6 NITROGEN METABOLISM 



chapters devoted firstly to the fermentation and oxidation 

 of amino-acids by heterotrophs, and secondly to the auto- 

 trophic nitrifying bacteria. It will be seen that in nature the 

 ammonia produced during the decomposition of amino- 

 acids may suffer one of three fates: (i) oxidation by the 

 nitrifying bacteria to nitrate (Chap. Ill), (2) after oxida- 

 tion to nitrate, conversion to molecular Ng ^^^ nitrous 

 oxide (Chap. Ill), (3) incorporation into organic molecules 

 (Chap. V). The anabolic aspects of nitrogen metabolism 

 culminate in the formation of two major groups of complex 

 substances, proteins and nucleic acids. The latter are con- 

 sidered in a separate chapter whilst protein synthesis is 

 traced step by step, beginning with the mode of incorpora- 

 tion of nitrogen from molecular Ng and NH3 into organic 

 molecules. After dealing with the synthesis of amino-acids 

 and with the mechanisms operative in the absorption of 

 these compounds from the environment, attention is next 

 directed to the significance of peptides in intermediary meta- 

 bolism, the problems of protein synthesis and how amino- 

 acids become joined together by peptide bonds. This part 

 of the monograph concludes with a chapter devoted to the 

 enzymes responsible for proteolysis, a process which ulti- 

 mately yields free amino-acids. The catabolism of the latter 

 is discussed at the beginning of the monograph, conse- 

 quently it will be appreciated that the metabolism of amino- 

 acids and proteins has been studied at various stages in a 

 cycle. The underlying theme of the monograph is none 

 other than that known to all biologists as the nitrogen 

 cycle, and an attempt has here been made to analyse some 

 of the component steps of the cycle from the standpoint of 

 the biochemistry of the various reactions and the micro- 

 organisms concerned (Fig. i.i). 



Purely for convenience, and in order to avoid possible 

 confusion, the microbial metabolism of nucleotides, nucleo- 

 sides, purines and pyrimidines is discussed in a separate 

 chapter. This field is now being studied intensively and there 

 has been little time to correlate many of the experimental 

 facts rapidly being placed at our disposal. Partly for this 

 reason, and partly because of limitations in the amount of 



