Comparison of the binding of Ca2+ and Mn2+ to bovine alpha-lactalbumin and equine lysozyme.

This research examines the energy changes that occur when calcium ions (Ca2+) and manganese ions (Mn2+) bind to two specific proteins, equine lysozyme and bovine alpha-lactalbumin, using a technique called batch microcalorimetry. The researchers found that these ions bind differently to these proteins, and that the removal of calcium ions subtly changes the structure of the proteins.

Method and Measurements

• The team of researchers measured the enthalpy change, which is the heat energy change, produced when Ca2+ and Mn2+ ions bind to equine lysozyme – a protein found in horse milk. The experiment was carried out at a temperature of 25 degrees Celsius and in a neutral pH environment of 7.5.
• Microcalorimetry, the technique used to make these measurements, allows for the precision detection of heat changes in small sample sizes. Through this method, the team could provide a quantified understanding of energy changes during ion binding.

Results and Findings

• The results demonstrated the specific enthalpy changes for each ion-protein interaction: -76 kJ/mol for Ca2+ and -21 kJ/mol for Mn2+. A negative enthalpy change suggests that the binding process was exothermic; energy was released when these ions interacted with the protein.
• From these data, the researchers calculated the binding constants, these tell researchers how strongly these ions attach to the protein. Ca2+ had a higher value (6.5) than Mn2+ (4.1), indicating stronger binding affinity for the calcium ion.
• The experiment also involved the calculation of entropy changes (delta S), which provide information about disorder in the system during these ion-protein interactions. The entropy changes were -131 JK-1mol-1 for Ca2+, suggesting a decrease in disorder upon binding whilst for Mn2+, the entropy change was positive, suggesting an increase in disorder.

Effects of Calcium Removal

• Removing Ca2+ from this environment led to small yet important shifts in the protein’s structure. According to circular dichroism spectroscopy, there was evidence of a partially unfolded protein structure, known as an apo-conformation, after the removal of Ca2+.
• The researchers found this new apo-conformation had similarities with, yet distinct differences from, the apo-conformation observed in another protein, the bovine alpha-lactalbumin. This insight could bring understanding into how ion binding influences the structure and function of proteins.