224 BACTERIOLOGICAL CHEMISTRY 



The uxiiiie is then rechiced to give aspartic acid, 

 COOH.CH.NH2.CH2.COOH, which can be decarboxylated 

 to give p-alanine, CH2.NH2.CH2.COOH. In presence of 

 the enzyme, transaminase, aspartic acid can transfer the 

 amino group to pyruvic acid to form a-alanine, 

 CH3.CH.NH2.COOH, a process by which other amino- 

 acids may be synthesised. 



That nitrogen fixation is intimately connected with 

 reduction processes involving molecular hydrogen, is 

 shown by the fact that when nitrogen fixation by 

 Azotobacter is inhibited by the presence of combined 

 nitrogen, such as nitrate, the activity of the enzyme 

 hydro genase is also stopped. The adaptation of the 

 bacteria to use nitrate more readily is accompanied by 

 increased inhibition of both nitrogen fixation and 

 hydro genase activity. This occurs even in the presence 

 of the substrate, hydrogen. Hydrogenase appears, 

 therefore, to be an adaptive enzyme whose formation 

 depends not on the presence of its substrate but on 

 nitrogen fixation. 



Protein and Amino -acid Breakdown 



The utilisation of proteins and amino -acids by bacteria 

 in th^ production of new cells during growth appears to 

 follow somewhat the same lines as in animal nutrition. 

 Complex proteins and polypeptides are broken down 

 outside the cell by the proteolytic enzymes in a manner 

 analogous to the digestion of the proteins in the stomach 

 and intestine by the enzymes pepsin and trypsin. The 

 breakdown products of this hydrolysis, the amino-acids 

 and the lower peptides, are then taken into the cell, where 

 they are either directly rebuilt into the proteins character- 

 istic of the particular bacterial species concerned or are 

 further broken down into ammonia and simple carbon 

 compounds, which are then used in the synthetic processes 

 involved in the growth of the cells. 



Protein Degradation. — As indicated above, tlie proteins 



