Detailed Explanation
Affinity describes how tightly a peptide binds to its target receptor or protein. It's quantified by the dissociation constant (Kd) — the concentration at which half of the receptors are occupied. A lower Kd means tighter binding: a peptide with a Kd of 1 nanomolar binds 1,000 times more tightly than one with a Kd of 1 micromolar.
In peptide drug development, affinity is the starting point for optimization. A peptide discovered through phage display or library screening might have micromolar affinity — too weak for a drug. Medicinal chemists then systematically modify residues to improve binding, a process called affinity maturation. The goal is typically low nanomolar to picomolar affinity.
Affinity is distinct from selectivity (binding the right target but not others) and efficacy (triggering the desired biological response after binding). A peptide can bind tightly to a receptor without activating it — that's how peptide antagonists work.
Key Facts
- Measured as Kd (dissociation constant); lower = tighter binding
- Drug-grade affinity: typically low nanomolar (nM) range
- Distinct from selectivity and efficacy
- Improved through structure-activity relationship (SAR) studies
- Measured by SPR, ITC, or fluorescence assays
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