284 UNITY AND DIVERSITY IN BIOCHEMISTRY 



of metabolism. Such a case are the stages of glycolysis between phospho- 

 glyceraldehyde and 3-phosphoglyceric acid in the course of which an 

 energy-rich bond is formed (p. 189). 1,3-Diphosphoglyceric acid undergoes 

 a spontaneous hydrolysis into 3-phosphoglyceric acid and inorganic 

 phosphate. The equilibrium constant for the enzymic conversion of 

 phosphoglyceraldehyde into 1,3 -dip hosphogly eerie acid is such that only a 

 small amount of this acid is present at equilibrium. For the step from 

 1,3 -dip hosphogly eerie acid to 3-phosphoglyceric acid, with the formation 

 of ATP, the equilibrium constant favours the formation of a relatively large 

 amount of the latter. When the two reactions are combined, a stationary 

 state is set up in which the concentration of 1,3-diphosphoglyceric acid is 

 very small. In this way the slow spontaneous hydrolysis of the latter is kept 

 to a point where it is insignificant, 



D. The Concentration of Coenzymes 



An example of the importance of this in the regulation of metabolic 

 processes is provided by the antagonistic effect of K+ and Na+ ions on the 

 second transfer of phosphate during fermentation. 



E. Temperature 



Since temperature does not affect all enzymes in the same manner its 

 influence is such as to cause changes in biochemical reactions. 



F. Permeability of the Outer Region of the Cytoplasm 



This acts as a regulator of metabolism by controlling the passage of 

 substrates or metabolites. An example of the first mode of control is the 

 fact that yeast ferments glucose but does not ferment F-1,6-PP, to which 

 the surface of the cell is impermeable. 



The second type of effect is illustrated by the ready permeability of the 

 outer region of the yeast cell to ethyl alcohol. But for this ready perme- 

 ability, the accumulation of alcohol inside the cell would soon cause 

 alcoholic fermentation to cease. 



G. Topobiochemistry 



The cellular topobiochemistry of enzymes, as it has been described in 

 the preceding chapter, offers numerous possibilities for intracellular 

 regulation of reactions; yet the study of this subject has hardly begun. 

 During alcoholic fermentation, for example, ATP is formed in the cyto- 

 plasm and transferred to cellular particles containing ATP-ase, from 

 whence the molecules return to the cytoplasm to replenish the stock of 

 adenine nucleotides acting as phosphate acceptors (AMP, ADP). The 

 diffusion of ATP away from the cytoplasm and the diffusion of AMP and 

 ADP towards the cytoplasm evidently form one of the regulating processes 

 in glycolysis. 



