AMINO ACIDS, PEPTIDES AND PROTEINS 223 



very extensively. The molecular weight of edestin, a seed protein, has been obtained by 

 electron microscopy (68). One of the problems encountered in molecular weight deter- 

 minations of proteins is that under different conditions the ^parent molecular weight of 

 a protein may vary greatly. This is due to the ability of proteins to form dissociable ag- 

 gregates. It is therefore necessary to state the conditions used in the molecular weight 

 determination very precisely and to bear in mind that a given experimental value may not 

 represent the minimum molecular weight. 



ME TABOLIC PA THWA YS 



striking advances have been made in the elucidation of the biosynthetic pathways of 

 amino acids and proteins. However, because of the ready availability of mutants of micro- 

 organisms most of the work has been carried out with these primitive plants. Unfortu- 

 nately, one has no assurance that these processes are universal and it is therefore ex- 

 tremely risky to attempt to apply the results obtained with micro-organisms to higher 

 plants. The pathways shown in Figures 1-3 represent a composite derived from many 

 different organisms, and in some cases intermediate steps are omitted in the transforma- 

 tions. Reviews of amino acid biosynthesis and breakdown appear rather frequently in the 

 literature. 



NITROGEN FIXATION AND NITRATE REDUCTION 



Molybdenum appears to be of prime significance in both the symbiotic and non-sym- 

 biotic nitrogen fixations (69). A great deal of evidence can be cited to support the belief 

 that atmospheric nitrogen is reduced stepwise to the ammonia level, and that it enters 

 the metabolic pools in the form of glutamine and glutamic acid. The exact nature of the 

 intermediates is not known, and they may remain bound to the enzyme until released as 

 ammonia. The suggestion of Bach (70) that 3, 4-dihydropyridazinone-5-carboxylic acid 

 (V) may be an intermediate has not been verified by other workers. Cell-free prepara- 

 tions with which nitrogen fixation can be carried out are available (71, 72) and should be 

 of great utility in the determination of the intermediates and enzymes involved in the 

 process. 



CO„H 



I 2 



CHp N 

 I I 



CH^ NH 



6 2 



Most species of higher plants possess the ability to reduce nitrate to the ammonia 

 level. The first intermediate in this process is nitrite. The enzyme involved in this 

 process in some organisms is a molybdenum flavoprotein (73) and the electron donor is 

 TPNH. Hyponitrous acid and hydroxy lamine or addition products formed with alpha keto 

 acids may be other intermediates in this reduction process. 



