SAXS Reconstruction of IgG1 Immunoglobulin — Application Note

Summary

Immunoglobulin G (IgG) antibodies are critical components of the immune system and represent the largest class of therapeutic biopharmaceuticals. IgG1, the most abundant subclass, is widely used as a scaffold for therapeutic monoclonal antibodies targeting cancer, autoimmune diseases, and infectious diseases.

This application note demonstrates SAXS ab initio reconstruction of IgG1's low-resolution tertiary structure in native buffer conditions. The characteristic Y-shaped structure with its flexible hinge region was successfully visualized, and structural parameters were determined for use in biopharmaceutical quality control and biosimilar development.

Key Achievement: Reconstructed the complete 3D envelope structure of IgG1 in solution at 0.25% wt concentration, validated against high-resolution crystallographic data from Protein Data Bank, confirming SAXS capability for therapeutic antibody characterization.

Background & Challenge

IgG1 Structure & Function

IgG1 is a ~150 kDa glycoprotein composed of:

Two identical heavy chains (~50 kDa each)
Two identical light chains (~25 kDa each)
Characteristic Y-shaped structure with antigen-binding Fab regions and Fc effector region
Flexible hinge region allowing conformational adaptability

Pharmaceutical Importance

Therapeutic antibodies must maintain proper higher-order structure for:

Antigen binding: Fab region orientation and flexibility
Effector functions: Fc region structure affects ADCC, CDC
Pharmacokinetics: FcRn binding and serum half-life
Stability: Resistance to aggregation and degradation
Biosimilarity: Structural comparability to reference products

Why SAXS for Antibodies? While crystal structures provide atomic detail, antibodies in solution exhibit flexibility and domain motions not captured by crystallography. SAXS provides the solution structure in native conditions — essential for understanding function and ensuring product quality.

Methods & Experimental Design

Sample Information

Sample: IgG₁ immunoglobulin

Concentration: 0.25% wt in buffer

Temperature: Room temperature

SAXS Measurement

Instrument Parameters

X-ray sourceCu Kα
InstrumentLaboratory SAXS at DANNALAB
Q range0.05-4.3 nm⁻¹

Ab-initio SAXS Reconstruction

Structural Parameters

Molecular Weight (SAXS)

145 kDa

From SAXS reconstruction

Crystal Structure (PDB)

146 kDa

1IGY high-resolution model

Agreement

99.3%

SAXS vs. crystal structure

Reconstruction Method

Ab-initio (DAMMIN)

Low-resolution envelope

IgG1 SAXS reconstruction compared to crystal structure

Figure 1. IgG₁ structure reconstructed from SAXS solution scattering experiment (top) compared with the 1IGY high-resolution crystallographic model (bottom). Molecular weight from SAXS: 145 kDa vs. 146 kDa from crystal structure.

The low-resolution envelope structure of IgG₁ was determined by ab-initio reconstruction using the DAMMIN software package (EMBL). The estimated molecular mass for the reconstructed structure was 145 kDa compared to 146 kDa of the high-resolution crystallographic structure (1IGY entry in PDB database).

Conclusion

One of DANNALAB's research directions relates to the reconstruction of protein and peptide structures in close-to-natural biological environments. The function of proteins is closely defined by their actual structure and conformations. The reconstruction of the IgG₁ structure in a close-to-natural environment is an important step in assessing its stability and conformational space.

Reference: [1] 1IGY entry in PDB database (www.pdb.org)

SAXS Reconstruction of IgG1 Immunoglobulin Tertiary Structure