Enzymes: How Cells Promote Chemical Activities 



101 



Groups of enzymes may also be specified 

 by the system, according to: (1) the class of 

 substrate involved — for example, proteases, 

 which catalyze protein reactions, and lipases, 

 for lipid reactions, etc.; or (2) the type of 

 reaction — for example, hydrolases, for hy- 

 drolysis reactions; oxidases, for reactions in 



which oxygen has a direct involvement; and 

 dehydrogenases, for the transfer of hydrogen 

 (p. 145) from one organic substrate to another 

 (an indirect type of oxidation). Metabolism, 

 however, is exceedingly complex and enzyme 

 nomenclature has developed certain other 

 usages, which will be encountered later. 



Table 5-1— Some Representative Enzymes 



General Types; 

 Specific Names 



Catalytic Action 



Comment 



Metabolic Enzymes 

 Dehydrogenases 

 1. Lactic dehy- 

 drogenase 



1. Liberates 2 hydrogens from 



lactic acid 



OH 



I 

 CHo— C— COOH 



I 



H 

 and from other molecules hav- 

 ing similar structure, namely 



OH 



-C- 



1. Hydrogen transferred to a 

 hydrogen acceptor (p. 145); 

 lactic acid oxidized to 

 pyruvic acid 

 O 



CH 3 C— COOH 



present in most, if not all, 

 cells 



2. Succinic dehy- 

 drogenase 



H 



2. Liberates 2 hydrogens from 



succinic acid 



H H 



I I 

 HOOC— C— C— COOH 



I I 



H H 

 and from other molecules 

 having 



H H 



I I 

 _C— C— 



2. Hydrogen transferred to 

 another type of hydrogen 

 acceptor (p. 145); succinic 

 acid oxidized to fumaric 

 acid 



HOOC— C=C— COOH 

 I I 

 H H 

 also widely distributed in 

 cells generally 



H H 



in their structure 



Transphosphorylases 

 1. Glucokinase 



2. Fructokinase 



1. Transfers phosphate 

 (H 2 PO-) from ATP (p. 

 149) to glucose 



2. Transfers phosphate from 

 ATP to fructose 



Activates (p. 149) glucose, 

 initiating its catabolic 

 breakdown; widely dis- 

 tributed 



Activates fructose; widely 

 distributed 



