ENZYMES. There are many chemical reactions which pro ceed alone at a very slow rate. By the addition of minute quan tities of certain inorganic substances, termed catalysts, the rate of such reactions can be tremendously increased. Enzymes may be regarded as catalysts of biological origin which are formed in all living cells. They enable the cell to carry out the chemical processes necessary for its existence at a sufficient speed and at temperatures much below those which would be required in the laboratory.
It is now realized that most of the changes going on in the living cell are due to constituents similar to the zymase of the yeast cell, which brings about the fermentation of sugar. In many cases the enzymes act within the cell, but in others the enzymes are discharged from the cells which formed them and carry out their particular changes outside the cells. Thus the enzyme ptyalin, elaborated in the salivary gland cells, is discharged into the mouth, where it commences its action on the starch of the food.
The action of all enzymes shows certain characteristic features which closely resemble those shown by inorganic catalysts. The chemical reactions in which enzymes play a part are, so far as is known, reversible in type. In such reactions, an equilibrium point is reached at which the rate of the forward or breakdown process equals that of the reverse or synthetic process. In the presence of an enzyme the same equilibrium point is reached much more rapidly. Since the actual point of equilibrium reached depends solely on the ratio of the velocities of the forward and reverse reactions, the enzyme must accelerate both reactions equally. Thus enzymes are capable of increasing the rate of synthetic as well as breakdown processes. Which change will predominate depends on the concentration of the molecules involved and is not deter mined by the enzyme.
Enzymes show great specificity--a given enzyme will act only on a single substance or group of closely related substances. In the case of enzymes acting on carbohydrates this specificity is so extreme that it has been suggested that there must be a "lock and key" relationship between the enzymes and the molecule on which it acts—the substrate.
The activity of enzymes is influenced in a remarkable degree by factors such as temperature, acidity or alkalinity, and the presence of certain metallic salts. There is an optimum temperature for the action of an enzyme ; above this temperature the enzyme is rapidly destroyed and its activity diminishes. Owing to this sensi tiveness to external factors, an enzyme is frequently able to change only a limited amount of substrate, for the products of its activity accumulate and by their accumulation gradually inhibit the activity of the enzyme.
Enzymes are active in extremely minute amounts. Thus the enzyme invertase can hydrolyse one million times its weight of cane sugar without appreciable loss of activity The majority of the reactions in which enzymes take part are hydrolytic, and involve the addition of the elements of water. Thus the enzymes of the alimentary canal break down the foodstuffs by hydrolysis. There are other types of enzyme action which are important, notably those concerned with oxidation processes. Enzymes ac celerating such processes are known as oxidases, and to enzymes of this class are due the colour changes seen when a cut potato or apple is exposed to the air.
The chemical structure of enzymes is not yet known. They were formerly regarded as proteins, but according to Euler and others they are not proteins, but are in colloidal combination with the cell proteins. Their sensitiveness to heat and other factors depends probably on this association. Willstatter conceives the molecule of an enzyme as consisting of a colloidal carrier and of a purely chemical active group. Complete separation of these is not yet possible. There is no doubt that, owing to its close asso ciation with a colloid, the enzyme molecule has an extensive surface area, and it appears certain that one of the first stages in the action of an enzyme is the concentration of molecules of the substrate on this surface by the physical process of adsorption. Whether a chemical combination between enzyme and substrate occurs has yet to be demonstrated.