PHOSPHORYLATION 55 



7. Phosphorylation and dephosphorylation 



Tiie anaerobic breakdown of carbohydrate in yeast and in 

 muscle involves the initial phosphorylation of the carbo- 

 hydrate by a series of reactions involving adenosine-tri- 

 phosphate and inorganic phosphate. The phosphorylated 

 compounds then undergo a series of changes resulting in the 

 formation of phosphopyruvic acid which is then dephos- 

 phorylated before the final stages of fermentation take place. 

 The fermentation of glucose by Esch. coli and related organisms 

 has now been investigated in considerable detail (Chap. VII) , 

 and appears to involve the same basic cycle of reactions as 

 those occurring in yeast fermentation, so that phosphorylation 

 of glucose to hexosediphosphate precedes breakdown to 

 simpler molecules. The intermediate stages of the phosphory- 

 lation and the enzymes involved have not yet been worked 

 out with bacteria, but it is highly probable that the first step 

 is a transfer of phosphate from adenosine- tri -phosphate to the 

 6-position of glucose by the enzyme Hexokinase: 



H 



-\ Adenosine-di-phosphate 



Many other bacterial fermentations will occur only in the 

 presence of phosphate and are accompanied by an uptake of 

 phosphate from the medium by the fermenting cells. The 

 changes have seldom been investigated in detail and it is not 

 possible to say whether phosphorylation is an invariable step 

 in anaerobic carbohydrate breakdown. Fermentation of 

 carbohydrate represents one of the main sources of energy for 

 anaerobic existence. In those cases which have been worked 

 out in detail, it appears that the incorporation of phosphate at 

 low energy levels into organic compounds, followed by its 

 removal at a later stage of the fermentation process from 

 compounds in which the phosphate bond has become 



