Chapter 10 



ENZYMES 



by G. W. E. PL ALT 



Assistant Professor, Institute for Enzyme Research 

 University of Wisconsin 



Enzymes may be defined as thermolabile organic catalysts elaborated 

 by living cells and capable of exerting their effects independently of 

 these cells. Certain topics, especially those concerned with digestion and 

 metabolism (Chaps. 11-16) , will necessitate mention of these biocatalysts ; 

 but nothing will be said there regarding their chemical nature, their 

 mode of action, factors affecting their rate of action, and their other 

 properties. 



Occurrence 



Great numbers of enzymes can be detected in all hving cells. If one 

 considers the quantity and diversity of enzymes present in a cell, it be- 

 comes evident that the cell contents must consist largely of enzymes. 

 Enzymes, such as oxidative enzymes, functioning normally within the 

 cell are usually called endo-enzymcs. If the usual site of action is out- 

 side the cell, as is the case with those involved in digestion, the enzymes 

 are designated exo-enzymes. 



Chemical nature 



All enzymes that have been obtained in a high degree of purity are 

 proteins. Many such enzymes have been obtained in the crystalline 

 state, e.g., urease, catalase, pepsin, trypsin, carboxypeptidase, a- and 

 ^-amylases, yellow enzyme, ribonuclease, aldolase, and alcohol-, lactic-, 

 and phosphoglyceraldehyde dehydrogenases. Such enzymes are similar 

 to other proteins in elementary composition, amino acid content, and prop- 

 erties, e.g., color tests, solubility, isoelectric point, thermolability, etc. 

 For example, aldolase recrystallized four to six times was found by Velick 

 and Ronzoni to consist of 18 amino acid residues. Complete accounting 

 of the nitrogen was obtained in the amino acid residues. The number 

 of residues per mole of aldolase (1267) was calculated, and from these 

 data the amino acid formula of aldolase could be expressed as Glyios 



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