLA showed similar heat capacity increments for the native to molten globule transition, indicating comparable conformational changes.
• Significant differences in heat capacity increments were observed for Ca(2+) binding between GLA and BLA.
• BLA exhibits a higher sensitivity to Ca(2+)-dependent conformational changes compared to GLA, linked to a greater accumulation of negative charges near the Ca(2+)-binding site.

Conclusions:

• The study highlights distinct Ca(2+) binding mechanisms in GLA and BLA.
• Differences in charge distribution, particularly the substitution of Glu-11 by Lys in GLA, contribute to the observed variations in Ca(2+)-dependent conformational changes.
• These findings provide insights into the structure-function relationships of alpha-lactalbumin variants.