AMINO-ACID BREAKDOWN 159 



decarboxylation; or (3) by splitting or hydrolysis of the 

 molecule in some other position. In some cases we find that a 

 single organism may attack an amino-acid by both deamination 

 and decarboxylation, but the two processes do not take place 

 together as the enzymes involved are not formed under the 

 same conditions of growth. When growth takes place in an 

 alkaline medium, the carboxyl group of the amino-acid is 

 ionised, R.CHNHg.COO', and if the particular amino-acid 

 can be attacked by that organism, the specific deaminase 

 is produced and the organism will attack the amino-acid by 

 removal of the unionised — -NHg group. Conversely, if growth 

 takes place in an acid medium, then the amino-group of the 

 amino-acid is ionised, R . CHNH3+ . COOH, and the specific 

 decarboxylase will be produced so that the organism will 

 attack the amino-acid by removal of the unionised — COOH 

 group. For example, Esch. coli, grown in an alkaline medium, 

 will attack L-glutamic acid with the formation of a-keto- 

 glutaric acid and ammonia: 



HOOC.CH2.CH2.CHNH2.COOH -f 



> HOOC . CH2 . CH2 . CO . COOH -f NH3, 



but the same organism grown in an acid medium can no 

 longer attack the molecule by deamination but does so, 

 if at all, by decarboxylation 



HOOC . CH2 . CH2 . CHNH2 . COOH 



> HOOC . CH2 . CH2 . CH2NH2 -f CO2. 



The products of amino-acid breakdown may thus be pro- 

 foundly influenced, not only by the particular organism, but 

 by the ^:>H of the medium in which that organism is grown 

 (see Chap. IV). 



DEAMINATION 



Removal of the — NHg group from an amino-acid may be 

 accomplished by different bacteria in different ways, such as by 

 oxidation, reduction, desaturation, hydrolysis, etc., of the 

 substrate molecule. 



