324 BIOLOGICAL ACTIVITIES OF YEASTS 



nique for the study of the mechanism of fermentation — a technique 

 whereby fermentive systems may be studied apart from the other 

 chemical processes (e.g., assimilatory processes) of the cell. It has 

 made possible the isolation and purification, at least partially, of the 

 individual components of this battery of enzymes and the determina- 

 tion of some of their chemical properties. 



The Role of Phosphates in the Fermentation. The studies of 

 Harden and his associates ^^ demonstrated the important fact that 

 phosphates exert a stimulatory effect on the zymase fermentation 

 of glucose, which proceeds more slowly than the fermentation carried 

 on by intact cells. Harden and Young -° found that inorganic phos- 

 phates disappear during the first stage of fermentation and are re- 

 placed by organic phosphates, esters of hexose. It was originally 

 believed that the primary purpose of phosphorylation was to intro- 

 duce two phosphate groups into the hexose molecule to form hexo- 

 sediphosphate and, thus, model the molecule for cleavage into two 

 triose phosphates. 



Subsequent investigations, however, have assigned an even more 

 important role to the phosphates. The phosphate ester bonds of 

 hexose are changed, as the fermentation proceeds, to different types 

 of linkages which serve as carriers of energy. Large quantities of 

 energy given off by the oxidation-reduction reactions occurring in the 

 fermentive breakdown of sugars are not dissipated haphazardly, 

 but accumulate instead in energy-rich phosphate bonds f (Lipmann's 

 terminology). The major portion of oxidation-reduction energy, 

 made available, is converted into phosphate bond energy. Once the 

 energy-rich phosphate bonds are produced, transphosphorylations, 

 i.e., migration of phosphate groups from one compound to another, 

 may occur through transport systems such as the adenylic acid 

 system.^ 



t Energy-rich phosphate bonds: Unkages which when cleaved yield large 

 amounts of energy, ca. lOj'^OO calories, e.g., 



0H^ \ O COOH 



R-CO-P-O-'POsHa, R-C-O^POsHa, RrCO^POsHz. 



O 

 Energy-rich phosphate bonds will be designated in the text as: '~P03H2. 



t Adenylic acid system: A coenzyme system consisting of two adenosine poly- 

 phosphates, adenosine triphosphate (ATP), and adenosine diphosphate (ADP); 

 and adenosine monophosphate (AMP) or adenylic acid. ATP may donate an 

 energy-rich phosphate group to a phosphate acceptor and itself become ADP. 

 The ADP may in like manner give up an energy-rich phosphate gi-oup and become 

 AMP. AMP, though containing a residual phosphate group, is incapable of giv- 



