LACTIC ACID FORMATION 133 



of formic acid as precursor of hydrogen. Formic hydrogen- 

 lyase is a reversible enzyme, so that synthesis of formic acid 

 will occur in the presence of hydrogen and carbon dioxide. 

 In the absence of any side-reactions, the final result of formic 

 hydrogenlyase action is to produce an equilibrium mixture 

 of formic acid, hydrogen, and carbon dioxide. Formic 

 hydrogenlyase is an iron-activated enzyme, and if the 

 organism is grown in an iron-deficient medium, the formation 

 of the enzyme is prevented and, as a result, no gas is liberated 

 during the fermentation of glucose, formic acid accumulating 

 instead (see p. 83). 



Lactic acid formation 



Esch. coli possesses the same series of enzymes as those 

 involved in the production of lactic acid by muscle. It 

 possesses lactic dehydrogenase which will bring about the 

 reduction of pyruvic acid to lactic acid in the presence of a 

 H-donator, which is itself supplied by the presence of reduced 

 coenzyme I. The series of reactions involved in the fermenta- 

 tion of glucose includes the oxidation of glyceraldehyde- 

 phosphate to phosphoglyceric acid by triose-phosphate 

 dehydrogenase, the oxidation involving the reduction of 

 coenzyme I. If, however, the fermentation substrate is 

 pyruvic acid instead of glucose, then triose-phosphate 

 dehydrogenase is not involved and coenzyme I is not reduced. 

 This is probably linked with the facts that whereas lactic 

 acid forms approximately 50 per cent, of the products of glucose 

 fermentation, it forms only about 5 per cent, of the products 

 when pyruvic acid is the fermentation substrate. The main 

 reaction giving rise to lactic acid is presumably therefore : 



CHO CHg COOH CH3 



CHOH -h H2O + CO ^ <CHOH + CHOH (5) 



I III 



CH2OPO3H2 COOH CH2OPO3H2 COOH 



the oxidation of glyceraldehyde-phosphate being linked to the 

 reduction of pyruvic acid by coenzyme I acting as H-carrier 



