DEPARTURE FROM THE BIOSPHERE 367 



In the same way aspartic acid is converted to fumaric acid by E. colt. 



CHNH2— COOH CH— COOH 



I ^ II +NH3 



CH2— COOH CH— COOH 



The removal of HgS from cysteine is followed by the same process, 

 which can be accomplished, in part, by Proteus vulgaris. 



CH2SH CH2 CH3 CH3 



1 — H2S II I H2O I 



CHNH2 -> CNH2 -» Cr^NH -> C=0 H- NH3 



COOH COOH COOH COOH 



Another type of deamination is reductive deamination as carried out 

 anaerobically by the Clostridia. Glycine gives acetic acid, alanine and 

 serine give propionic acid, etc. Although evidently hydrogen plays some 

 part in the reaction, only amino acids are necessary. For example, two 

 moles of glycine are reductively deaminated in the presence of CI. sporo- 

 genes whilst at the same time a mole of alanine is oxidized to acetic acid. 



CHa— CHNH2— COOH + H2O -^CHa— CO— COOH + NH3 + 2 (H) 

 CH3— CO— COOH + H2O -> CH3— COOH + CO2 + 2 (H) 

 The hydrogen is then accepted by the glycine 



2 CH2NH2— COOH + 4 (H) -^ 2 CH3— COOH + 2 NH3 



The amino group of amino acids is never hydrolysed directly by bacteria, 

 but the latter can liberate NH3 by hydrolysis of amides such as asparagine 

 and glutamine. 



R— CONH2 + H2O -> R— COOH + NH3 



A source of ammonia in the soil is the urea arising from the hydrolysis 

 of arginine by various micro-organisms to ornithine and urea (micrococci, 

 Bacillus subtilis). Certain bacteria (Streptococcus fecalis, Streptococcus 

 hemolyticus, Micrococcus aureus; these organisms are parasites and play 

 no part in the soil economy) contain an arginine-hydrolase and can 

 perform a double hydrolysis of arginine to produce ornithine, two molecules 

 of ammonia, and COg. 



Certain other bacteria can form citrulline by a hydrolytic deamination 

 of the =NH of arginine, but the citrulline formed is only metabolized 

 very slowly. 



Because they contain urease in their cells, many bacteria can hydrolyse 

 urea, either derived as outlined above, or produced as an animal excretory 

 product. 



