Cation bindint to alpha-sl-casein B. A comparison of electrostatic models

Area of Science:

• Biochemistry
• Protein Chemistry
• Physical Chemistry

Background:

• Alpha-sl-casein B is a key protein involved in calcium binding.
• Understanding calcium binding and proton release is crucial for casein functionality.

Purpose of the Study:

• To investigate system characteristics determining calcium binding and proton release from alpha-sl-casein B.
• To construct and evaluate charge models for predicting these interactions.

Main Methods:

• Experiments were conducted at pH 6.6 with varying sodium ion concentrations (0.04, 0.0, and 0.16M).
• Distributed and discrete charge models were developed using protein solvation, site-bound calcium, and net monomer charge values.
• Rodlet models of the whole molecule and the acidic peptide portion were created based on the amino acid sequence.

Main Results:

• Distributed charge models were inadequate for predicting proton release within acceptable dielectric constant limits.
• Three discrete charge models achieved good agreement with experimental data at a dielectric constant of 4, especially as site-bound calcium increased.
• The acidic peptide portion electrostatically dominates calcium binding, with a relatively constant electrostatic environment for other sites.
• A toroidal model of the acidic peptide showed better data matching with less assumed strain under high molecular charge conditions.

Conclusions:

• Discrete charge models, particularly those considering molecular conformation like the toroidal model, are effective for calculating electrostatic free energy in alpha-sl-casein B.
• Conformation and association play significant roles in the electrostatic interactions during calcium binding to casein.