The most prominent characteristic of enzymes is that they bring about very great chemical changes without themselves being appreciably affected. This property is also shown by many inorganic substances which are spoken of as “catalytic agents” or “catalyzers” so that enzymes are sometimes called “organic catalyzers.” The function of catalytic agents seems to be to hasten the rate of a reaction which would occur spontaneously, though in a great many cases with extreme slowness.
Just how enzymes act is not certain and probably will not be until their composition and constitution are known. Most probably they form a combination with the substance acted on (the substrate) as a result of which there is a rearrangement of the atoms in such a way that new compounds are formed, nearly always at least two, and the enzyme is at the same time set free. It is rather remarkable that chiefly optically active substances are split up by enzymes and where two modifications exist it is usually the dextro-rotatory one which is attacked. No single enzyme attacks both. This probably means that the structure of the enzyme corresponds to that of the substrate, “fits it as a key fits a lock,” as Emil Fischer says.
The production of enzymes is by no means restricted to bacteria since all kinds of living cells that have been investigated have been shown to produce them and presumably all living cells do. Hence the number of different kinds of enzymes and of substances acted upon is practically unlimited. Nevertheless they may be grouped into a comparatively few classes based on the general character of the change brought about by them.
I. Class I is the so-called “splitting” enzymes whose action is for the most part hydrolytic, that is, the substance takes up water and then splits into compounds that were apparently constituents of the original molecule. As examples may be mentioned diastase, the enzyme first discovered, which changes starch into a malt-sugar, hence is more commonly called amylase[16] (starch-splitting enzyme); invertase,[16] which splits cane-sugar into dextrose and levulose: C12H22O11 + H2O = C6H12O6 + C6H12O6. Lipase[16] or a fat-splitting enzyme, which decomposes fat into glycerin and fatty acid:
Fat
C3H5(OCnH2n-1O)3 + 3H2O = Glycerin
C3H5(OH)3 + Fatty acid
3CnH2nO2.
Proteases, which split up proteins into proteoses and peptones.
Other classes of “splitting enzymes” break up the products of complex protein decomposition, such as proteoses, peptones and amino-acids. A variety of the “splitting enzymes” is the group of
“Coagulases” or coagulating enzymes as the rennet (lab, chymosin) which curdles milk; fibrin ferment (thrombin, thrombase) which causes the coagulation of blood. These apparently act by splitting up a substance in the fluids mentioned, after which splitting one of the new products formed combines with other compounds present (usually a mineral salt, and in the cases mentioned a calcium salt) to form an insoluble compound, the curd or coagulum.
Another variety is the “activating” enzymes or “kinases” such as the enterokinase of the intestine. The action here is a splitting of the zymogen or mother substance or form in which the enzyme is built up by the cell so as to liberate the active enzyme.
Of a character quite distinct, from the splitting enzymes are