Oxaloacetate reaction stoichiometry

This type of stoichiometry is obvious and is tacitly assumed in any discussion of metabolism. It is mentioned explicitly here only for the sake of completeness. For example, 1 /imole of acetyl-SCoA will condense with 1 /zmole of oxaloacetate to form 1 //mole of citrate. Summation of such reaction stoichiometries leads to carbon balances and similar stoichiometries of sequences thus, 1 /imole of glucose can be converted to 2 //moles of lactate or 6 jumoles of CO2. 

The transformation of pyruvate to carbon dioxide is achieved by the several steps in a cyclical series of reactions known as the tricarboxylic acid (TCA) cycle. The name of the cycle comes from the first step where acetyl-CoA is condensed with oxaloacetic acid to form citric acid, a tricarboxylic acid. Once citrate is formed the material is converted back to oxaloacetate through a series of 10 reactions, as illustrated in Fig. 5.22, with the net production of 2 molecules of carbon dioxide and reducing equivalents in the form of 4 molecules of NADH + H and 1 molecule of FADH2, together with 1 mole of ATP. The overall stoichiometry of the TCA cycle from pyruvate is ... 

The answer in the text is for the conversion of fumarate to oxaloacetate rather than to aspartate. The equation above gives the correct stoichiometry for the synthesis of urea from NH4 and glutamate. It includes a non-energy-requiring transamination reaction. Hence, the number of P spent remains at four. Note that aspartate does not appear in this equation, since it is resynthesized. Thus aspartate can be considered a nitrogencarrying colactor in the synthesis of urea. 

This enzyme catalyses the decarboxylation of the ) -ketoacid oxaloacetate, with the same stoichiometry as acetoacetate decarboxylase. The former however, requires a Mn ion for activity and is insensitive to the action of sodium borohydride. This duality of mechanism is not unlike the one observed for enzymatic aldol condensation, where enzymes of Class 1 react by forming Schiff-base intermediates, whereas enzymes of Class II show metal ion requirements [47]. Oxaloacetate decarboxylase from cod also catalyses the reduction by borohydride of the enzymatic reaction product pyruvate. This is evidenced by the accumulation of D-lactate in presence of enzyme, reducing agent, and manganous ions. It has been proposed that both reduction and decarboxylation occur by way of an enzyme-metal ion-substrate complex in which the metal ion acts as an electron sink, thereby stabilizing the enolate ion formed in the decarboxylation reaction [48] ... 

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