Pyruvate carboxylase partial reactions

Pyruvic acid is an intermediate in the fermentation of grains. During fermentation the enzyme pyruvate carboxylase causes the pyruvate ion to release carbon dioxide. In one experiment a 200.-mL aqueous solution of the pyruvate in a sealed, rigid 500.-mL flask at 293 K had an initial concentration of 3.23 mmol-L -l. Because the concentration of the enzyme was kept constant, the reaction was pseudo-first order in pyruvate ion. The elimination of CU2 by the reaction was monitored by measuring the partial pressure of the C02 gas. The pressure of the gas was found to rise from zero to 100. Pa in 522 s. What is the rate constant of the pseudo-first order reaction ... 

The first partial reaction of pyruvate carboxylase, the formation of carboxybiotin, depends on the presence of acetyl CoA. Biotin is not carboxylated unless acetyl CoA is bound to the enzyme. Acetyl CoA has no effect on the second partial reaction. The allosteric activation of pyruvate carboxylase by acetyl CoA is an important physiological control mechanism that will be discussed in Section 17.3.1. 

Fig. 9. Effect of glucagon on hepatic gluconeogenesis. Reversible reactions are indicated by double arrows, irreversible reactions by simple arrows. A crossed arrow indicates an inhibited reaction and an asterisk an enhanced reaction. PC = pyruvate carboxylase PK = pyruvate kinase PDH = pyruvate dehydrogenase PEPCK = phospho-enol pyruvate cocarboxykinase PFK = phosphofructokinase. (Partially redrawn from Ruderman et al Arch. Internal. Med., 123, 299,1969.)...

Certain microorganisms synthesize acetoin by a modified pathway. The enzymes involved have been partially purified from extracts of Aerobacter aerogenes. The first step resembles the carboxylase reaction of pig heart, in that acetaldehyde is not found, but all of the two-carbon fragment is transferred to another carbonyl group, forming an acyloin. The only known acceptor is pyruvate, and the product is d-a-acetolactate. 

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