ENZYMES AND THEIR ACTION
The characteristic difference between the reactions of inorganic compounds and those of organic substances lies in the rapidity, or velocity, of the chemical changes involved. Speaking generally chemical reactions take place between substances which are in solution, so that they may come into sufficiently intimate contact that chemical action between them can take place. There are, of course, occasional examples of reactions between dry solids, such as the explosion of gunpowder, etc., but the general rule is that reacting materials must be in either colloidal or true solutions.
Inorganic materials, when dissolved in water, usually ionize very readily. That is, they are not only disintegrated into individual molecules, but a considerable proportion of these molecules separate into their constituent ions. When solutions containing ionized compounds are brought together, conditions for chemical interaction are ideal, and the reaction proceeds with such tremendous rapidity as to be completed almost instantaneously, in most cases.
Organic compounds, on the other hand, ionize only very slowly, if at all. Hence, reactions between organic compounds, even when they are in solution, proceed very slowly unless carried on at high temperatures, under increased pressure, or under the influence of some catalytic agent. Even under the stimulation of these reaction-accelerating agencies, most chemical changes in organic compounds when carried on in the laboratory, require several hours or even days and sometimes weeks, for their completion. But when similar reactions take place in living organisms, they proceed with velocities which resemble those of inorganic compounds in the laboratory. This difference between the velocity of organic reactions when carried on under artificial conditions in the laboratory (often spoken of as "in vitro") as compared with that of the same reactions when they take place in a living organism ("in vivo"), is due to the universal presence in the living protoplasm of certain organic catalysts, known as enzymes.
ENZYMES AS CATALYSTS
The phenomenon known as "catalysis" is of common occurrence in both inorganic and organic chemistry. The effect of a small amount of manganese dioxide in aiding in the liberation of oxygen from potassium chlorate is an example which is familiar to all students of elementary chemistry. Similarly, spongy platinum accelerates the oxidation of sulfur dioxide to sulfur trioxide, in the commercial manufacture of sulfuric acid. Again, the hydrolysis of sucrose into fructose and glucose proceeds very slowly in the presence of water alone, but if a little hydrochloric acid or sulfuric acid be added to the solution, the velocity of the hydrolysis is enormously accelerated. Many other examples of the accelerating effect of various chemicals upon reactions into which they do not themselves enter, might be cited.
The essential features of all such catalytic actions are: (1) the velocity of the reaction is greatly altered, usually accelerated; (2) the catalytic agent does not appear as one of the initial substances, or end-products, of the reaction, and is not itself altered by the chemical change which is taking place; (3) the accelerating effect is directly proportional to the amount of the catalyst which is present; (4) relatively small amounts of the catalyst produce very large results in the reacting mixture; and (5) the catalysts cannot themselves initiate reactions, but only influence the velocity of reactions which would otherwise take place at a different rate (usually much more slowly) in the absence of any catalytic agent.
Enzymes conform to all of these properties of catalysts, and are commonly defined as the "catalysts of living matter." They are almost universally present in living organs of every kind, and perform exceedingly important functions, both in the building-up of synthetic materials and in the rendering soluble of the food of both plants and animals, so that it can be translocated from place to place through the tissues of the organism.
Enzymes differ from inorganic catalysts in being destroyed by heat, in not always carrying the reaction to the same stage as does the inorganic catalyst which may accelerate the same reaction, and in producing different changes in the same substance by different enzymes.
The name "enzyme" comes from Greek words meaning "in yeast," as the nature and effect of the enzyme involved in the alcoholic fermentation of sugars by yeast were those which were first recognized and understood. It was at first thought, by Pasteur and his students, that fermentation is the direct result of the life activities of the yeast plant. Later, it was found that water extracts from sprouted barley, from almond seeds, and from the stomach, pancreas, etc., were able to bring about the decomposition of starch, of amygdalin, and of proteins, respectively, in a way which seemed to be quite comparable to the fermentative action of yeasts. Hence, it was thought that there were two varieties of active agents of this kind, one composed of living cells and the other non-living chemical compounds, and these were called the "organized ferments" and the "unorganized ferments," respectively. However, in 1897, Büchner found that by grinding yeast cells with sharp sand until they were completely disintegrated and then submitting the mass to hydraulic pressure, he could obtain a clear liquid, entirely free from living cells, which was just as active in producing fermentation as was the yeast itself. This discovery paved the way for a long series of investigations, which have conclusively demonstrated that there is no distinction between "organized" and "unorganized" ferments, that all living organisms perform their characteristic functions by means of the enzymes which they contain, and that these enzymes can bring about their characteristic catalytic effects outside the cell, or tissue which elaborates them, just as well as within it, provided only that the conditions of temperature, acidity or alkalinity of the medium, etc., are suitable for the particular enzyme action which is under consideration.