An enzyme is a biological catalyst that accelerates a chemical reaction without being consumed overall. Most enzymes are proteins whose three dimensional structures create specialised active sites, although some RNA molecules called ribozymes also possess catalytic activity. Enzymes are essential to life because many biochemical reactions would proceed far too slowly under normal cellular conditions without catalysis. They participate in metabolism, DNA replication, transcription, protein synthesis, digestion, cellular signalling, energy production and countless other processes.
An enzyme binds one or more substrates and facilitates their conversion into products through a specific molecular mechanism. Enzymes do not change the overall thermodynamic difference between reactants and products, but they lower the activation energy required to reach the transition state. Their activity depends on molecular structure and environmental conditions such as temperature, pH, substrate concentration and the presence of cofactors or inhibitors.
The active site is the region of an enzyme where substrate binding and catalysis occur. Its three dimensional arrangement creates a chemical environment suited to particular substrates and reactions. The older lock and key model describes the active site as having a shape complementary to the substrate, whereas the induced fit model emphasises that enzyme and substrate can change conformation when they interact.
In reality, enzyme recognition often involves a dynamic combination of shape, electrostatics, hydrogen bonding, hydrophobic interactions, van der Waals forces and conformational flexibility. Substrate binding can position reactive groups precisely, strain particular bonds, stabilise charged intermediates and create acid base or metal assisted catalytic environments. Enzyme specificity can be extremely high, although some enzymes act on families of related substrates rather than one unique molecule.
Enzymes accelerate reactions by lowering the activation energy through specific catalytic mechanisms. They can use general acid base catalysis, in which amino acid side chains donate or accept protons. They can use covalent catalysis, in which the enzyme temporarily forms a covalent bond with a substrate or intermediate. Metal ion catalysis can stabilise charges, activate water or facilitate electron transfer. Electrostatic catalysis can stabilise developing charges in transition states.
Enzymes can also bring substrates together in favourable orientations, reducing the entropic and geometric barriers to reaction. The transition state is particularly important because an enzyme that binds and stabilises the transition state effectively can dramatically increase reaction rate. Enzyme catalysis therefore depends on precise molecular interactions rather than simply holding a substrate in place.
Enzyme kinetics describes how reaction rate depends on substrate concentration and other variables. For many simple enzymes, the Michaelis Menten model relates initial reaction velocity to substrate concentration using parameters called Vmax and Km. Vmax represents the maximum rate under specified conditions when the enzyme is saturated with substrate, while Km is the substrate concentration corresponding to half of Vmax in the simplest Michaelis Menten model.
Km can sometimes be interpreted as an apparent measure related to substrate affinity, but it is not universally identical to a binding constant. The catalytic constant, kcat, describes the turnover rate of enzyme molecules under saturating substrate conditions. The ratio kcat divided by Km is commonly used as a measure of catalytic efficiency under low substrate concentrations. More complex enzymes may display cooperative binding, allostery or kinetics that do not follow simple Michaelis Menten behaviour.
Enzyme activity is strongly affected by temperature and pH. Increasing temperature generally increases molecular motion and reaction rate over a moderate range, but excessive temperature can disrupt the protein's three dimensional structure and cause denaturation. Each enzyme has a range of pH values in which its structure and catalytic residues function effectively. Changes in pH can alter the protonation state of amino acid side chains and substrates, changing binding and catalytic activity.
Some enzymes are adapted to extreme environments and remain stable at temperatures or pH values that would damage ordinary proteins. Cellular organisms regulate enzyme activity through compartmentalisation, reversible modification, changes in substrate availability, inhibitors and signalling pathways. Enzyme function is therefore inseparable from the physical and chemical environment in which the enzyme operates.
Many enzymes require cofactors or coenzymes. Cofactors can be inorganic ions such as magnesium, zinc or iron, or organic molecules often derived from vitamins. A protein component without its required cofactor may be inactive, while the complete functional enzyme is sometimes called a holoenzyme. Coenzymes can transfer chemical groups, electrons or atoms between reactions. Nicotinamide adenine dinucleotide participates in redox reactions, while coenzyme A carries acyl groups. Metal ions can stabilise charged intermediates or participate directly in catalytic chemistry. Some cofactors remain tightly bound, while others associate transiently during the catalytic cycle. These additional components expand the chemical capabilities of proteins beyond what amino acid side chains alone can accomplish.
Enzyme inhibitors reduce enzyme activity through different mechanisms. Competitive inhibitors bind to or compete for the active site and can often be overcome by increasing substrate concentration under the simplest kinetic conditions. Noncompetitive and mixed inhibition involve interactions at other sites or with different enzyme states and can affect catalytic capacity in more complex ways. Irreversible inhibitors form stable interactions that permanently or effectively disable enzyme molecules.
Allosteric regulators bind at sites distinct from the active site and change enzyme conformation or dynamics. Cells use inhibition naturally to control metabolic pathways, preventing excessive production of metabolites. Drugs also exploit enzyme inhibition, including medicines that target enzymes involved in blood clotting, cholesterol synthesis, infections and other diseases. Enzyme inhibition is therefore both a fundamental biological regulatory mechanism and a major principle of pharmacology.
Enzymes are used extensively outside living cells. Industrial biotechnology uses enzymes in food processing, detergents, textiles, paper production, biofuels and pharmaceutical manufacturing. Proteases help break down proteins, amylases break down starch, lipases act on fats and cellulases degrade cellulose. Enzymes can provide high selectivity under relatively mild temperatures and pressures, reducing the need for harsh chemical conditions. Their disadvantages include sensitivity to temperature and pH, possible inhibition and limited stability in some industrial environments. Modern biotechnology can improve enzyme performance through directed evolution, protein engineering and computational design. Enzymes illustrate the general principle of catalysis in an especially sophisticated biological form, combining molecular recognition, structural dynamics and chemical reactivity to accelerate essential reactions with remarkable specificity.
