Structural thermodynamics by cryo-EM resolves minute energy differences in heterogeneous protein complexes.

Abstract

Hetero-oligomeric protein complexes play crucial roles across biology, providing combinatorial diversity and context-specific functionality in membrane receptors, molecular chaperones and beyond. Yet, existing methods are limited in their ability to resolve and quantify the assembly energetics of complex biomolecular populations. Here, we introduce a comprehensive method combining iterative 3D classification, error-rate quantification via simulated particles, and global thermodynamic modeling to extract interaction energies from cryo-EM data. Using hexameric cruciferin (the primary rapeseed storage protein) as a model, we reconstruct a 1.83 [A] structure and distinguish three highly similar 50 kDa isoforms. We resolve the complex compositional heterogeneity of the hexameric species at the single-particle level and fit a global binding model revealing pairwise interaction energies between isoforms differing by fractions of the thermal energy (kB T ). This method offers a powerful, broadly applicable tool to characterize the compositional heterogeneity and energetics of biomolecular complexes.