ENZYMES 53 



comprise all those enzymes which bring about hydrolytic 

 reactions of various sorts. They are further divided into 

 the carbohydrases, the proteases, the lipases (attacking 

 fats), esterases, amidases, and so on. Their main function 

 is the breaking down of complex food materials, proteins, 

 polysaccharides and fats, into simpler units readily 

 utilisable by the organism for its nutrition. Generally 

 speaking, their action involves only very small energy 

 changes. Since their action is on more or less non- 

 diffusible substrates they would be virtually useless if 

 they were retained \vithin the cell, so Nature has decreed 

 that the hydrolases as a class shall be secreted into the 

 medium outside the cell ; they are extra-cellular or 

 Exo -enzymes. 



The other large group of enzymes comprises those 

 involved in the processes of respiration and metabolism. 

 They are kno^Ti as Desmolases. Most of the reactions 

 with which they are concerned involve considerable 

 energy changes and, in fact, it is their function to supply 

 the energy requirements of the cell. To this group belong 

 the oxidases and reductases, zymase (the system of 

 enzymes in yeast responsible for alcoholic fermentation), 

 catalase and other enzymes involved in anaerobic fer- 

 mentation. Their activity would not benefit the cell if 

 it were carried on outside its confines, and normally such 

 enzymes are held within the cell and are not liberated into 

 the surrounding medium unless the cell becomes damaged. 

 These enzymes are Endo-enzjrmes. 



Theories of Enzyme Action. — At present our ideas as 

 to the mode of action of enzymes are somewhat nebulous, 

 but depend, as is to be expected, on our conception of 

 the mechanism of catalysis in general. Catalysts may act 

 in two ways, either reacting chemically to give unstable 

 intermediate compounds which then break down Avith 

 formation of the end product and setting free the catalyst 

 again (as is the case with the oxides of nitrogen in the 

 manufacture of sulphuric acid), or the catalyst may act 



