CHAPTER V 

 SYNTHESIS OF AMINO-ACIDS 



From the vast amount of information now available it is 

 clear that glutamic acid and aspartic acid occupy a key 

 position in amino-acid metabolism, and, of the two, the 

 former is the more important. Glutamic acid can be synthe- 

 sized from NH3 and a-ketoglutaric acid by the glutamic 

 dehydrogenase system (p. 12) and aspartic acid from NH3 

 and fumaric acid by aspartase (p. 23). The direct addition 

 of an inorganic compound of nitrogen to the appropriate 

 carbon skeleton does not appear to be a general route for 

 the synthesis of amino-acids, and the importance of the 

 dicarboxylic amino-acids is in part due to the fact that they 

 contain nitrogen in a form which can be transferred to suit- 

 able acceptors and thus utilized for the synthesis of other 

 nitrogenous compounds, e.g. the conversion of citrulline to 

 arginine (p. 70) and the synthesis of amino-acids by trans- 

 amination. Moreover, it will become clear from the following 

 paragraphs that several amino-acids may be derived from 

 other preformed amino-acids — for example, proline and 

 ornithine can be synthesized from glutamic acid (pp. 69, 70). 

 Many organisms use an inorganic form of nitrogen, such as 

 molecular N2 , NH3 and NOJ, as a source of this element, 

 and it is now generally held that the first steps in the 

 utilization of molecular Ng and nitrate for this purpose 

 involves their conversion to NH3 (see Chaps. IV and III). 

 It is therefore worthy of note that the glutamic dehydro- 

 genase system and aspartase are mechanisms which enable 

 inorganic nitrogen in the form of NH3 to be incorporated 

 into an organic molecule. 



Transamination and amino-acid synthesis 



A transaminase catalyses the reversible transfer of the 

 amino group of one amino-acid to the a-keto acid corre- 

 sponding to another amino-acid: 



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