Common Pathways of Cellular Metabolism - 151 



olism for rebuilding its high-energy reserves 

 (p. 147). Meanwhile, however, glycolysis pro- 

 vides energy more quickly, and this helps to 

 prevent any very rapid depletion of phos- 

 phate reserves. 



The first step in glycolysis is the liberation 

 of many glucose-phosphate units as the bonds 

 of the huge glycogen molecule are ruptured. 

 As may be seen in Figure 8-6, part of this 

 initial breakdown is hydrolytic, but a larger 

 part is phosphorolytic. For the rupture of 

 the 1-4 bonds of the glycogen structure the 

 cell employs inorganic phosphate (H 3 P0 4 ), 

 rather than water (Fig. 8-6). 



The further steps of glycolysis are quite 

 complex and hence will not be explained in 

 lull detail. It should be noted, however, that 

 the pathway of glycolysis has been thoroughly 

 explored by the intensive investigations of 

 many biochemists, in many countries. 



In brief summary, glucose phosphate un- 

 dergoes rearrangement and phosphorylation, 

 forming fructose diphosphate, at the expense 

 of one molecule of ATP (Fig. 8-5). But now 

 energy begins to appear on the positive side 

 of the ledger. Each molecule of fructose di- 

 phosphate fragments into two molecules of 

 triose phosphate (Fig. 8-5). Each triose phos- 

 phate undergoes oxidation, gaining energy 

 as it passes hydrogen to a primary acceptor 

 (DPN). This energy goes into the formation 

 of diphosphoglyceric acid, to which inor- 

 ganic phosphate has been bonded (Fig. 8-5). 

 The newly formed phosphate bond (in the 

 diphosphoglyceric acid) is of the high-energy 

 type. It is destined to be transferred to ADP 

 — in the reaction that converts diphospho- 

 glyceric to monophosphoglyceric acid (Fig. 

 8-5) — thus generating a new molecule of 

 ATP. If we remember that two triose di- 

 phosphates were formed from each original 

 glucose molecule, it is plain that two new 

 molecules of ATP have now been formed, at 

 the expense of the one molecule originally 

 sacrificed. In short, energy from oxidation 

 has been used to achieve high-energy phos- 

 phorylation. 



Two additional high-energy bonds are 



still to be gained before glycolysis reaches the 

 end point (pyruvic acid; Fig. 8-5). By a com- 

 plex set of reactions (Fig. 8-7) phosphogly- 

 ceric acid is transformed into pyruvic acid, 

 generating energy for the conversion of one 

 molecule of ADP to ATP for each of the 

 two reacting molecules. Thus there has been 

 a net gain of 3 molecules of ATP for each 

 glucose-phosphate unit consumed in gly- 

 colytic catabolism. 



Fig. 8-7. Set of reactions that builds up the high- 

 energy phosphate (ATP) reserves of the cell. Note that 

 energy for building a new high energy bond (~~) is 

 provided by reaction 1 and that a transphosphoryla- 

 tion (reaction 2) shifts the high-energy phosphate to 

 ADP, forming ATP. Enolpyruvic acid, being unstable, 

 quickly passes to pyruvic (reaction 3). 



The final end product of glycolysis as it 

 occurs in a working muscle (p. 435) is lactic 

 rather than pyruvic acid. The conversion of 

 pyruvic to lactic acid, however, involves a 

 relatively simple type of reaction, such as is 

 shown by the equation on page 145. 



Alcoholic Fermentation. Alcoholic fermen- 

 tation is a process utilized by various yeast 

 cells (p. 174) in obtaining energy under 

 anaerobic conditions. Much of the pathway 



