ligandbound

Ligandbound is a term used to describe the state of a biomolecule, typically a protein, in which one or more ligands are bound at defined binding sites. The ligandbound state is opposite to the ligandfree or apo form. Binding events are governed by affinity and kinetics, with the equilibrium described by the dissociation constant (Kd) and on/off rates. Binding can be reversible or, less commonly, covalent and irreversible.

Binding may induce conformational changes, a phenomenon known as induced fit, or stabilize a preexisting active conformation (conformational selection). The ligandbound state can modulate activity, specificity, stability, and allosteric regulation. In enzymes, ligand binding often controls catalytic efficiency; in receptors, it triggers signal transduction.

Experimentally, ligandbound states are studied using techniques such as isothermal titration calorimetry, surface plasmon resonance, X-ray crystallography, nuclear magnetic resonance spectroscopy, cryo-electron microscopy, and fluorescence assays. Structural biology often aims to solve ligandbound structures, providing coordinates of the ligand and its interactions with amino acid residues, water molecules, and metal cofactors. In structural databases, entries may be annotated as ligandbound or apo, and multiple ligandbound states can be observed for allosteric sites or different binding poses.

In pharmacology and drug design, understanding ligandbound states helps identify binding pockets, optimize affinity and selectivity, and predict pharmacodynamics. The term highlights the biological and structural consequences of ligand association and supports modeling of binding equilibria in cellular contexts.