Other meanings of Active site
Biochemistry
The active site is the region of an enzyme where substrate binding and catalysis occur, typically a three-dimensional cleft or pocket formed by specific amino acid residues. It is the functional heart of an enzyme, dictating substrate specificity and reaction rate, and is often the target of drug design.
The active site is typically a small, three-dimensional pocket formed by residues that may be distant in the primary sequence but converge upon protein folding1. Binding is governed by complementarity of shape, charge, and hydrophobicity, often described by the lock-and-key model or the induced-fit model, in which the enzyme undergoes conformational changes upon substrate binding2. The specificity of an active site for its substrate arises from precise hydrogen bonding, van der Waals contacts, and electrostatic interactions with the substrate.
In many enzymes, the active site contains a catalytic triad or other functional groups that participate directly in bond making and breaking. For example, serine proteases feature a triad of serine, histidine, and aspartate residues essential for peptide hydrolysis3.
The active site accelerates reactions by stabilizing the transition state more than the ground state, thereby lowering the activation energy4. This stabilization is achieved through mechanisms such as acid–base catalysis, covalent catalysis, metal ion catalysis, and proximity/orientation effects that bring reactants into optimal alignment5.
Enzymes often employ multiple catalytic strategies simultaneously. For instance, chymotrypsin uses a covalent acyl-enzyme intermediate, while lysozyme stabilizes an oxocarbenium-ion-like transition state via electrostatic interactions. The active site microenvironment may also differ in pH and polarity from the bulk solvent, enhancing reactivity6.
Because the active site controls enzyme activity, it is the primary target for many drugs and inhibitors. Competitive inhibitors resemble the substrate and bind reversibly to the active site, whereas noncompetitive inhibitors bind elsewhere and alter active site function7. Suicide inhibitors (mechanism-based inactivators) form covalent bonds with the active site, leading to irreversible inhibition.
Rational drug design often exploits active site structure, as exemplified by the development of HIV protease inhibitors, which mimic the transition state of the viral protease8. Similarly, aspirin irreversibly acetylates a serine residue in the active site of cyclooxygenase, blocking prostaglandin synthesis.
Some enzymes have active sites that are regulated by allosteric effectors binding to remote sites, causing conformational changes that affect substrate affinity or catalysis. This is crucial in metabolic feedback regulation, such as the inhibition of aspartate transcarbamoylase by CTP.
Many active sites are not perfectly specific; they can catalyze secondary reactions unrelated to their primary function. This phenomenon, known as catalytic promiscuity, is thought to be a driving force in enzyme evolution, enabling the acquisition of new functions.
Although the term is most associated with enzymes, analogous active sites exist in ribozymes such as the ribosome, which uses RNA residues to catalyze peptide bond formation. Also, some antibodies (abzymes) have been engineered to possess catalytic activity in their antigen-binding sites.
Many enzymes are synthesized as inactive precursors (zymogens) with an extra peptide that blocks the active site. Proteolytic cleavage removes this peptide, exposing the active site. For example, trypsinogen becomes trypsin after cleavage by enteropeptidase.
The concept of the active site evolved from Emil Fischer's lock-and-key hypothesis (1894) and was refined by Koshland's induced-fit model (1958).
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