170 GENERAL BIOCHEMISTRY 



reactants and products. As usual the ionizable compounds are shown 

 in a form existing under physiological conditions. 



These reactions are all enzymatic, catalyzed by the enzymes named. 

 The known cofactors are also indicated. Two of these substances have 

 not been discussed previously, and both function in the same way. 



This mechanism provides for the transfer of the phosphate group 

 without consuming 2,3-diphosphoglycerate. Glucose-6-phosphate is 

 formed from glucose-1-phosphate in the same way, with glucose-1,6- 

 diphosphate as the cofactor and a similar transfer of phosphate. 



The interconversion of glycogen or amylopectin and glucose-1-phos- 

 phate requires a step not shown because of the branches in these poly- 

 saccharides. An enzyme, 4,6-transglucosidase, reversibly shifts the 

 branches from the 4 to the 6 positions, where they can be phoshory- 

 lated in the reaction shown. 



Anaerobic processes yield lactate, ethanol, and related compounds 

 from pyruvate as indicated. In air much of the pyruvate enters the 

 citric acid cycle. The remainder is converted to acetate, fats, and 

 alanine in either the presence or the absence of oxygen. 



The citric acid cycle. This group of reactions (Figure 7-2) provides 

 for the further utilization of pyruvate by respiration, converting the 

 carbon atoms to carbon dioxide. Most of the energy arising from the 

 respiration of plants and animals is derived ultimately from the oxida- 

 tions and reductions of this cyclic process. For purposes of con- 

 venience, pyruvate is shown on the inside of the circular array of reac- 

 tions. 



There are still differences of opinion about several of these reactions, 

 especially some of the more complex transformations. Continuing re- 

 search will doubtless produce some revisions if the past is any index. 

 Nevertheless, the general form of the citric acid cycle seems soundly 

 established. 



The isocitrate to oxalosuccinate conversion involves a different 

 coenzyme than any considered heretofore. Written in the oxidized 

 and reduced forms as TPN+ and TPNH, respectively, the structures 

 correspond to DPN+ and DPNH. The name triphosphopyridine 



