ENERGY TRANSFERS AND BIOLOGICAL OXIDATIONS 



171 



oxaloacetale 



DPNH 



malic 

 dehydrogenase, 



DPN"*"- 



CH. _,("()()" 



HOCCOOH 



I 



y CH2COO 



\ citrate 



i~ \ aconitasp 



CH3CO-SC0A 



1 

 CoA- 



SH 



"OOCCH2CHOHCOO- 



i malate ■'^ 



I DPNH + COo 



fumarase 



pyruvic 



dehydrogenase 



TPP 



^ 



H20 



fiiniarato 



■OOCCH=CHCOO" 



k 



\ succinic dehydrogenase 



"OOCCHaCHgCOO" 



succinate 



^^^ ^CoA-SH 



GTP- 



DPN" 



CHoCOCOO" 



pyruvate 



'-H.A) 

 CHzCOO" 



CCOOH 



II 

 CHCOO" 



r/s-aconitate 



\ 

 aconitasp ->-H«>0 



T 

 CH2COO- 



isocitrate CHCOOH 



I 



CHOHCOO- 

 -TPN+ 



isocitru' 

 dehydrogenase 



TPP \Mg++COO- 

 succinyl CoA 



transphosphorylase y^ pir 



HPOf + GDP 



SCoA 

 succinyl CoA 



TPNH 



oxalosuccinatc 



CH9COO" 



CHCOOH 

 COCOO" 



. + + 



Mu- 



isocitric 



dehydrogenase 



COO- 



a-ketoglutarate 



FIGURE 7-2. The Krebs tricarboxylic acid cycle as written in 1959. GTP and GDP 

 represent guanosine triphosphate and guanosine diphosphate, respectively, com- 

 pounds similar to ATP and ADP l)ut containing guanine instead of adenine. TPP 

 represents thiamine pyrophosphate (page 330), known also as cocarboxylase, and 

 probably acts in coml)ination with a second coenzyme called lipoic acid in the 

 two processes indicated. 



nucleotide denotes the presence of a third phosphate group which is 

 attached to the second position of the ribose attached in turn to the 

 adenine. Otherwise DPN+ and TPN+ have the same structures. 



Each hexose unit metabolized by the pathway of glycolysis yields two 

 molecules of pyruvate. Each pyruvate in turn yields three molecules 



