144 UNITY AND DIVERSITY IN BIOCHEMISTRY 



from the 1,3-diphosphoglyceric acid and reappears in ATP. 

 1,3-diphosphoglyceric acid + ADP:<^ 3-phosphoglycericid + ATP. 



This transfer of energy of the energy-rich acyl-phosphate bond to the 

 energy-rich pyrophosphate bond is catalysed by a highly specific enzyme, 

 3-phosphoglyceric phosphokinase ; this enzyme, also, has been crystallized. 



An example of the transformation of an ordinary phosphate ester into an 

 energy-rich phosphate is provided by another reaction of the glycolysis 

 chain : the conversion of 2-phosphoglyceric acid into phosphoenolpyruvic 

 acid followed by a transphosphorylation. The reaction (a dehydration), 

 catalysed by enolase, results in the redistribution of the internal energy of 

 the molecule with concentration of around 16,000 calories in the enol- 

 phosphate bond. 



The rapid transfer of this energy-rich phosphate to ADP with formation 

 of ATP is brought about by pyruvic phosphokinase. 



(b) Phosphorylation in the Respiratory Chain 



As we have just seen, the formation of ~P bonds is coupled with 

 oxidation-reductions taking place at substrate level. Others may be formed 

 in a way which depends on the energy liberated by electrons removed from 

 the substrate as they pass along a series of carriers of increasing potential 

 until finally oxygen is reached. Acetyl-CoA resulting from a number of 

 metabolic reactions (see Part 3) is oxidized by a common pathway in the 

 presence of oxygen. Here, unlike the previously cited examples taken from 

 anaerobic glycolysis, the phosphorylations which yield pyrophosphate 

 bonds are not coupled to oxidation-reductions at substrate level. This 

 concept is based on a number of experimental findings. The quantitative 

 study of this process was initiated by Belitzer in 1939. As we shall see, 

 during the respiratory cycle which transforms substrates into COg and 

 HgO, a series of dehydrogenations occur. If each of these dehydrogen- 

 ations was accompanied by a phosphorylation of the substrate, then for 

 each of these a molecule of phosphate would be removed from the solution 

 and an atom of oxygen would be consumed (P/0 would be equal to 1). 

 Now, Belitzer calculated the ratio P/0 and obtained values greater than 

 unity. This expresses the fact that for each pair of electrons (or of electrons 

 accompanied by protons) transferred to oxygen several phosphorylations 

 occur. Theoretically there is nothing very astonishing about this since 

 when we defined oxidation-reduction potentials we showed that when 

 lactate is dehydrogenated to pyruvate, the transfer of electrons or hydrogen 

 atoms (electron + proton) to oxygen liberates much more energy than is 

 required for the formation of an energy-rich pyrophosphate bond. 



Belitzer put forward the following hypothesis, which has been confirmed 

 by numerous observations : the reaction producing the energy which is 

 coupled to phosphorylation is the oxidation of the carrier, which was re- 



