PRIMING REACTIONS 



189 



system of high negative JF, such that the energy of the energy-rich bond 

 of ATP is not conserved and the reaction is irreversible. 



F— 6— P + ATP -> F— 1,6— PP + ADP 



4. The formation of phosphotriose, the substrate for glycolysis, takes 

 place by the splitting, in the presence of aldolase, of F — 1,6 — PP into a 

 mixture of two triosephosphates. Equilibrium between the hexose- 

 phosphate and the mixture of triosephosphates is established in the cell. 



CH20PO(OH)2 

 Phosphodihydroxyacecone | 



CO 



F - 1,6 - PP;=i + CH2OH 



aldolase \[ 



J-phosphoglyceraldchyde CHO 



I 

 CHOH 



96 7o 



CH20PO(OH)2 



} 4 Vo 



89 % 



11 % 



70 



5. In the course of reactions 1 to 4, a molecule of glucose has been 

 transformed into a mixture of F — 1,6 — PP and two triosephosphates. 

 Now occurs the first anaerobic oxido-reduction (see p. 142) in which, in 

 the presence of triosephosphate dehydrogenase and its coenzyme DPN, an 

 internal oxido-reduction takes place forming 1,3-diphosphoglyceric acid, 

 a molecule containing an energy-rich acylphosphate bond. 



H 0-P0(0H)2 



/ / 



c=o c=o 



I + DPN+ + H3PO, - I -\- DPNH +H+ 



CHOH CHOH 



CHoOPOCOH), CH20PO(OH)o 



3-phosphoglyceraldehyde 1,3-diphosphoglyceric acid 



6. This reaction causes the F — 1,6 — PP to dissociate into triose phos- 

 phates, since it uses up one of the products of that reaction. Reaction 5 

 itself is pushed completely to the right (although it is a reversible reaction) 

 by the fact that, in the presence of the very specific enzyme 3-phospho- 

 glycerate phosphokinase, the energy-rich compound 1,3-diphosphoglyceric 

 acid transfers its energy-rich bond to ADP to form ATP. The phos- 

 phokinase acts almost at equilibrium so that the energy-rich bond is 

 transferred with little loss (1 acylphosphate bond -^ 1 pyrophosphate 

 bond) (low —AF). 



