Camelid heavy-chain variable domains provide efficient combining sites to haptens

Area of Science:

• Biochemistry
• Structural Biology
• Immunology

Background:

• Camelids produce unique antibodies lacking light chains and CH1 domains.
• Camelid heavy-chain variable domains (VHH) exhibit high affinity for protein antigens.
• The capacity of small, monomeric VHH fragments to bind haptens remained uncertain.

Purpose of the Study:

• To investigate the ability of VHH fragments to bind haptens.
• To determine the structural basis for VHH-hapten interaction.
• To explore the biotechnological applications of VHH fragments in hapten binding.

Main Methods:

• Generation of llama antibodies against the azo-dye hapten Reactive Red (RR6).
• Determination of the crystal structure of a VHH fragment complexed with a dimeric RR6 hapten.
• Measurement of the dissociation constant for the VHH-hapten interaction.

Main Results:

• Successfully raised llama antibodies against RR6.
• Determined the crystal structure of the VHH-RR6 complex, revealing a large interaction surface (approx. 300 A²).
• Measured a low dissociation constant (22 nM) for the VHH-monomer interaction, indicating high affinity.
• Identified specific interactions, including histidine residues in CDR1 binding to copper atoms of the hapten.

Conclusions:

• VHH fragments possess an efficient combining site for haptens, utilizing their CDR loops.
• VHH fragments demonstrate significant potential for selecting, removing, or capturing haptens.
• These findings suggest VHH fragments have broad biotechnological applications beyond protein recognition.