340 BACTERIOLOGICAL CHEMISTRY 



/OH 



R.Ce— COOH > R.C.COOH + H,0 



^NHa NH 



The imino-acid then becomes reduced to give the amino - 

 acid : — 



R.C.COOH + 2H > R.CH.COOH. 



II I 



NH NH2 



The last step is one half of a coupled oxidation-reduction 

 process, the other half being the dehydrogenation of a 

 sugar breakdown product, in all probability the formation 

 of pyruvic acid from glyceraldehyde phosphate or from 

 methylglyoxal. If the group R is a methyl group the 

 above scheme illustrates the formation of alanine from 

 pyruvic acid. Aspartic acid has been shown to be 

 synthesised by many bacteria from fumaric acid and 

 ammonium salts : — 



COOH.CH=CH.COOH + NH3 > COOH.CH.CHgCOOH. 



I 

 NH2 



The amino -acids formed in this way condense with one 

 another to give peptides : — 



Rj Rj 



R.CH.COOH + NH2— CH > R.CH.CO.NH.CH + HgO 



I III 



NH2 COOH NH2 COOH 



and the process is repeated with formation of polypeptides 

 and ultimately of proteins. 



The conversion of an a-keto-acid to an amino-acid 

 may proceed by two other routes instead of via the 

 imino-acid. The keto-acid might react with hydroxyl- 

 amine (which is a probable intermediate in nitrogen 

 fixation, see p. 223) to give the oxime which, on reduction, 

 would iiive an amino-acid :— - 



