g6 NITROGEN METABOLISM 



is widely distributed [46]. Several bacterial peptides, especi- 

 ally those secreted into the environment, contain D-amino- 

 acids and, in addition, amino-acids not yet found in peptide 

 combination in animals and plants (Table 7.1). Whether 

 these unusual amino-acids are also present in bacterial pro- 

 teins remains to be proved. On the basis of their biological 

 activity, the peptides associated with the metabolism of 

 micro-organisms can be arranged into three groups: 



(i) Peptides which are or may be co-factors in inter- 

 mediary metabolism, e.g. glutathione, the folic acid 

 factors, biocytin (p. 24), glutamine and aspara- 

 gine. Though the latter three compounds are not 

 true peptides, they can be regarded as containing 

 the 'peptidic bond', — CO — NH— [25]. 



(ii) Peptides which serve as a source of amino-acids 

 essential for growth. In natural environments, 

 these peptides arise as the result of autolysis and 

 the action of extracellular proteases (pp. 11 5-1 6). 



(iii) Extracellular peptides. Among the bacteria, members 

 of the Bacillaceae are especially active in the forma- 

 tion of this type of peptide, the majority of which 

 possess antibiotic activity, an exception being the 

 polypeptides of D-glutamic acid which may be 

 attached to the organism in the form of a capsule 

 {B. anthracis and B. mesentericus) or free in the 

 medium {B. suhtilis and B. mesentericus) [2]. The 

 capsular material of B. anthracis consists of chains 

 of glutamic acid residues linked by gamma peptide 

 bonds together with chains linked by alpha peptide 

 bonds. Such a capsule may confer immunity from 

 attack by proteases of the infected host [15]. 



Co-factors containing peptide bonds 



The available evidence indicates that glutamine and as- 

 paragine play important roles in intermediary metabolism 

 and it is to be noted that several co-factors are known which, 

 like glutamine, contain the y-glutamyl radical. Glutamine 

 may be essential for initiating the growth of certain organ- 

 isms, but in many instances it is replaceable by glutamic 



