Human IgG1 Fc pH-dependent optimization from a constant pH molecular dynamics simulation analysis.

Area of Science:

• Biochemistry and Molecular Biology
• Immunology
• Computational Chemistry

Background:

• The neonatal Fc receptor (FcRn) binds to the IgG Fc region, mediating IgG's extended serum half-life through pH-dependent interactions.
• Optimizing Fc-FcRn interactions is crucial for enhancing the pharmacokinetic properties of IgG-based therapeutics.
• Previous studies focused on IgG Fc mutations to modulate FcRn binding, but a deeper molecular understanding of the pH-dependent mechanism is needed.

Purpose of the Study:

• To investigate the molecular mechanisms underlying human Fc-FcRn binding and dissociation at physiologically relevant pH values (6.0 and 7.5).
• To computationally design novel Fc variants with improved FcRn binding affinity at acidic pH while retaining dissociation at neutral pH.
• To evaluate the binding free energy of a designed Fc mutant using molecular modeling approaches.

Main Methods:

• Constant pH molecular dynamics (CpHMD) simulations were employed to analyze the Fc-FcRn complex at pH 6.0 (binding) and pH 7.5 (dissociation).
• Analysis of titratable residue protonation states and electrostatic interactions was performed to understand the pH-dependent binding.
• The Molecular Mechanics/Generalized Born Surface Area (MM/GBSA) method was used for binding free energy calculations of Fc variants.

Main Results:

• CpHMD simulations revealed significant changes in the protonation states of titratable residues between pH 6.0 and 7.5, influencing Fc-FcRn complex stability.
• Electrostatic repulsion was identified as a key factor disrupting Fc-FcRn complex formation at neutral pH.
• A computationally designed Fc mutant (MutM4) demonstrated enhanced binding affinity at pH 6.0 and maintained FcRn dissociation at pH 7.5 compared to the YTE mutant.

Conclusions:

• The study elucidates the molecular basis of pH-dependent Fc-FcRn interactions, highlighting the role of titratable residues and electrostatic forces.
• A novel Fc variant (MutM4) was successfully designed in silico, exhibiting improved binding characteristics for FcRn.
• This work provides a foundation for future experimental validation and offers a promising strategy for engineering Fc-based biologics with optimized pharmacokinetics.