Temperature effect on calcium binding to aspartate and glutamate

Area of Science:

• Biochemistry
• Computational Chemistry

Background:

• Calcium ions are crucial for numerous biological processes.
• Amino acids like aspartate and glutamate play vital roles in biological systems.
• Understanding the binding mechanisms of calcium to amino acids is essential for deciphering biological functions.

Purpose of the Study:

• To investigate and compare the binding mechanisms of calcium ions to aspartate (Asp) and glutamate (Glu) mononegative ions.
• To elucidate the thermodynamic and structural differences in calcium binding between Asp and Glu.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to model the binding interactions.
• Electrochemical methods were used to determine binding affinities.
• Temperature-dependent binding constants were measured to analyze thermodynamic parameters (ΔH⁰).
• Isothermal Titration Calorimetry (ITC) was used for qualitative validation.

Main Results:

• Aspartate binds calcium through both carboxylates, while glutamate binds through only its side chain carboxylate.
• Electrochemical measurements confirmed stronger calcium binding to Asp (Kass,c = 5.3 M⁻¹ at 37 °C) compared to Glu (Kass,c = 3.6 M⁻¹ at 37 °C).
• Thermodynamic analysis revealed a less negative enthalpy of binding for Asp (ΔH⁰ = -17 kJ·mol⁻¹) compared to Glu (ΔH⁰ = -21 kJ·mol⁻¹).

Conclusions:

• The stronger binding of calcium to Asp, despite a less favorable enthalpy, is attributed to a significant entropy gain.
• This entropy gain in Asp-calcium complexation is likely due to the formation of a stable ring structure.
• DFT calculations and experimental data consistently support these findings, highlighting the distinct calcium-binding modes of Asp and Glu.