142 UNITY AND DIVERSITY IN BIOCHEMISTRY 



B. Formation of Energy-rich Bonds during Oxidation-reduction 



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Although the passage of electrons, either from one point to another in 

 the same molecule, or by a conductor made up of a series of systems of 

 higher and higher oxidation potential, is accompanied by a change in free 

 energy, these free energies, like that resulting from the direct oxidation of 

 glucose, are not in a form which can be used by the cell. These free ener- 

 gies will be degraded into heat and lost to the cell unless some mechanism 

 exists to store them in the form of energy-rich bonds of ATP, the universal 

 source of cellular work. 



It is, therefore, important now to consider this mechanism — the coupling 

 of electron transfer and phosphorylation. There are two aspects to be 

 considered : phosphorylation at substrate level and phosphorylation during 

 the transfer of electrons through a series of intermediates. 



(a) Phosphorylation at Substrate Level 



An example of this type of genesis of pyrophosphate linkages is furnished 

 by the oxidation of phosphoglyceraldehyde to 1,3-diphosphoglyceric acid, 

 an oxidation which occurs during glycolysis. 



Phosphoglyceraldehyde dehydrogenase (PGAD) is the enzyme catalys- 

 ing the oxidation (anaerobic) of phosphoglyceraldehyde which takes place 

 with an internal redistribution of electrons and the accumulation of 16,000 

 calories in the acyl-phosphate bond. The enzyme, whose coenzyme is 



C C 



^H 



^ 0-P0(0H)2 



CHOH -t-DPN^ +H3PO4 ^ CHOH +DPNH 



CH20PO(OH)2 CH,OPO(OH)2 



Phosphoglyceryldehyde 1,3-diphosphoglyceric acid 



DPN, has been crystallized. It is poor in cysteine but rich in basic amino 

 acids. Near to an — SH group on the surface of the enzyme protein 

 molecule there is attached a molecule of the coenzyme DPN+. An addition 

 complex is formed between the — SH grouping and the — N = C — of 

 DPN+. When phosphoglyceraldehyde is added, this bond is broken and 

 an energy rich thioester is formed and at the same time the DPN+ is 

 reduced to DPNH. A phosphate residue which is also attached to the 

 protein molecule takes the place of the thiol group and the energy of the 



