Enzymes 53 



Energy is liberated at each oxidative step of the cycle, i.e. 

 wherever electrons are transferred. As in glycolysis the generation 

 of energy is associated with the synthesis of ATP from ADP. A mole- 

 cule of ATP is synthesised every time a pair of electrons is trans- 

 ferred from one hydrogen carrier to another. This process of energy 

 production by way of oxidation and ATP synthesis is known as 

 oxidative phosphorylation. 



Although glucose (& therefore by implication carbohydrate 

 in general) was chosen as a starting point, both fats and proteins 

 feed the respiratory pathway just described. Fatty acids (derived 

 from neutral fats) are converted to acetyl CoA by /? oxidation. 

 Amino acids (derived from proteins) are converted to the corre- 

 sponding keto acid by deamination or transamination. Thus glu- 

 tamic acid becomes a ketoglutaric acid and alanine becomes pyruvic 

 acid. The advantage of the cyclical system is that any of the car- 

 boxylic acids which act as intermediate substrates can be used by 

 the cell to maintain oxidative phosphorylation. Another point 

 worth making is that the most basic of metabolic processes, respira- 

 tion (the uptake of oxygen and the formation of carbon dioxide) 

 is essentially a device for generating the energy needed to maintain 

 all the multiple processes that together constitute life. The cell 

 achieves this by synthesising energy-rich ATP in exchange for 

 acetyl CoA. Each step in the long series of reactions leading to this 

 end is catalysed and depends upon one of the respiratory enzymes. 



THE EFFECTS OF INJURY ON RESPIRATION, 

 GLYCOLYSIS AND OXIDATIVE PHOSPHORYLATION 



The effects on respiratory enzymes and the reactions they con- 

 trol of injury in the general sense are best considered by a study of 

 events in cells which are "dying" because they have been removed 

 from the living animal. It is known that death of cells is accom- 

 panied by the dissolution of their structure (autolysis) and that 

 this change is associated with activity of cellular proteolytic enzymes 

 and with a rise in intracellular acidity, phenomena which are con- 

 sidered elsewhere (see under Autolysis) . 



The sequence of events has been examined more carefully by 

 perfusing the rat liver following its removal from the body 



