a Overall structure of Stx2a holotoxin in complex with MMβA-mono. Stx2a is shown as a charge distribution surface model. The surface of the Stx2a A-subunit is colored by charge (blue, positive; red, negative). MMβA-mono is shown as a stick model. A close-up view shows the catalytic cavity of the Stx2a A-subunit, which is where MMβA-mono binds. b Structural view of the binding between the A-subunit and MMβA-mono. Interacting residues are shown as stick models, hydrogen bonds are shown as broken lines, and water molecules are shown as spheres. c AlphaScreen assay to examine the binding between the Stx2a A-subunit and MMβA-mono with amino acid substitution(s). Data are presented as a percentage of the maximum binding value (mean ± SE, n = 3). ***P < 0.001, **P < 0.01, *P < 0.05 (compared with MMβA-mono by Dunnett’s test).