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3D Molecular Visualization of a Human Antibody by MAXS Measurement Reveals Significant Differences Between the Solution and Crystalline States

Takashi Matsumoto, Akihito Yamano, and Takashi Sato

Winter 2023 Volume 39, No. 1 , 10-14

X-ray solution scattering experiments have been utilized to analyze structures and conformational changes of biological macromolecules. Those experiments employ X-ray scattering data in a small-angle region, a technique called Small Angle X-ray Scattering (SAXS). SAXS experiments often focus on the macroscopic shapes and sizes of molecules. However, in principle, the scattering data in the higher scattering angle region contains more detailed structural information. For example, scattering data corresponding to q values (=4π sin θ/λ) between 0.30 and 0.65 Å⁻¹ reflects distances among domains, secondary structure modules, and/or adjunct chemical groups in Antibody–Drug Conjugates (ADCs). These Middle Angle X-ray Scattering (MAXS) experiments should give us a novel picture of complex molecular behaviors, as well as conformational changes in flexible biomolecules. In this article, we will discuss solution structure analysis of human Immunoglobulin G (IgG) by MAXS that revealed significant differences between the solution and crystalline states, as well as a novel observation of its flexibility.

Highlights

  • Middle Angle X-ray Scattering (MAXS) extends beyond conventional SAXS by capturing structural information that reveals domain-level conformational changes in proteins while they remain in solution.
  • Human IgG antibodies adopt significantly more open and flexible conformations in solution than suggested by crystal structures, particularly in the Fab regions.
  • Removal of Fc sugar chains increases antibody flexibility and alters the relative positions of functional domains, providing a structural explanation for reduced Fc receptor binding.

Summary

Structural analysis of antibodies has traditionally relied on X-ray crystallography or cryo-electron microscopy, but these techniques capture molecules under constrained conditions that may not accurately reflect their native behavior. MAXS expands the range of solution scattering measurements into the middle-angle region, enabling reconstruction of three-dimensional electron density maps that preserve information about molecular flexibility while avoiding the structural constraints imposed by crystallization.

Application of this approach to human immunoglobulin G (IgG) demonstrated that antibodies in solution are considerably more extended than their crystal structures suggest. The Fab arms exhibited asymmetric behavior, with one arm showing substantially greater flexibility than the other despite being genetically identical. This observation suggests that antibody molecules may use unequal movements of their binding arms to improve antigen recognition and capture.

The technique was also used to investigate the role of Fc glycosylation. Antibodies lacking their Fc sugar chains adopted a more open conformation and exhibited increased mobility throughout both the Fc and Fab regions. These structural changes altered the spatial relationship between domains involved in Fc receptor recognition, providing a plausible explanation for the well-known reduction in receptor binding after deglycosylation.

For researchers familiar with X-ray fluorescence and battery materials, MAXS represents a complementary structural characterization method that extracts additional information from scattering data by extending the measured q-range. Rather than identifying elemental composition, it provides insight into molecular architecture, conformational heterogeneity, and dynamic structural behavior in solution. These capabilities have significant potential for biopharmaceutical development, where understanding molecular flexibility can improve therapeutic design, manufacturing consistency, stability assessment, and quality control.

Frequently asked questions

MAXS extends solution scattering measurements into a higher scattering-angle (higher q) region than traditional Small Angle X-ray Scattering. While SAXS primarily provides information about overall molecular size and shape, MAXS captures additional structural detail related to domain organization, secondary structure arrangements, and distances between functional regions. This allows reconstruction of more detailed three-dimensional models while retaining the advantages of solution-based measurements.

Antibodies are highly flexible molecules whose structures continuously change in solution. Crystal structures represent only a single, constrained conformation influenced by crystal packing, while cryogenic methods capture frozen snapshots. Solution-state measurements reveal the range of conformations that antibodies naturally adopt during biological function, providing a more realistic picture of their behavior.

Solution-state measurements show that antibodies are generally more open and extended than their crystal counterparts. The Fab arms exhibit substantially greater freedom of movement, producing conformations that are rarely observed in crystal structures. These differences indicate that crystallization can mask important aspects of antibody flexibility that are relevant to biological activity.

Three-dimensional electron density reconstructions revealed that one Fab arm closely matched its crystallographic structure, while the other displayed a broader, more diffuse density consistent with increased flexibility. This unexpected asymmetry suggests that identical Fab domains can perform different dynamic roles, potentially improving the efficiency of antigen searching and binding.

Sugar chains attached to the Fc region help stabilize antibody conformation and regulate molecular flexibility. Removing these glycans results in a more open Fc structure and greater mobility throughout the molecule. These conformational changes alter the relative positions of important structural domains and can significantly reduce binding affinity for Fc receptors.

The increased flexibility caused by removal of Fc sugar chains changes both the distance and orientation between domains involved in receptor recognition. In solution, these structural rearrangements can either separate critical interaction sites too far apart or bring them too close together, reducing the likelihood of forming a stable interaction with the Fc receptor.

MAXS enables researchers to evaluate intact therapeutic proteins under solution conditions, providing insight into conformational flexibility, structural heterogeneity, and the effects of modifications such as glycosylation. This information can improve molecular design, formulation development, manufacturing consistency, stability assessment, and analytical characterization of antibody-based therapeutics.

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