INTERCONVERSION OF SUGARS 



and would be different from the transminase reaction (16), in 

 which there is an exchange of — NHo for =0. Since one of the 

 reactants is an amide, the name transaminase is not applicable. 

 A suggestion is to call the enzyme transamidase. The mechanism 

 of the reaction is at present obscure. No cofactor has been found 



^ P t' P 



HG-OH + C-NHo *■ HCNHo + C-OH (15) 



Ri' R2 Ri R2 



Ri R2 Ri R2 



C=0 + HCNH2 ^ HCNH2 + G=0 (16) 



Ri' R2' Ri' R2' 



to be necessary, but a further search in this direction should be 

 carried out, especially on recalling the difficulties which at- 

 tended the recognition of pyridoxal phosphate as the coenzyme 

 of transaminases. 



Deoxy Sugars 



Only in one case has an enzyme been found which will syn- 

 thesize a deoxy sugar (22). It has been obtained from animal 

 tissues and from microorganisms and catalyzes the following re- 

 action : 



glyceraldehyde-3-phosphate + acetaldehyde > 



deoxyribose-5-phosphate (17) 



Deoxyribose-5-phosphate (XXV) can be converted by an- 

 other enzyme of the phosphoglucomutase type into the 1 -phos- 

 phate, which in turn can react with certain purines to yield de- 

 oxyribosides. Other deoxy sugars which occur fairly frequently 

 are L-fucose (XXVI) and L-rhamnose (XXVII). L-Fucose is 

 related (on paper) to L-galactose by reduction of the CH2OH to 

 CH3; L-rhamnose is similarly related to L-mannose. Both 

 methyl pentoses are found in plants, and L-fucose is a constituent 



601 



