Enzymes are biological catalysts that accelerate chemical reactions without being consumed overall in the reaction. Most Enzymes are proteins, although some RNA molecules called ribozymes also possess catalytic activity. Enzymes increase reaction rates primarily by providing an alternative pathway with a lower activation energy. They do not change the overall equilibrium constant of a reaction and do not make an energetically unfavourable reaction favourable merely by being present. Enzymes achieve catalysis through specific interactions with substrates and transition states. Their active sites provide chemical environments that can orient reactants, stabilise transition states, transfer protons, form temporary covalent bonds or use metal ions and other cofactors.
Enzyme specificity arises from the structure and chemical properties of the active site. A substrate interacts with amino acid residues positioned to recognise particular molecular features. Modern understanding emphasises induced fit and conformational dynamics rather than treating the active site as a completely rigid lock. Binding can alter enzyme conformation and promote a catalytically productive arrangement. Specificity can be absolute, group based or broad depending on the enzyme. Some Enzymes recognise a family of related molecules, while others are highly selective for one substrate. Binding affinity and catalytic efficiency are separate concepts because a molecule can bind strongly without necessarily being converted rapidly.
Enzyme kinetics describes how reaction rate depends on substrate concentration and other variables. For many simple Enzymes, the Michaelis Menten model relates reaction velocity to substrate concentration using parameters such as Vmax and Km. Km is not universally equivalent to binding affinity, although under particular mechanistic assumptions it can approximate a dissociation related quantity. Vmax represents the limiting rate when the enzyme is effectively saturated with substrate under the specified conditions. Catalytic efficiency is often expressed using kcat divided by Km. Real enzymes can deviate from simple Michaelis Menten behaviour because of multiple substrates, cooperativity, allosteric regulation, product inhibition or complex reaction mechanisms.
Enzymes can be inhibited in several ways. Competitive inhibitors bind at or near the substrate binding site and can often be overcome by increasing substrate concentration under the simplest kinetic model. Noncompetitive and mixed inhibition involve other relationships between inhibitor binding and enzyme states. Irreversible inhibitors form stable interactions that permanently or effectively disable enzyme molecules. Allosteric inhibitors bind at regulatory sites and change enzyme conformation. Enzyme inhibitors are important in medicine because selectively reducing the activity of a particular enzyme can alter a pathological or physiological pathway. Drug design frequently exploits differences between a target enzyme and related enzymes to improve specificity and reduce unwanted effects.
Many Enzymes require cofactors for activity. Cofactors can be inorganic ions such as magnesium or zinc or organic molecules called coenzymes. Some coenzymes participate in electron transfer, group transfer or other chemical transformations and may be derived from vitamins. A protein component without its required cofactor can be inactive and is often called an apoenzyme, while the complete active complex is referred to as a holoenzyme. Metal ions can stabilise charged intermediates, orient substrates or participate directly in redox chemistry. The presence and identity of cofactors therefore greatly expands the chemical reactions that biological catalysts can perform.
Enzymes are essential in metabolism because metabolic pathways depend on controlled rates of chemical transformation. Rather than allowing reactions to proceed solely according to their uncatalysed rates, cells regulate enzyme concentrations and activities. Feedback inhibition allows a downstream product to reduce activity of an earlier enzyme, preventing unnecessary accumulation. Covalent modification such as phosphorylation can activate or inhibit particular enzymes. Compartmentalisation can also separate substrates and enzymes spatially. These regulatory mechanisms permit cells to respond rapidly to changes in energy availability, nutrient concentration and signalling state while maintaining overall metabolic coordination.
Enzymes are widely used outside living organisms. Industrial biotechnology uses enzymes in food production, detergents, textiles, pharmaceuticals, biofuels and chemical manufacturing. Enzymatic processes can operate under relatively mild temperatures and pressures and can provide high stereochemical selectivity. However, enzymes can be sensitive to temperature, pH, solvents and inhibitors, and industrial processes may require immobilisation or protein engineering to improve stability. Immobilised enzymes can be recovered and reused and can facilitate continuous processing. Directed evolution and rational protein engineering can alter substrate specificity, stability and catalytic performance for industrial purposes.
Enzymes are sometimes incorrectly described as substances that make all reactions faster regardless of direction. In reality, an enzyme accelerates the approach to equilibrium for reactions that it catalyses in either direction and does not determine the thermodynamic equilibrium itself. Enzymes also do not necessarily bind substrates permanently or with maximum possible strength because excessive binding can interfere with product release or transition state chemistry. Their catalytic power arises from precise molecular interactions and dynamic conformational behaviour. Enzyme research therefore connects thermodynamics, kinetics, molecular structure, genetics, medicine and industrial biotechnology.
