228 UNITY AND DIVERSITY IN BIOCHEMISTRY 



C. The Fatty Acid Cycle 



Here, the calculation of the energy yield is less certain. It is believed that 

 5 energy-rich bonds are formed as each acetyl group is split off. Now each 

 of the latter as it traverses the tricarboxylic acid cycle will give twelve 

 energy-rich bonds (one complete turn of the cycle). 



One mole of palmitic acid gives eight acetyls, so that the production of 

 ATP during the passage of a mole of palmitic acid through the priming 

 reactions will be as follows : 



palmitic acid + 10^ -> 8 acetyls 7 X 5 = 35 



8 acetyls + 16 Og -> I6H2O + I6CO2 8 x 12 = 96 



Total 131 



The overall reaction can be written : 



C16H32O2 23O2 + 131ADP + 131? -^ I6H2O + 16COa + 131ATP 



D. Degradation of Amino Acids 



Let us take the example of alanine, the heat of combustion of which is 

 392,000 cal per mole: on deamination it will yield a mole of pyruvate. The 

 latter will be oxidized by the respiratory mechanism yielding energy-rich 

 bonds calculated as follows : 



pyruvate + oxaloacetate + O -> citrate+C02 3 



citrate + 23 O2 -> oxaloacetate + 2 H2O + 2 CO2 12 



15 



E. Remarks on the Preceding Calculations 



These calculations must be accepted with caution. In fact they imply 

 values which may in some cases only be approached rather than attained. 

 For example the P/0 ratio in the course of oxidative phosphorylations. 

 Moreover they assume a perfect coupling between phosphorylations and 

 oxido-reductions. In practice numerous factors bring about the uncoupling 

 of these two processes, examples are dinitrophenol and thyroxine. There 

 are good reasons to believe that even in a given species the efficiency of 

 coupling varies from one individual to another. 



Furthermore, the values used in the above calculations have been 

 obtained from measurements made in vitro on systems of the purified 

 substances. This is not the situation in the cell. Most important is the fact 

 that it contains an enzyme, adenosine triphosphatase, which hydrolyses 

 ATP into ADP and inorganic phosphate, and it is not impossible that in 

 vivo part of the ATP formed is lost through the action of this enzyme, also 

 this enzyme is involved in a specific way in the transformation of ATP 

 bond energy into work. 



