346 



Enzymes: Kinetics of Oxidations /I8 : 4 



and active transport. The second is to produce heat to maintain the 

 cellular temperature in an optimum range. Because all the energy- 

 conversion processes are less than 100 per cent efficient, some heat is 

 always a by-product. The warm-blooded animals have internal 

 systems to regulate the efficiency of energy conversion so as to maintain 

 a more or less constant internal temperature. Some cold-blooded 

 animals also tend to regulate their internal temperature but must vary 

 their muscular activity to do so. 



4. Oxidative Phosphorylation 



In most animal cells, a major part of the energy released by oxidations 

 is used to drive the reaction 



ADP + P -> ATP 



which may be written in structural form as 



H— N— H 



O O O 



o — p — o— p— cr + o — p — oh 



H-C-H OH OH 

 H 



OH 



H-N-H 



H-CH OH OH 



OH 



The ATP is formed primarily by the cytochrome chain which is located 

 in intracellular organelles called mitochondria; these are shown in 

 Figure 1, Chapter 15. The mitochondria also contain a set of enzymes 

 which catalyze a cyclic process called Krebs' cycle. Two and three 

 carbon fragments of glucose, fatty acids, and amino acids are coupled 

 into Krebs' cycle. It, in turn, drives the cytochrome chain, thereby 

 causing the conversion of the energy of oxidation to ATP. The mole- 

 cule ATP can diffuse throughout the cell and couple with many 

 enzyme systems. The reverse action 



ATP -> ADP + P 



can provide the energy to drive chemical syntheses, to cause active 



