An enzyme is a biological catalyst that increases the rate of a chemical reaction without being consumed in the overall process. Most enzymes are proteins, although some RNA molecules also possess catalytic activity. Enzymes enable reactions to proceed rapidly under conditions compatible with life and organize the chemical processes of cells. Their collective activities support metabolism, growth, reproduction, and the maintenance of biological structures. The study of enzymes is a central field of biochemistry. (ncbi.nlm.nih.gov)
Structure and substrate recognition
Protein enzymes consist of chains of amino acids folded into three-dimensional structures. A relatively small region, the active site, binds the reactant or reactants, called substrates, and provides the chemical environment required for catalysis. Residues brought together by protein folding may participate in binding or chemical transformation even when they are widely separated along the protein sequence. (ncbi.nlm.nih.gov)
Enzymes exhibit specificity for particular substrates and reactions, although this specificity is not always absolute. Recognition depends on shape, charge, and other molecular properties. The historical lock-and-key model describes a substrate fitting a complementary active site; the induced-fit model additionally recognizes that binding can alter the enzyme’s conformation. These changes help position reactive groups for the subsequent reaction. (ncbi.nlm.nih.gov)
Catalytic RNA molecules are called ribozymes. Examples include RNA involved in precursor-RNA processing. RNA also forms the catalytic center responsible for peptide-bond formation in the ribosome, demonstrating that biological catalysis is not restricted to proteins. (nobelprize.org)
Catalytic mechanism
Enzymatic catalysis provides a reaction pathway with a lower activation free-energy barrier. Enzymes can orient substrates, facilitate proton transfer, form temporary covalent intermediates, or use metal ions to assist chemical transformations. A central principle is stabilization of the transition state relative to the reactants. After product release, the enzyme can undertake another catalytic cycle. (ncbi.nlm.nih.gov)
An enzyme does not change the overall Gibbs free-energy difference between reactants and products or the position of chemical equilibrium. It accelerates the approach to equilibrium rather than making an otherwise unfavorable reaction favorable. Cells can drive unfavorable transformations by coupling them to favorable reactions, such as the breakdown of adenosine triphosphate (ATP). (ncbi.nlm.nih.gov)
Some enzymes require a cofactor, a nonprotein component necessary for activity. Cofactors include inorganic ions and organic molecules called coenzymes. They may help transfer electrons or chemical groups, or support the active site’s catalytic chemistry. Their availability can therefore influence enzyme function. (openstax.org)
Kinetics and environmental conditions
Enzyme kinetics examines how reaction rates depend on substrate concentration, enzyme concentration, and experimental conditions. Many simple enzyme-catalyzed reactions approximately follow Michaelis–Menten kinetics:
[ v_0=\frac{V_{\max}[S]}{K_m+[S]}, ]
where (v_0) is the initial reaction rate, ([S]) is substrate concentration, and (V_{\max}) is the limiting rate at saturating substrate concentration. The Michaelis constant, (K_m), is the substrate concentration at which the rate equals half of (V_{\max}). This equation applies under defined assumptions and does not describe every enzyme system. (ncbi.nlm.nih.gov)
At low substrate concentrations, increasing substrate generally increases reaction rate substantially. As active sites become occupied more frequently, the rate approaches saturation. The turnover number, (k_{\mathrm{cat}}), expresses the maximum number of substrate molecules converted per catalytic site per unit time. These kinetic measurements characterize an enzyme under specified conditions rather than independently of its environment. (ncbi.nlm.nih.gov)
Temperature and pH affect catalytic activity. Warming often increases reaction rates, but sufficiently high temperatures can disrupt protein structure and reduce activity. Changes in pH alter the ionization of substrates and active-site residues; extreme conditions may cause denaturation. Optimal conditions differ among enzymes and depend partly on how activity is measured. (openstax.org)
Regulation and inhibition
Cells regulate enzymes to coordinate metabolic pathways. Molecules binding outside the active site can change activity through allosteric regulation. In feedback inhibition, a pathway’s product inhibits an earlier enzyme, helping prevent unnecessary accumulation. Enzyme activity can also be controlled by reversible chemical modifications or by changing enzyme abundance. (ncbi.nlm.nih.gov)
Enzyme inhibition reduces catalytic activity. Competitive inhibitors compete with substrates for binding, commonly at the active site. Other inhibitors interact with different enzyme states or sites and alter catalysis through different mechanisms. Allosteric regulation and kinetic inhibition categories are related but are not interchangeable descriptions. (ncbi.nlm.nih.gov)
Classification and nomenclature
The International Union of Biochemistry and Molecular Biology classifies enzymes by the reactions they catalyze, using four-part Enzyme Commission numbers. The seven main classes are oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases, and translocases. They cover oxidation–reduction, group transfer, hydrolysis, nonhydrolytic cleavage or addition, rearrangement, molecular joining, and catalytically coupled translocation, respectively. Translocases became a separate class in 2018. An EC number identifies an enzyme activity, not a unique protein sequence. (iubmb.org)
Historical development and applications
In 1897, Eduard Buchner demonstrated that cell-free yeast extracts could carry out alcoholic fermentation, establishing that this process did not require intact living cells. In 1926, James B. Sumner crystallized urease, providing evidence that enzymes could be proteins. The later discovery of catalytic RNA extended the molecular definition of an enzyme. (nobelprize.org)
Enzymes are important experimental and manufacturing tools. Polymerase chain reaction uses DNA polymerases, including heat-resistant Taq polymerase, to amplify selected DNA segments. Industrial biocatalysis uses enzymes to produce chemical compounds, including pharmaceutical intermediates. Directed evolution modifies enzymes through repeated cycles of genetic variation and selection or screening, producing catalysts adapted to particular substrates and processing conditions. (genome.gov)