=Sucrase= (or invertase) is present in almost all species of yeasts, where it serves to convert unfermentable sucrose into glucose and fructose, which are readily fermentable. Invertase is also present in moulds and other microorganisms; and in the buds, leaves, flowers, and rootlets of those higher order plants which store their carbohydrate reserves in the form of sucrose. It appears that sucrose, while easily soluble, is not readily translocated, or utilized, by plants until after it has been hydrolyzed into its constituent hexoses. The optimum temperature for invertase is 50° to 54°; it is killed if heated, in the moist condition, to 70°. Its activity is increased by the presence of small amounts of free acids; but is inhibited by free alkalies. =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, C_{6}H_{12}O_{6} = 2C_{2}H_{5}OH + 2CO_{2}. 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.