132 PROVISION OF energy: fermentation 



Under anaerobic conditions the first breakdown of pyruvic 

 acid is to acetic and formic acids by what was originally called 

 the " hydroclastic split ": 



CH3.CO.COOH + H2O— >CH3.C00H + H.COOH (1) 



However, it was found that when cell-free enzyme preparations 

 were used, this reaction would not take place unless phosphate 

 was present. This suggests that the formation of acetyl 

 phosphate occurs as an intermediate step in the breakdown, 



CH3.CO.COOH + H3PO4 ^^ CH3.COOPO3H2 + H.COOH 



(2) 



If the enzyme preparation is freed from adenylic acid or 

 adenosine-di-phosphate (ADP), then acetyl phosphate is 

 actually found to accumulate, but in the presence of either 

 adenylic acid or ADP, the acetyl phosphate breaks down 

 to acetic acid iand the phosphate group is transferred to the 

 adenine compound with the synthesis of ATP: 



CH3.COOPO3H2 + ADP > CH3.COOH + ATP . .(3) 



In the intact organism the over-all result of reactions 2 and 3 

 is reaction 1, with the additional result that inorganic phos- 

 phate is taken up during the course of the reaction and 

 ATP synthesised. 



Hydrogen and carbon dioxide formation 



The phosphoclastic split described above (reaction 2), 

 releases formic acid as a product of pyruvic acid breakdown. 

 Formic acid is further broken down to hydrogen and carbon 

 dioxide by an enzyme called formic hydrogenlyase, 



Formic 

 H.COOH ^Ha + COg (4) 



hydrogenlyase 



The hydrogen formed during fermentation is all produced 

 as a result of this reaction, though, as we shall see later, the 

 phosphoclastic splitting of pyruvic acid is not the sole source 



