ENZIMI. — The name e., meaning “in the yeast,” from the Greek ἐν Ῥυμῇ, was proposed by Kühne in 1878. Together with vitamins and hormones, e. constitute the so-called “biocatalysts,” substances
that promote many physiobiological processes of great importance for life.
The first enzymatic action was discovered by Spallanzani in 1720, when, while studying the phenomena of digestion, he noticed that gastric juice possessed the property of digesting meat. In 1830 Dubrunfaut found that an extract of germinated barley transformed starch into maltose. In 1836 Schwann gave the name pepsin to the active principle contained in gastric juice and capable of digesting meat, and a year later Woehler and Liebig called emulsin the active principle contained in almond preparations, acting in the hydrolysis of amygdalin. Insteur had discovered and demonstrated that fermentations are the work of microorganisms and that alcoholic fermentation was due to yeasts. But in 1897 Buchner succeeded in obtaining alcoholic fermentation by means of a substance extracted from yeast, a substance that was called zymase and was subsequently recognized as consisting of several e. Until Buchner found zymase, e. were divided into figurate e. (or ferments), consisting of living cells, and non-figurate e., which act, like pepsin, without requiring the presence of a living cell. But after Buchner’s discovery, a new classification was reached: intracellular e., contained within cells and extractable from them, and extracellular e., which flow spontaneously out of cells.
Duclaux proposed forming the name of the enzyme by adding the ending -ase to the name of the compound upon which the e. acts; for example, amylase, lipase, protease, etc., according as the compound undergoing enzymatic action is starch, fat, protein, etc. The names pepsin, emulsin, etc., remain unchanged because they had entered common usage before Duclaux’s proposal. In general, hydrolases are extracellular and desmolases intracellular. Hydrolases are e. that govern phenomena of hydrolysis similar to those produced by acids and bases: by adding one or more molecules of water, they split the molecules upon which they act into other, simpler ones. Desmolases, on the other hand, bring about the breakdown of complex molecules into simpler ones through reactions different from hydrolytic reactions.
The e. are further divided into various subgroups: carbohydrases (action on sugars or carbohydrates), proteases (action on proteins), lipases (action on fats), glucosidases (action on glucosides), dehydrogenases and oxidases (dehydrogenation and oxidation), and others.
The activity of e. is regulated by various factors: the nature of the substrate upon which the e. acts, the concentration of the e. itself, temperature, and pH. Temperature is of great importance: within certain limits, an increase in temperature promotes enzymatic action. Almost all e. are irreversibly destroyed at 80°, that is, without any possibility of being reconstituted; a few e. are not destroyed at 80°; examples include crystalline ribonuclease and trypsin crystallized in N/10 hydrochloric acid. pH, that is, the concentration of hydrogen ions, or more simply the degree of acidity or basicity of the environment in which the e. acts, is also of great importance. If one exceeds, or falls even slightly below, the optimum pH, the e. loses its activity, at first slowly and then more rapidly, until it becomes completely inactive, usually reversibly at first, before subsequently becoming definitively and irreversibly inactive.
Generally, the e. consists of two parts: the coenzyme and the apoenzyme. The coenzyme is the stimulant of the apoenzyme; that is, it functions as an activator of the apoenzyme, which without the former would be incapable of carrying out any activity. The apoenzyme is generally a colloidal protein and therefore has a high molecular weight. The coenzyme, on the other hand, consists of one or more different simple and complex substances. Even in small quantities, e. are capable of catalyzing transformations of large quantities of substrate (v. CATALISI).
E. are found in all living cells. The most important are distributed as follows: 1) pepsin, present only in gastric juice, hydrolyzes proteins into peptones and, in some cases only, as far as amino acids. 2) Trypsin, present only in pancreatic juice after the latter has entered the intestine—that is, after enteropeptidase of the intestinal mucosa has activated trypsinogen by transforming it into trypsin—differs from pepsin in that it cannot attack proteins in their native state, but only after they have already been split into peptones; it then continues their hydrolysis as far as amino acids. 3) Lipase, abundant in the pancreas but also present in the stomach, fatty tissues, and small intestine; other esterases are the phosphatases of the liver, kidneys, intestinal epithelium, milk, yeast, etc., and cholinesterase, probably consisting of a mixture of two e., one specific and one nonspecific, which are sometimes found separately and sometimes together, in the pancreas, liver, serum, and nervous tissues. The action of this e. is to hydrolyze acetylcholine; this is a process directly connected with the transmission of nerve impulses. 4) Carbohydrases, which regulate the fermentative processes of sugars: lysozyme belongs to this group; present in the nasal mucosa, it has the property of destroying certain bacteria. 5) Amylase, which hydrolyzes starch and is abundant in the pancreas and salivary glands, but is also present in the blood, muscles, and other tissues. Amylase and carbohydrases are also of great industrial importance, especially in the production of alcohol from starch.
Some e. have a vitamin function, and conversely some vitamins have enzymatic action. This occurs, for example, with the yellow enzyme, ribonuclease, ascorbic-acid oxidase, and the conjugase of vitamin B.
E. may be regarded as the regulators of all the biological functions of animal and plant organisms. It is truly interesting and marvelous to observe how these substances regulate biochemical reactions and bring them to their final result without appearing among the products of the reaction itself, and how, from the harmonious concatenation of numerous successive enzymatic actions, complicated biological processes of great importance for life and industry result: glycolysis, or lactic fermentation, carried out by muscle cells, is a fine example of the succession of enzymatic phenomena and of their orderly concatenation for the purposes of muscular contraction.