ENZYMES 155 



Isomerizing Enzymes 



These enzymes fall into two categories, those inducing changes in 

 the configurations of carbon atoms and those shifting groups from 

 one carbon atom of the substrate to another. The interconversion of 

 the stereoisomers of alanine and the mutarotation of glucose are 

 examples of the first type. 



alanine 



D-Alanine ' L-alanine 



racemase 

 niutarotase 



a-D-Glucose 7 /3-D-glucose 



Among the second group of isomerizing enzymes is found the reaction 



HOCH2COCH20P03= ' OHCCHOHCHsOPOs^ 



dihydroxyacetone triosephosphate isonierase 3-phosphoglycer- 



phosphate aldehyde 



Carboxylation Enzymes 



These enzymes include many systems in which carbon dioxide is a 

 reactant or a product. The reactions involve carboxyl groups and 

 lead to either the formation or the removal of such groups. 



pyruvic 

 CH3COCOO- + H3O+ ' CH3CHO + CO2 + H2O 



carboxylase 



I VSI TIP 



H3N+(CH2)4CHCOO- > H3N+(CH.2)5NH2 + CO2 



I decarboxylase , 



cadavenne 



NH3+ 



The second reaction requires pyridoxal phosphate as the coenzyme. 

 A number of these carboxylation-decarboxylation systems combine 

 to form the carbon dioxide evolved during respiration. One or more 

 reversible steps utilize the carbon dioxide taken up during photo- 

 synthesis. 



Practical Utilization of Enzymes 



Enzymes were used as catalysts for various reactions for hundreds 

 of years before they were recognized as definite chemical substances. 

 The description of the agents (ferments) which produced fermenta- 

 tions precedes our knowledge of the existence of enzymes in living 

 cells. Enzymatic processes such as bread-making, brewing, alcohol 

 manufacture, and wine-making have been known from antiquity. 



One of the older applications of enzymes still employed is the use 

 of rennin or rennet to make cheese with the enzyme coagulating the 



