Phosphoenolpyruvate cleavage

Figure 3. Examples of major types of uptake mechanisms realised in prokaryotic outer membranes (a to c) and cytoplasmic membranes (a, and d to 1). The solutes to be transported are shown by filled circles x symbolises another solute which is transported in the same or in the opposite direction. In systems h-k, uptake is driven by the cleavage of ATP to ADP and phosphate. One type of uptake system, 1, depends on the energy-rich molecule phosphoenolpyruvate shown as PEP .

In the presence of excess phosphohexose isomerase and glucose-6-phos-phate dehydrogenase the rate of reduction of TPN is proportional to the rate of cleavage of fructose diphosphate. For cases when small, quantities of fructose diphosphate must be used, a second spectrophoto-metric assay, in which fructose diphosphate is regenerated, has been proposed 20). Fructose 6-phosphate is phosphorylated with ATP and phosphofructokinase, and the adenosine diphosphate (ADP) produced is measured with phosphoenolpyruvate and lactic dehydrogenase ... 

Carboxylation is, in the first place, a thermodynamic problem. Nature solves this problem by associating the carboxylation with some exergonic process. This may be reduction of a product (as in the case of malic enzyme and isocitrate dehydrogenase), hydrolysis of ATP (as in the case of pyruvate carboxylase), hydrolysis of phosphoenolpyruvate (as in the case of phosphoenolpyruvate carboxylase), or cleavage of a carbon-carbon bond (as in the case of ribulose-bisphosphate carboxylase). 

PEP-Utilizing Enzymes that Catalyze C-O Bond Cleavage of Phosphoenolpyruvate 671... 

The carboxylation reaction produces an activated carboxyl group in the form of a high-energy carboxybiotin intermediate. The cleavage of this bond and release of CO2 in the phosphoenolpyruvate carboxykinase reaction or the transfer of the CO2 to acceptors in other reactions in which biotin participates allows endergonic reactions to proceed. Thus, the formation of phosphoenolpyruvate from oxaloacetate is driven by the release of CO2 (AG° = -4.7 kcal/mol) and the hydrolysis of GTP (AG° = -7.3 kcal/mol). 

The reaction requires an energy expenditure of 7 kcal/mole, which has to be provided by some other exergonic reaction. This may come, e.g. from the cleavage of phosphoenolpyruvate ... 

Although I have chosen to describe in some detail our experiments on the inactivation of phosphoenolpyruvate carboxykinase, the conclusions are compatible with inactivation measurements on tryptophan oxygenase (Li and Knox, 1972) and, with the exception of an early proteolytic cleavage, on ornithine aminotransferase (Kominami and Katun-uma, 1976). 

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