124 NITROGEN METABOLISM 



'apoenzyme', and that the cation acted as a bridge joining 

 the enzyme to the substrate. Smith [36] has proposed that 

 the cation chelates with the substrate, and thus causes a 

 redistribution of electrons with the result that the suscep- 

 tible peptide bond becomes unstable and easily broken: 



— NH.CH(R).C NH.CH(R')COO— 



II 

 O 



e.g. for a 

 carboxypeptidase, 



Enzyme protein 



The co-ordination is believed to be between the — CO — 

 group of the peptide bond and the terminal free carboxyl 

 group for carboxypeptidases and the — NH — group and 

 the free amino group for aminopeptidases. Examples are 

 known in which co-ordination between cations and peptides 

 is extremely specific, and this may explain why peptidases 

 attacking different substrates are activated by different 

 metal ions. The aerobic bacteria Ps. aeruginosa, Ps. fluores- 

 cens and Serratia marcescens contain a peptidase activated 

 by Mg"*"^ and comparable in specificity with the leucine 

 aminopolypeptidase of animal tissues [5, 7]. Dipeptidases, 

 or enzymes with activity against dipeptides, have been found 

 in preparations from Mycobacterium tuberculosis [35], Leuco- 

 nostoc mesenteroides [6], Phytomonas tumifaciens [5], 5. mega- 

 therium [8], Sac. cerevisiae [20, 21] and Aspergillus para- 

 siticus [7, 26]. A polypeptidase from Sac. cerevisiae and Asp. 

 parasiticus was activated by Zn"^"*", and both the di- and 

 poly-peptidase of yeast also required chloride ions [24] 

 (Table 8.2). In general, the optimum pH for peptidase 

 activity is in the range 8-9, but some peptidases of Ln. 

 mesenteroides, propionibacteria and lactobacilli are most 

 active at an acid pH (5-5-6-0), and they are not activated 

 by divalent cations [5, 6]. 



