Structural Basis of Hemoglobin Amyloid Fibrils Revealed by cryo-EM and Molecular Dynamics Simulations.

Hemoglobin has recently gained attention as a potential building block for amyloid-based biomaterials. However, the lack of atomic-level structural information has hindered its rational engineering. Here, we present atomic structures of hemoglobin amyloid fibrils determined by cryo-electron microscopy (cryo-EM). The structure of a new polymorph (PM2), together with the previously reported PM1, reveals that hemoglobin fibrillization is driven by the β-subunit. Using virtual fitting and molecular dynamics simulations, we demonstrate that the homologous α-subunit cannot adopt the amyloid fold due to steric clashes and electrostatic incompatibilities under acidic conditions (pH 2.0), particularly the introduction of positively charged histidine residues within the amyloid core. In contrast, the β-subunit forms stable fibrils, as its sequence enables favorable hydrophobic packing and electrostatic compatibility. Our findings thus provide the atomic-level explanation for subunit-specific amyloid formation in hemoglobin and establish a structural foundation for designing nanomaterials from this widely available agricultural byproduct.