STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 83 



oxidative decarboxylation. Thus, pyruvate is converted via the pyruvic 

 oxidase of mitochondria to the thioester, acetyl coA, in the follovviTig 

 sequence: 



pyruvate + thiamine pyrophosphate (TPP) ^-acetaldehyde . TPP -}- CO 2 



acetaldehyde . TPP + oxidized lipoate 5>-acetyl Hpoate -|- TPP 



acetyl lipoate -\- coASH ^acetyl — SCoA -f" reduced lipoate 



A hpoate dehydrogenase requiring DPN and hnked to the cytocliromes 

 regenerates oxidized lipoate. In a comparable series of reactions, a-keto- 

 glutarate forms succinyl — SCoA plus COg. 



These thioesters may participate in the following synthetic reactions to 

 form ATP: 



acetyl-SCoA + AMP ^ acetyl - A^IP + CoASH (la) 



acetyl — AMP + pyrophosphate ^ acetate + ATP (lb) 



In this group of reactions, the energy conserved in the thioester linkage is 

 stored in the carboxy-phosphoanhydride of acetyl — AJVIP prior to formation 

 of ATP. The apparent role of such reactions in protein synthesis will be 

 discussed in a later section. Eeaction (2a) takes place in heart muscle and 

 (2b) in spinach. 



succinyl — SCoA + P + guanosine diphosphate (GDP) ^ GTP + CoASH + 



succinate (2a) 

 succinyl - SCoA + P + ADP ^ ATP + CoASH + succinate (2b) 



Myokinase is also present in mitochondria and catalyzes the reaction 



2 ADP ^ AMP + ATP 

 In addition to generating ATP, acetyl coA can participate in the following 

 reactions: 



Acetyl — SCoA + oxalacetate ^ citrate + coASH ( 1 ) 



starting the turn of a tricarboxyhc acid cycle as seen in formula (I) which 

 can degrade two carbons of acetate to CO 2 : 



Pyruvate -^^^^ acetyl -ScoA 



^,JI^^^^^="*' Citrate — >■ c/s - aconltate 



Oxalacetate ^^ 



/Isocitrate 

 \ \TPN 

 Malate DPn\ ^ , • ,^ 



Fumarate ct-ketoglutarate 



^ —C02^^ \+NH3 



Succinate 



'*■ — Succinyl -ScoA 



Porphobilinogen 

 (I) 



