Zymase is the active alcoholic fermentation enzyme of yeasts. It accelerates the well-known reaction for the conversion of hexose sugars into alcohol and carbon dioxide, namely,
C6H12O6 = 2C2H5OH + 2CO2.
Because of its scientific interest and industrial importance in the fermentation industries, its action has been extensively studied. It acts only in the presence of soluble phosphates and of a coenzyme (see below) which is dialyzable and not destroyed, which is probably an organic ester of phosphoric acid. The significance of the molecular configuration of the hexose sugars in their susceptibility to action by zymase has already been discussed in detail (see [page 56]).
The optimum temperature for zymase action is 28° to 30°. The enzyme is killed by heating to 45° to 50° in solution, or to 85° if in dry preparation.
Proteases of the erepsin type, i.e., those which break proteins down to amino-acids instead of only to the proteose or peptone stage, as is characteristic of the enzymes of the trypsin type, are widely distributed in plants. Except in the case of the two which occur in large amounts in certain special fruits (papain in papaws, and bromelin in pineapples), they are very difficult to prepare in pure form for study. In general, all proteolytic actions, even when accelerated by active enzymes, proceed much more slowly than do the hydrolyses of carbohydrates or fats. It seems that metabolic changes of the complex protein molecules are much more difficult to bring about and take place much more slowly than do those of the energy-producing types of compounds.
The presence of proteolytic enzymes in most vegetative cells, and in seeds, may be demonstrated, however, by studying the action of extracts of these tissues upon soluble proteins. The best-known example of this type of enzymes is the protease of yeast; but similar ones may be found in germinating seeds. These vegetable proteases are usually most active in neutral or only faintly alkaline solutions, and their activity is nearly always inhibited by even traces of free acids.
Most laboratory studies of proteolytic enzymes are carried on with preparations of the powerful members of this class of enzymes which are found in the digestive tract of animals, namely, the pepsin of the gastric juice, which acts in the acid medium, in the stomach, and the trypsin of the pancreatic juice, which acts in the alkaline medium of the intestinal tract. But even these powerful proteases require several hours for the transformation of an amount of soluble albumin into its amino-acid constituents which is equivalent to the amount of starch which is hydrolyzed to maltose by diastase in a very few minutes.
Enzymes which govern oxidative changes, known respectively, as catalases and oxidases, are almost universally present in plants. Catalase decomposes peroxides, with the liberation of free oxygen. It is, therefore, necessary to the final step in the process of photosynthesis, as elucidated by Usher and Priestley (see [page 26]), and serves to prevent the destructive action of hydrogen peroxide upon chlorophyll. The almost universal presence of oxidases in plant tissues has been repeatedly demonstrated. They are present in especially large amounts in tissues which are being acted upon by parasitic fungi or are combating unfavorable conditions of growth. The oxidases, in such cases, seem to be the agents by which the plant is able to stimulate its metabolic activities to overcome the unfavorable environment for its normal development.
In vegetables and fruits, the common browning, or blackening, of the tissues when cut surfaces are exposed to the air has been demonstrated to be due to the catalytic oxidation of the tannins or of certain amino-acids, especially tyrosine, under the influence of the oxidases which are present in the tissues. In fact, most pigmentation phenomena are due to changes in the oxygen content of the chromogens of the cells of the plant, under the influence of the oxidases which are present in the protoplasm of the cells in question. Hence, the oxidases may be said to be the controlling agencies for both the energy-absorbing activities and for respiration in plants.