NITROGEN METABOLISM 237 



3. The Character of the Group R. — Jiong chaiii amino - 

 acids are more easily attacked than those containing 

 ring structures. The ease of attack appears to increase 

 with the length of the chain ; glycine, the simplest amino - 

 acid, CH2.NH2.COOH, is very resistant to bacterial 

 action, but alanine, CH3.CH.NH2.COOH, is much less 

 resistant. Ring structures containing nitrogen can be 

 attacked by some organisms but not others. Putre- 

 factive organisms can open the ring of proline with 

 formation of w -amino -valeric acid and of n- valeric 

 acid : — 



CH2— — CHo 



I I 



CH, CH.COOH > 



NH 



(n-valeric acid) (o-amino-valeric acid) 



The iminazole ring of histidine is opened by Esch. coli, 

 Sal. 2)aratyphi and Sal. schottmidleri and Ps. fluorescens, 

 but not by Proteus vulgaris. Tryptophane is similarly 

 attacked. Moulds can break down any type of nitrogen 

 ring if no other source of nitrogen is available. 



The deamination of serine by Esch. coli, which can 

 occur aerobically or anaerobically, appears to involve the 

 hydroxyl group since if it is masked by substitution 

 deamination does not occur. The suggested mechanism 

 is via the imino acid and pyruvic acid : — 



CH2OH.CH.COOH -H2O CH2=C.C00H CH3.C.COOH 



I — -> I — > li 



NH2 NHo NH 



(serine) (imino acid) 



NH 



+ H2O 



> CH3CO.COOH + NH. 



The sulphur containing amino-acids like cysteine, 

 CH2.SH.CH.NH2.COOH, appear to undergo reductive 



