Chapter 5 

 ENZYMES 



RESPIRATION AND THE SUPPLY OF ENERGY 



T, 



he integrity of the cell, its continued existence and the main- 

 tenance of its specialised functions depend ultimately on a supply 

 of energy. Interference with this supply occurs as a result of a large 

 variety of injuries including toxic substances, some of which are 

 discussed elsewhere, and if sufficiently severe lead to cell death 

 (necrosis) . 



The vital supply of energy is produced by the cell itself and is 

 derived from the metabolism of carbohydrate, fat and protein. In 

 effect, the energy is derived from the conversion of glucose to 

 pyruvate (glycolysis) and the oxidation of pyruvate to carbon 

 dioxide and water. Glycolysis provides about 20 per cent of the 

 cell's energy requirements and the oxidation of pyruvate the re- 

 mainder. 



To release the energy of glucose and pyruvate under the ordi- 

 nary conditions of the chemical laboratory would require high 

 temperatures or extremes of hydrogen ion concentration. The body 

 does so at 37° and at physiological pH. This is achieved with the aid 

 of enzymes, which are very efficient organic catalysts usually pro- 

 tein in nature and characteristically specific for the reaction they 

 control. Again, energy liberated by a usual chemical reaction in 

 vitro is obtained in the form of heat. Energy in this form would be 

 useless to the cell which instead possesses a specialised compound, 

 adenosine triphosphate (ATP) capable of trapping energy (in the 

 shape of high energy phosphate bonds) and passing it on to those 

 reactors which require it. Finally, to allow oxidation to occur under 

 physiological conditions, the cell does not add oxygen to the sub- 

 stance being oxidised, but instead subtracts hydrogen ions. The 

 hydrogen ions (i.e. electrons) are passed on to specialised com- 

 pounds with an affinity for hydrogen known as hydrogen carriers. 

 Cytochrome, flavoproteins and pyridine nucleotides are the most 



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