Affibody Complex Formation: An In-Depth Thermodynamic Analysis Using Isothermal Titration Calorimetry

Area of Science:

• Biochemistry
• Protein Engineering
• Molecular Recognition

Background:

• Affibody proteins are small, stable, engineered proteins valuable for molecular recognition.
• ZTaq and anti-ZTaq are anti-idiotypic binders used as models for studying protein interactions.
• Understanding affibody binding thermodynamics is crucial for protein engineering applications.

Purpose of the Study:

• To investigate the thermodynamics of ZTaq:anti-ZTaq binding across a wide temperature range.
• To elucidate the mechanisms behind the salt-independent binding of these affibody proteins.
• To correlate thermodynamic properties with sequence features for improved protein design.

Main Methods:

• Expression and purification of ZTaq and anti-ZTaq proteins.
• Characterization of protein-protein interaction using isothermal titration calorimetry (ITC).
• Analysis of thermodynamic parameters (ΔG_b, ΔC_p) and their dependence on temperature and salt concentration.

Main Results:

• The ZTaq:anti-ZTaq complex formation exhibits a large negative free energy of binding (ΔG_b).
• Binding affinity is remarkably unaffected by salt concentration, unlike typical polyelectrolyte systems.
• A significant negative heat capacity change (ΔC_p) is observed, linked to conformational transitions and molten-globule state disruption.

Conclusions:

• The salt-insensitivity of ZTaq:anti-ZTaq binding is driven by specific thermodynamic contributions.
• Disruption of the anti-ZTaq molten-globule-like state above 303 K significantly impacts binding thermodynamics.
• Insights gained can guide the engineering of affibodies with tailored high-affinity molecular recognition properties.