Sometimes as active enzymes, sometimes as pro-enzymes or zymogens which are converted into enzymes outside the cell by acids, other inorganic substances or other enzymes.
3. They produce very great chemical changes without themselves being appreciably affected.
Enzymes will not continue to act indefinitely, but are used up in the process (combination with products?). The amount of change is so great in proportion to the amount of enzyme that the above statement is justified in the relative sense. Thus a milk-curdling enzyme has been prepared that would precipitate 100,000,000 times its own weight of caseinogen.
4. Their action is specific in that each enzyme acts on one kind of chemical substance only, and the products are always the same.
The substance may be combined with a variety of other chemical substances so that the action appears to be on several, but in reality it is on a definite group of molecules in each instance. For example, emulsin attacks several different glucosides but always sets free dextrose from them.
5. The action is inhibited and eventually stopped, and in some cases the enzyme is destroyed by an accumulation of the products of the action. If the products are removed, the action will continue, if the enzyme is not destroyed. This effect is explained partly because the enzyme probably combines with some of the products, since it does not act indefinitely, and partly because of the reversibility of the reaction.
6. Like many chemical reactions those of enzymes are reversible, that is, the substance broken up may be reformed by it from the products produced in many instances. Thus:
maltose + maltase ⇆ glucose + glucose + maltase.
fat + lipase ⇆ glycerin + fatty acid + lipase.
7. The presence of certain mineral salts seems to be essential for their action. These and other substances which are necessary are sometimes called co-enzymes. A salt of calcium is most favorable for a great many.