464 ANIMAL BIOCHEMISTRY 



form the proteins needed by the animal. These cataboh'c (break- 

 down) and anabolic (synthetic) processes are closely interlinked in a 

 complex reaction network to make up the whole pattern of protein 

 utilization. Some of the information concerning the systems involved 

 is summarized in the following sections forming the remainder of this 

 chapter. 



Amino Acid Metabolism 



Although a large number of amino acids are known to occur in 

 cells, most of the biochemical effort has been expended on those 

 occurring in proteins and on certain others having a close metabolic 

 relationship. Even with this limitation the volume of information 

 transcends the size of this book. Therefore, the presentation herein 

 must be confined to particular schematic pathways and general 

 features of processes rather than covering individual reactions. The 

 pathways chosen are presented as partial examples of the reaction 

 networks believed to exist. Such systems are still incompletely imder- 

 stood, but even the information available is too extensive for inclusion 

 in a general textbook at the elementary level. 



Perhaps the most extensively studied pattern of metabolic reactions 

 of amino acids is that of Figure 20-1. This system of reactions ac- 

 counts for synthesis of the l isomers of six amino acids commonly 

 found in proteins and of others known only as metabolic interme- 

 diates. Moreover, most of the systems are either cyclic or reversible 

 and provide mechanisms connecting with the tricarboxylic acid cycle, 

 thus allowing use of these amino acids as sources of energy. Formation 

 of urea by the "urea cycle" is believed to be the major source of this 

 compound, which serves as the principal means employed by animals 

 in excreting nitrogen. 



Three of the reactions of Figure 20-1 are transaminations leading 

 to the formation of alanine, aspartate, and glutamate by the mecha- 

 nism of page 228. In addition, animals have the ability to utilize 

 ammonium ion for the synthesis of glutamate according to the scheme 

 shown. Although for simplicity the names have been omitted, the 



-OOCGH2CH2COCOO- + DPNH + NH4+ + H3O+ < ^^"^^"'"' ) 



dehydrogenase 



-OOCCH2GH2CHCOO- + DPN+ + 2H2O 



1 

 NH3+ 



reactions of the figure are all enzymatic. In addition, it is probable 

 that further study will reveal other connections and intermediates 

 for processes written here as single reactions. 



