Zymase

ZYMASE (from the Greek zyme—yeast), a name proposed by E. Buchner to designate the enzyme of yeast that causes fermentation. At present, it is known that fermentation is caused by the action not of a single enzyme, but by the sequential participation of a whole series of them. Nevertheless, the term zymase is still retained today to designate the entire complex of enzymes participating in the anaerobic breakdown of carbohydrates. Animal tissues and cells also possess the ability to anaerobically break down carbohydrates; therefore, one also speaks of "animal zymase." The zymase complex includes: 1) Hexases, enzymes that are still completely unstudied, the function of which apparently consists in converting ordinary forms of carbohydrates into more labile, "alloiomorphic" isomers, thereby preparing them for further breakdown. 2) Phosphatases, participating in the processes of formation and breakdown of carbohydrate esters of phosphoric acid (see Hexosephosphoric acids). 3) Oxidoreductases, enzymes causing numerous oxidation-reduction processes that lie at the basis of fermentation, including the formation of one of the final products of fermentation, alcohol, by way of the reduction of acetaldehyde. 4) Carboxylase, causing the formation of the second final product, CO₂, by way of the decarboxylation of pyruvic acid. Zymase was first obtained by Buchner by squeezing cell juice from yeast under a pressure of about 300 atmospheres (see Buchner press). At present, the so-called maceration juice (A. Lebedev) is more often used: yeast, washed, pressed, and dried for two days at 25–30° (preferably bottom fermentation yeast), is infused for 2 hours with three volumes of water at 35°, then filtered through an ordinary filter. The activity of the juice is preserved for no more than 1–2 days, because zymase is rapidly destroyed by the proteolytic enzyme of the yeast, which also passes into the juice. Zymase exhibits its action within a wide range of pH, between pH = 3 and up to pH = 7, with an optimum at pH = 4–5. Upon dialysis of the zymase solution or upon washing dry yeast with water, zymase loses its activity; the addition of boiled yeast juice restores the latter. Neuberg proposed calling the thermostable, non-diffusible enzyme complex apozymase, and the thermostable, diffusing substance is called cozymase. Only apozymase and cozymase together provide a complete zymase capable of causing the breakdown of hexoses. The participation of cozymase is necessary in the first stages of the process, in particular during the phosphorylation of the carbohydrate. Phosphorylated carbohydrates, as well as further intermediate products like pyruvic acid, are fermented even in the absence of cozymase, by apozymase alone. Judging by the latest works of Euler, cozymase is an adenine nucleotide. It is widely distributed in various tissues and cells of both plant and animal organisms; in particular, it is rich in muscles; it has been found in erythrocytes and in cancerous tumors. Animal zymase is in many respects similar to yeast zymase. Here, too, the presence of cozymase is necessary, and phosphatases and oxidation-reduction enzymes (ketone-aldehyde mutase) participate; the main difference consists in the fact that the final product of the action of oxidoreductases here is not alcohol, as in yeast fermentation, but lactic acid (see Glycolysis).