English: Botulinum toxin A1 (BoNT/A1) in complex with a clostridial non-toxic non-hemagglutinin (NTNHA or NTNH). PDB: 3V0A. Naturally occurring botulinum toxins are always combined with their corresponding NTNHA into a minimally functional progenitor toxin complex (M-PTC). Botulinum toxins always consist of 2 peptide chains: light and heavy.

NTNHA (orange). Light chain (green) binds a zinc ion (red) as a cofactor. Light chain is linked to heavy chain's N-terminal "belt" by a disulfide bond (yellow, -S-S-). Heavy chain (H) has 3 parts:

• magenta: "belt" (N-terminal)
• N-terminal translocation domain AKA H_N
• receptor-binding domain AKA H_C (C-terminal)

Sources:

• Dong M, Stenmark P. (2019) The Structure and Classification of Botulinum Toxins. In: Handbook of experimental pharmacology. Springer. doi:10.1007/164_2019_342 (Fig. 5)
• Nat Commun. 2018; 9: 5367. doi: 10.1038/s41467-018-07789-4 (Fig. 1)