Active site

An active site is the region of an enzyme where substrates bind and chemical reactions are catalyzed. Its three-dimensional arrangement of amino-acid residues creates a specific chemical environment for catalysis.

Enzyme–substrate complex: A transient molecular complex formed when a substrate binds an enzyme. It is the bound state in which an active site positions substrate for reaction.
Protein tertiary structure: The overall three-dimensional shape of a single folded protein chain. An active site is assembled by the enzyme’s folded structure, often from separated sequence regions.
Enzyme inhibitor: A molecule that reduces or prevents an enzyme’s catalytic activity. Many inhibitors act by occupying the active site or disrupting its catalytic chemistry.
Allosteric site: A site on a protein where ligand binding changes activity without occupying the active site. Allosteric regulation alters catalysis from a distinct location rather than blocking substrate directly.
Induced fit: A binding model in which ligand association changes a protein's shape to improve complementarity. Active-site residues can shift around a substrate after initial binding.
Amino acid: An organic molecule that serves as a building block of proteins. Active-site residues contribute chemical groups that bind substrates and promote reactions.
Competitive inhibition: Enzyme inhibition in which an inhibitor competes with substrate for the same binding site. It directly demonstrates how active-site occupancy can control reaction rate.
Lock-and-key model: A model of enzyme specificity in which a substrate fits a pre-shaped binding site. It contrasts with accounts in which active-site shape changes during substrate binding.
Transition state: A fleeting, highest-energy molecular configuration along a reaction pathway. Enzymes accelerate reactions by preferentially stabilizing this configuration.
Protein side chain: The variable chemical group attached to an amino acid’s common backbone. Side-chain charge, size, and reactivity help determine active-site properties.

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