IV 



BIOSYNTHESIS OF AMINO ACIDS 



8l 



2HN0 



Jt^ A16 

 1063 

 UVS2 

 2HNO, M13-579 



HzN-NHz 



M13-477 



Bg-al-P= Pyridoxal phosphate 



Bg- oxime - P = Oxime of pyridoxal phosphate 



85- amine -P= Pyridoxamine phosphate 



Fig. 38. Proposed pathways of nitrogen assimilation. 



with ^''C in the amidine group (Horner et al., 1956). Kidney enzymes also catalyze 



an amidine group transfer reaction between ornithine and arginine (Walker, 



1956). 



i) Arginine + ornithine-2-'-*C ^ — »• ornithine + arginine-2-i''C 

 2) Guanidoacetic-'^C + ornithine " r arginine-i-iC + glycine 



C. Fixation of Inorganic Nitrogen and Transfer 



I. Nitrogen assimilation 



We have previously considered the process of denitrification in connection with 

 the role of inorganic nitrogen compounds as alternative hydrogen and electron 

 acceptors in microorganisms and plants. These reductive processes are, however, 

 of particular significance for the growth process, in that molecular nitrogen or 

 inorganic nitrate is reduced to compounds which can be combined in organic 

 linkage. The ability to fix or reduce molecular nitrogen is apparently restricted to 

 microorganisms, the only exception being the remarkable system represented by 

 the root nodule bacteria and their plant symbionts. The free living nitrogen fixers 

 appear to be dispersed at random throughout the microbial kingdom from the 

 photosynthetic algae and bacteria to heterotrophic Clostridia with complex 

 nutritional requirements. The capacity to reduce nitrates and nitrites is, however, 

 shared by many microorganisms and plants. The reduction of nitrogen and of 

 nitrate proceeds through a number of intermediate steps, some of which are still 

 admittedly speculative, until hydroxylamine or NH3 are formed (Fig. 38). 



It is not as yet certain whether the form of nitrogen which is first fixed into 

 organic linkage is NH3 or hydroxylamine. An analysis of the nutritional require- 

 ments o{ Neurospora mutants has led to the suggestion that organic oxime compounds 



I.ileralure p. 124 



