Enzyme

Enzymes are proteins that function as biological catalysts, accelerating chemical reactions essential for life. They act on molecules known as substrates and convert them into products through highly specific interactions within their active sites. Nearly all metabolic pathways in living cells require enzyme catalysis for reactions to proceed at rates sufficient to sustain biological processes. The study of these proteins is known as enzymology, and a related field examines pseudoenzymes, which resemble enzymes in structure but have lost catalytic function through evolutionary change.

Fundamental Characteristics and Catalytic Properties

Enzymes exhibit remarkable chemical specificity, derived from their unique three-dimensional structures. Their active sites precisely accommodate substrates, ensuring that only certain molecules participate in a given reaction. By lowering the activation energy, enzymes significantly increase reaction rates. Some enzymes accelerate reactions by many millions of times, with extreme cases reducing reaction times from geological timescales to milliseconds.
Enzymes are not consumed during reactions and do not alter the equilibrium position, distinguishing them as true catalysts. Their activity is strongly influenced by temperature, pH, and the presence of other molecules. Inhibitors reduce enzymatic activity, while activators enhance it. Excessive heat may cause denaturation, permanently altering an enzyme’s structure and disabling its catalytic function.
Although enzymes are proteins, other biological catalysts exist. Catalytic RNA molecules, known as ribozymes, perform specific reactions in processes such as RNA splicing. In addition, certain biomolecular condensates also display catalytic abilities, forming a third category of natural biocatalysts.
Enzymes have numerous practical applications. They are used industrially in antibiotic synthesis and widely in household products such as detergents, where proteases, amylases, and lipases break down stains. Papain, an enzyme from papaya, is used to tenderise meat by breaking down proteins.

Etymology and Historical Development

The earliest observations of enzymatic processes date back to the seventeenth and eighteenth centuries, when scientists noted the digestion of meat by gastric juices and the conversion of starch to sugar by plant extracts and saliva. However, the underlying mechanisms remained unknown.
In 1833, Anselme Payen isolated diastase, the first enzyme to be identified. Later in the nineteenth century, Louis Pasteur studied the fermentation of sugar by yeast and proposed that a vital force within living cells, termed “ferments”, caused these reactions. He believed such processes could occur only in living organisms.
In 1877, Wilhelm Kühne introduced the term enzyme, derived from Greek, to describe ferments capable of acting outside living cells. This distinction separated non-living catalytic substances such as pepsin from cellular ferments.
A major breakthrough occurred in 1897 when Eduard Buchner demonstrated cell-free fermentation, showing that yeast extracts could ferment sugar without intact cells. This discovery established that enzymes themselves, rather than living organisms, carried out catalysis. Buchner named the sugar-fermenting enzyme “zymase” and later received the Nobel Prize in Chemistry for this work.
Naming conventions for enzymes evolved in the late nineteenth and early twentieth centuries. The practice of adding the suffix –ase to substrate names or reaction types became standard, producing names such as lactase and DNA polymerase. Despite these advances, the chemical identity of enzymes remained uncertain for many years.
In 1926, James B. Sumner crystallised urease, proving that enzymes could be pure proteins. Subsequent studies by John Howard Northrop and Wendell Meredith Stanley on digestive enzymes reinforced this conclusion. The crystallisation of enzymes made it possible to determine their structures by X-ray crystallography, beginning with lysozyme in 1965, which marked the emergence of modern structural biology.

Classification and Nomenclature

Enzymes can be classified on the basis of either sequence similarity, which reflects evolutionary relationships, or catalytic activity, which describes the reaction they perform. The International Union of Biochemistry and Molecular Biology established a systematic system of enzyme nomenclature, assigning each enzyme an Enzyme Commission (EC) number. This four-part numerical code categorises enzymes by reaction type, including:

• EC 1 – Oxidoreductases: catalyse oxidation–reduction reactions.
• EC 2 – Transferases: transfer functional groups such as methyl or phosphate groups.
• EC 3 – Hydrolases: catalyse hydrolysis of chemical bonds.
• EC 4 – Lyases: cleave bonds by means other than hydrolysis and oxidation.
• EC 5 – Isomerases: catalyse structural rearrangements within molecules.
• EC 6 – Ligases: join molecules together with covalent bonds.
• EC 7 – Translocases: transport ions or molecules across membranes or cause their separation within membranes.

An enzyme’s full EC number specifies its function with increasing precision from the first to the fourth digit. For example, hexokinase is designated EC 2.7.1.1, identifying it as a transferase that adds a phosphate group to a hexose sugar.
Enzyme activity classifications do not necessarily correspond to similarities in amino acid sequences. Genes encoding enzymes that catalyse identical reactions can differ greatly in sequence and evolutionary origin. These unrelated catalytic proteins are examples of non-homologous isofunctional enzymes. Sequence-based classifications group proteins into families that reflect shared ancestry and structural features, documented across protein family databases.