dehydration; hydrolysis 49 



the way to ammonia with the probable intermediate formation 

 of hydroxylamine, 



HNO2 + 4H > NH2OH + H2O 



NH2OH + 2H > NH3 + H2O, 



the sum total of the reduction being represented, 

 HNO3 + 4H2 > NH3 + 3H2O. 



3. Dehydration 



The removal of HgO from a substrate molecule is com- 

 paratively rare within our knowledge. In those cases where 

 such a reaction is the result of the action of a single enzyme, 

 that enzyme is called a dehydrase. Such an enzyme has been 

 postulated to explain the breakdown of L-serine to pyruvic 

 acid by Esch. coli : 



CH2OH CH2 ^^3 CJHg 



I Serine ll | | 



CHNH2-H2O > C . NH2 ^^ C :NH ^-^ CO + NH3 



I dehydrase | | H2O | 



COOH COOH COOH COOH 



The first step in the postulated breakdown being a dehydration 

 of serine to the corresponding imino-acid which hydrolyses 

 spontaneously to pyruvic acid. A very similar reaction takes 

 place with cysteine, in which the first step is the removal of 

 H2S (instead of HgO) from the molecule, after which the course 

 of the breakdown is the same (Chap. IX). 



4. Hydrolysis 



Hydrolytic enzymes are responsible for the processes known 

 as digestion, whereby proteins are broken down to amino- 

 acids, fats to fatty acids and glycerol, aomplex polysaccharides 

 to simpler polysaccharides and monosaccharides, etc. 



Proteolytic enzymes of many types are known, and have in 

 common the power to hydrolyse the linkage — CO — NH — , 

 splitting the peptide containing that linkage into two sub- 

 stances, one with a free — COOH and the other mth a free 



CHEM. A. B. 4 



