Protein binding

Protein binding describes the reversible attachment of a substance's molecules to proteins circulating in the bloodstream — chiefly albumin and alpha-1-acid glycoprotein — rather than travelling freely through the fluid. The bound fraction rides alongside these proteins; the unbound, or free, fraction floats independently.

Only the unbound fraction crosses biological membranes, reaches target tissues, and exerts pharmacological effects. It is also the only fraction available for metabolism and elimination. Protein binding therefore acts as a buffer — holding a reservoir of substance in the blood, releasing it gradually as the free fraction is consumed or cleared.

How it works · its role

The binding is reversible and governed by equilibrium. As the free fraction is used up — metabolised by the liver, filtered by the kidneys, or taken up by target tissue — bound molecules detach and replenish it. This continuous exchange keeps the ratio of bound to unbound relatively stable, though the balance is not fixed.

Several factors can shift it. Competing substances can displace a drug from its binding site, abruptly raising the free fraction without any change in dose. Low albumin levels — common in liver disease, malnutrition, or serious illness — reduce the total number of available sites, leaving more substance unbound. Changes in blood pH alter the conformation of binding proteins and can tighten or loosen their grip on a molecule.

Acidic drugs tend to bind albumin; basic drugs tend toward alpha-1-acid glycoprotein. Most substances distribute across both to some extent.

Relevance to substances & effects

Protein binding shapes three properties that matter across substance pages on this encyclopedia: onset, duration, and drug interaction.

A heavily bound substance has only a small free fraction doing the pharmacological work at any given moment. This tends to slow onset and extend duration, because the reservoir refills the active pool gradually rather than flooding it at once. It also means a measured blood concentration is not the same as an active dose — only the unbound portion counts.

Interaction risk rises when two highly bound substances compete for the same proteins. A drug displaced from its binding sites by a second compound may see its effective concentration rise sharply with no change in the amount taken. This mechanism underlies certain flagged interactions among anticoagulants, anticonvulsants, and some psychiatric medications.

Psychoactive substances span a wide range of binding behaviour. Benzodiazepines tend toward strong protein binding; alcohol is essentially unbound and distributes freely. The character of a substance's binding helps explain why two compounds at similar doses can behave very differently in the body — and why a clinical blood level alone does not always predict effect.