g6 GENERAL CONCEPTS 



indeed. Water is an excellent catalyst for many reactions. Pure, dry 

 hydrogen gas and dry chlorine gas do not react when mixed, but if a 

 slight trace of water is present they react with explosive violence to form 

 hydrogen chloride. Metals such as iron, nickel, platinum and palladium, 

 when ground into a fine powder, are widely used as catalysts in indus- 

 trial processes such as the hydrogenation of cottonseed and other vege- 

 table oils to make margarine or the cracking of petroleum to make 

 gasoline. A minute amount of catalyst will speed up the reaction of vast 

 quantities of reactants, for the molecules of catalyst are not exhausted 

 in the reaction but are used again and again. 



20. Enzymes 



The speed and specificity of the myriad chemical reactions that 

 occur in protoplasm are regulated by the catalysts called enzymes, pro- 

 duced by the cell. Man has used the fermenting of grape juice and the 

 souring of milk, which are enzymatic processes, for thousands of years. 

 Pasteur showed about 100 years ago that these processes occur only when 

 specific microorganisms are present and inferred that the enzymes (he 

 called them "ferments") were active catalysts only when they were a part 

 of the living cell. In his experiments he was unable to separate the active 

 catalysts from the living cell and concluded that enzymes were living 

 things which lost activity when separated from -the cell. Liebig, in con- 

 trast, believed that enzymes were simply complex organic compounds 

 that did not require a living cell in order to function, but he, too, was 

 unable to remove an enzyme from a cell and have it retain its activity. 

 Pasteur and Liebig had a classic, long-lasting argument over their diver- 

 gent views. The question was finally settled, after both Liebig and 

 Pasteur had died, when Eduard Buchner in 1897 extracted an enzyme 

 preparation from yeast which, though completely devoid of cells, was 

 able to decompose glucose. In the succeeding years, hundreds of other 

 enzymes have been extracted and shown to have their activity unim- 

 paired; some have been purified and prepared as pure crystalline sub- 

 stances. We can now define enzymes as organic catalysts which are 

 produced by living cells but which are active independently of the cell. 

 Enzyme-controlled reactions are basic to all the phenomena of life: 

 respiration, digestion, excretion, growth, muscle contraction, nerve con- 

 duction, and so on. There is no need to postulate some mysterious vital 

 force, as Pasteur did, to account for these phenomena. 



Properties of Enzyrries. AH of the enzymes that have been isolated 

 and crystallized to date have proved to be proteins. They are usually 

 colorless, but may be yellow, green, blue, brown or red. Most enzymes 

 are soluble in water or dilute salt solution, but some, for example the 

 enzymes located in the mitochondria, are bound together by lipoproteins 

 and are insoluble in water. Enzymes are usually named by adding the 

 suffix "-ase" to the name of the substance acted upon, called the sub- 

 strate. Thus, sucrose is split by the enzyme sucrase and urease is the 

 enzyme which attacks urea. 



The catalytic ability of enzymes is truly phenomenal; without them 



