Enzymes

ENZYMES. - The name e., which means "in yeast," from the Greek ἐν Ῥυμῇ, was proposed by Kühne in 1878. Together with vitamins and hormones, enzymes constitute the so-called "biocatalysts," substances that promote many physio-biological processes of great importance for life.

The first enzymatic action was discovered by Spallanzani in 1720, when he noticed, while studying the phenomena of digestion, that gastric juice had the property of digesting meat. In 1830 Dubrunfaut found that the extract of germinated barley transformed starch into maltose. In 1836 Schwann gave the name pepsin to the active principle contained in gastric juice capable of digesting meat, and a year later Woehler and Liebig called emulsin the active principle contained in preparations of bitter almonds that acts as the agent of the hydrolysis of amygdalin. Pasteur 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 which was later recognized to be constituted by several enzymes. Up until the moment when Buchner found zymase, enzymes were divided into figured enzymes (or ferments) (constituted by living cells) and non-figured ones (which act, like pepsin, without the need for the presence of a living cell). But after Buchner's discovery a new classification was reached, into intracellular enzymes, which are contained in cells from which they can be extracted, and extracellular enzymes, which flow out spontaneously from cells.

Duclaux proposed forming the name of the enzyme by adding the suffix -ase to the name of the compound on which the enzyme acts; for example, amylase, lipase, protease, etc., depending on whether the compound undergoing the enzymatic action is starch, fat, protein, etc. The names pepsin, emulsin, etc., remain as they are because they entered current usage before Duclaux's proposal. Generally speaking, hydrolyses are extracellular, and desmolases are intracellular. Hydrolyses are enzymes that preside over hydrolysis phenomena similar to those produced by acids and bases: by the addition of one or more molecules of water they break down the molecules on which they act into other simpler ones. Desmolases, on the other hand, bring about the breakdown of complex molecules into other simpler ones through reactions different from hydrolytic ones.

Enzymes are further divided into several subgroups: carbohydrases (action on sugars or carbohydrates), proteases (action on proteins), lipases (action on fats), glucosidases (action on glucosides), dehydrogenases and oxidases (action of dehydrogenation and oxidation), and others.

The activity of enzymes is regulated by several factors: the nature of the substrate on which the enzyme acts, the concentration of the enzymes themselves, temperature, pH. Temperature is of great importance: within certain limits an increase in temperature favors enzymatic action. Almost all enzymes are irreversibly destroyed at 80°, that is, without the possibility of reconstituting themselves; a few enzymes are not destroyed at 80°; examples are crystalline ribonuclease and trypsin crystallized in N/10 hydrochloric acid. The pH, that is, the concentration of hydrogen ions or more simply the degree of acidity or basicity of the environment in which the enzyme acts, is also of great importance. If in fact one exceeds or remains even slightly below the optimum pH, the enzyme loses its activity, at first slowly then more rapidly, until it becomes completely inactivated, usually reversibly at first, and then becoming definitively and irreversibly inactive.

Generally the enzyme is constituted by two parts: the coenzyme and the apoenzyme. The coenzyme is the stimulant of the apoenzyme, that is, it has the function of activator of the apoenzyme, which latter without the former would not be capable of carrying out any activity. The apoenzyme is generally a protein of colloidal nature and, therefore, of high molecular weight. The coenzyme, on the other hand, is constituted by one or more different substances, simple and complex. Enzymes, even in small quantities, are capable of catalyzing transformations of large quantities of substrate (v. CATALISI).

Enzymes are found in all living cells. The most important are distributed as follows: 1) pepsin, which, present only in gastric juice, hydrolyzes proteins into peptones and only in some cases down to amino acids. 2) Trypsin, present only in pancreatic juice after this has entered the intestine, that is, after the enterokinase of the intestinal mucosa has activated trypsinogen, transforming it into trypsin: unlike pepsin, trypsin cannot attack proteins in their native state but only after they have already been split into peptones, and it continues their hydrolysis down to amino acids. 3) Lipase, abundant in the pancreas but also present in the stomach, in fatty tissues and in the small intestine; other esterases are the phosphatases of serum, kidneys, intestinal epithelium, milk, yeast, etc., and cholinesterase, probably constituted by a mixture of two enzymes, one specific and one non-specific, which are found sometimes separate and sometimes united, in the pancreas, liver, serum and nervous tissues. The action of this enzyme is to hydrolyze acetylcholine: this is a process directly connected to the transmission of nerve impulses. 4) The carbohydrases, which regulate the fermentative processes of sugars: lysozyme belongs to them, which, present in the nasal mucosa, has the property of destroying certain bacteria. 5) Amylase, which hydrolyzes starch and abounds in the pancreas and salivary glands but is also present in the blood, in muscles and in other tissues. Amylase and the carbohydrases also have great industrial importance, especially in the production of alcohol from starch.

Some enzymes have a vitamin function and conversely some vitamins have enzymatic action. This, for example, happens for the yellow enzyme, for ribonuclease, for ascorbic acid oxidase and for vitamin B conjugase.

Enzymes can be considered as the regulators of all biological functions of animal and plant organisms. It is truly interesting and wonderful to observe how these substances regulate and lead to the final result the biochemical reactions without appearing among the products of the reaction itself, and how from the harmonious concatenation of numerous and successive enzymatic actions there result complicated biological processes of high importance for life and for industry: glycolysis or lactic fermentation carried out by muscle cells is a fine example of a succession of enzymatic phenomena and of their orderly concatenation for the purposes of muscle contraction.

Filippo Dentico di Accadia