Binding site

A binding site is the specific region on a protein — a receptor, enzyme, transporter, or ion channel — where a molecule physically docks. It is typically a pocket or cleft in the protein's three-dimensional structure, shaped by the folded arrangement of amino acids into a cavity with particular chemical properties: charge, polarity, and contour.

The concept is foundational because pharmacological action is almost always site-specific. A drug does not simply hit a receptor — it attaches at a defined location, and the site's geometry and chemistry determine which molecules fit, how tightly, and what happens when they do.

A single protein often carries more than one binding site, and two compounds acting at different sites on the same target can produce very different — sometimes opposite — effects. This is why pharmacology names the site rather than only the receptor.

How it works · its role

Binding is driven by non-covalent forces — hydrogen bonds, electrostatic attraction, and hydrophobic interactions — rather than permanent chemical bonds. This reversibility is what allows a drug's effect to diminish as concentrations fall.

The orthosteric site is where the protein's natural signalling molecule, its endogenous ligand, attaches. An allosteric site is a distinct location elsewhere on the protein; molecules binding there reshape the protein and change how well the orthosteric ligand works, without occupying its position.

Affinity describes how tightly a molecule binds at a given site — a high-affinity drug holds on longer and competes more effectively at low concentrations. Selectivity describes how much a molecule prefers one site over others; a substance that binds many different sites tends to produce a broader, less predictable effect profile.

Relevance to substances & effects

Most psychoactive substances exert their effects through site-specific binding. Classic psychedelics such as LSD and psilocin bind at the orthosteric site of the 5-HT₂A receptor, which drives their perceptual and cognitive effects.

Benzodiazepines illustrate the allosteric case: they bind at a site on the GABA-A ion channel distinct from where GABA itself attaches, and amplify the channel's response to its endogenous ligand without replacing it. This is why benzodiazepines cannot fully open the channel on their own — they can only modulate.

Opioids bind at the orthosteric sites of mu, kappa, and delta opioid receptors; each subtype has a distinct binding pocket, which partly explains why different opioids vary in their analgesic, euphoric, and respiratory effects.

Ketamine and PCP occupy a site inside the NMDA receptor's ion channel itself — accessible only when the channel is already open — producing the dissociation and anaesthesia associated with NMDA blockade.

Understanding which site a substance occupies clarifies why combining two drugs acting on the same target can be additive, synergistic, or mutually antagonistic. A competitive antagonist at the orthosteric site and a modulator at an allosteric pocket do not simply cancel out; their combined effect depends on how each changes the protein's configuration.