100 - TheCel 



that the mixture froths like beer. Moreover, 

 it can be determined that none of the man- 

 ganese compound disappears from the test 

 tube, and consequently the same small sam- 

 ple of Mn0 2 can foster the decomposition of 

 peroxide in a quantity that is virtually with- 

 out limit. 



In the foregoing reaction, manganese diox- 

 ide may be recognized as a catalyst, and its 

 action upon the peroxide is a typical example 

 of catalysis. A catalyst is any reagent that 

 decelerates a chemical reaction without 

 affecting the end point, and without being 

 destroyed when the end [joint is reached. A 

 wide variety of catalysts are known in inor- 

 ganic chemistry, and there is an even greater 

 number of organic catalysts. 



ENZYMES: DEFINITION AND EXAMPLE 



Enzymes are organic (essentially protein) 

 catalysts produced by living cells. Typically 

 each cell contains some 2 to 3 thousand dif- 

 ferent enzymes and these enable the proto- 

 plasm to carry on a very complex and vigor- 

 ous traffic of chemical activity. 



Many enzymes can be extracted from the 

 protoplasm without impairing their activity. 

 For example, catalase, the peroxide-activat- 

 ing enzyme, can be obtained in dry crystalline 

 form practically devoid of contaminating im- 

 purities. Such a purified enzyme displays tre- 

 mendous activity. A small fraction of a gram 

 of catalase, dropped into a test tube contain- 

 ing peroxide solution, gives rise to a most 

 vigorous evolution ol oxygen. In fact, cata- 

 lase accelerates the decomposition of perox- 

 ide much more effectively than manganese 

 dioxide when equivalent concentrations of 

 the two catalysts are employed in the reaction 

 (p. 108). 



Small amounts of catalase are present in a 

 wide variety of animal and plant tissues, and 

 catalase activity can be demonstrated in many 

 crude preparations. Thus when peroxide is 

 poured upon an open wound, the frothing 

 of the solution indicates that catalase is 

 present in the blood and serum that oozes 



from the wound. Also, one may add the 

 scrapings from a raw potato to a peroxide 

 solution and observe the bubbling, which re- 

 sults from catalase liberated by the dam- 

 aged potato cells. 



General Importance of Enzymes. Virtually 

 all the many chemical reactions that sustain 

 the life of the cell are activated by enzymes. 

 Each different reaction generally depends 

 upon a different enzyme. Most of these fid- 

 fill their duties inside the cell, speeding the 

 reactions of metabolism, but some may be 

 extruded from the protoplasm to foster the 

 reactions of digestion. In other words, neither 

 metabolism nor digestion could occur in the 

 absence of the wide variety of enzymes that 

 are produced by every living cell. 



The Naming of Enzymes. Progress in the 

 field of enzymology has been very rapid since 

 1926, when James B. Sumner of Cornell Uni- 

 versity first succeeded in isolating pure crys- 

 tals of urease, the enzyme that catalyzes the 

 hydrolytic breakdown of urea, CO(N T H 2 ) 2 , 

 into carbon dioxide (CO..) and ammonia 

 (NH-j). This progress gave rise to the modern 

 system of enzyme nomenclature, which is 

 exemplified in Table 5-1. However, the 

 names of some enzymes, especially those of 

 the human digestive tract, do not conform to 

 the system. These enzymes were very familiar 

 before the modern terminology came into 

 use and hence their old names have tended 

 to persist (Table 5-1). 



In modern enzyme nomenclature, the 

 suffix -ase indicates the name of an enzyme 

 or group of enzymes. Specific individual en- 

 zymes are usually named according to the 

 principal substance, called the substrate, the 

 chemical activity of which is being catalyzed. 

 Thus the principal substrate of urease is 

 urea, and that of starch (Latin = amy him) is 

 amylase. However, not only the substrate but 

 also the type of reaction may be indicated 

 by the specific name, as in lactic dehydro- 

 genase, which catalyzes the splitting off of 

 hydrogen atoms from lactic acid (p. 101), ami 

 fructokinase, which catalyzes the phosphory- 

 lation of fructose (p. 101). 



