Phosphoenolpyruvate carboxylase, reaction catalyzed

Compartmentation of these reactions to prevent photorespiration involves the interaction of two cell types, mescrphyll cells and bundle sheath cells. The meso-phyll cells take up COg at the leaf surface, where Og is abundant, and use it to carboxylate phosphoenolpyruvate to yield OAA in a reaction catalyzed by PEP carboxylase (Figure 22.30). This four-carbon dicarboxylic acid is then either reduced to malate by an NADPH-specific malate dehydrogenase or transaminated to give aspartate in the mesophyll cells. The 4-C COg carrier (malate or aspartate) then is transported to the bundle sheath cells, where it is decarboxylated to yield COg and a 3-C product. The COg is then fixed into organic carbon by the Calvin cycle localized within the bundle sheath cells, and the 3-C product is returned to the mesophyll cells, where it is reconverted to PEP in preparation to accept another COg (Figure 22.30). Plants that use the C-4 pathway are termed C4 plants, in contrast to those plants with the conventional pathway of COg uptake (C3 plants). 

This enzyme, similar to all C02 assimilating enzymes, contains biotin for a cofactor. Oxaloacetate is released from the mitochondria into the cytoplasm to enter gluconeogenesis. In the cytoplasm, oxaloacetate converts to phosphoenolpyruvate via a reaction catalyzed by phosphoenolpyruvate carboxylase ... 

Phosphoenolpyruvate carboxykinase (GTP) [EC 4.1.1.32], also known as phosphoenolpyruvate carboxylase and phosphopyruvate carboxylase, catalyzes the reaction of GTP with oxaloacetate to produce GDP, phosphoenolpyruvate, and carbon dioxide. ITP can replace GTP as the phosphorylating substrate. 

This enzyme [EC 4.1.1.38] (also known as phosphoenolpyruvate carboxytransphosphorylase, phosphopyruvate carboxylase, and phosphoenolpyruvate carboxylase) catalyzes the reaction of phosphoenolpyruvate with orthophosphate and carbon dioxide to produce oxaloacetate and pyrophosphate (or diphosphate). The enzyme also catalyzes the reaction of phosphoenolpyruvate with orthophosphate to produce pyruvate and pyrophosphate. 

If we add the equations for the reactions catalyzed by pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and nucleoside diphosphate kinase, we obtain the overall reaction for conversion of pyruvate to phosphoenolpyruvate. 

The Q pathway for the transport of CO2 starts in a mesophyll cell with the condensation of CO2 and phosphoenolpyruvate to form oxaloacetate, in a reaction catalyzed by phosphoenolpyruvate carboxylase. In some species, oxaloacetate is converted into malate by an NADP+-linked malate dehydrogenase. Malate goes into the bundle-sheath cell and is oxidatively decarboxylated within the chloroplasts by an NADP+-linked malate dehydrogenase. The released CO2 enters the Calvin cycle in the usual way by condensing with ribulose 1,5-bisphosphate. Pyruvate formed in this decarboxylation reaction returns to the mesophyll cell. Finally, phosphoenolpyruvate is formed from pyruvate by pyruvate-Pi dikinase. 

Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) catalyzes the reaction of phosphoenolpyruvate (PEP) with HC03 to form oxaloacetate (OAA) and orthophosphate in... 

The C4 pathway for the transport of CO2 starts in a mesophyll cell with the condensation of CO2 and phosphoenolpyruvate to form oxaloacetate, in a reaction catalyzed by phosphoenolpyruvate carboxylase. In some species, oxaloacetate is converted into malate by an NADP -linked... 

Comparison of the reactions of glycolysis and gluconeogenesis. All the reactions of glycolysis occur in the cytoplasm of the cell. However, in many human cells, pyruvate carboxylase is found in the mitochondria and phosphoenolpyruvate carboxykinase is located in the cytoplasm. Oxaloacetate, the product of the reaction catalyzed by pyruvate carboxylase, is shuttled out of the mitochondria and into the cytoplasm by a complex set of reactions. 

Two important aspects of the carbon isotopic fractionation imposed by the C4 pathway are summarized graphically in Figures 9 and 10. The first schematically indicates the carbon-isotopic relationships between dissolved CO2, bicarbonate, and the carbon that is added to phosphoenolpyruvate in the reaction catalyzed by phosphoenolpyruvate carboxylase. It shows that, because the kinetic isotope effect associated with PEP carboxylase is smaller than the equilibrium isotope effect between dissolved CO2 and bicarbonate, the fixed carbon is enriched in relative to that in the dissolved CO2. As resi lt, the CO2 subsequently made available to rubisco in cam and C4 plants is enriched in relative to atmospheric CO2. If that p arbon were fixed with perfect efficiency, the biomass of the plant would be enriched in relative to CO2 from the atmosphere—Sp4 would be negative, indicating an inverse fractionation. 

C4 plants concentrate their Calvin cycle photosynthesis in specialized bundle sheath cells, which lie below a layer of mesophyll cells (Figure 17.25). The mesophyll cells, which are most directly exposed to external C02, contain the enzymes of the C4 cycle. See Figure 17.26 for the reactions of the C4 cycle. The key reaction, which is the capture of C02 into oxaloacetate, is catalyzed by the enzyme phosphoenolpyruvate carboxylase and occurs in the mesophyll cells. 

Finally, oxaloacetate is simultaneously decarboxylated and phosphorylated by phosphoenolpyruvate carboxykinase in the cytosol. The CO2 that was added to pyruvate by pyruvate carboxylase comes off in this step. Recall that, in glycolysis, the presence of a phosphoryl group traps the unstable enol isomer of pyruvate as phosphoenolpyruvate (Section 16.1.7). In gluconeogenesis, the formation of the unstable enol is driven by decarboxylation—the oxidation of the carboxylic acid to CO2—and trapped by the addition of a phosphate to carbon 2 from GTP. The two-step pathway for the formation of phosphoenolpyruvate from pyruvate has a AG° of + 0.2 kcal mol ( + 0.13 kj moP ) in contrast with +7.5 kcal mol ( + 31 kj mol ) for the reaction catalyzed by pyruvate kinase. The much more favorable AG° for the two-step pathway results from the use of a molecule of ATP to add a molecule of CO2 in the carboxylation step that can be removed to power the formation of phosphoenolpyruvate in the decarboxylation step. Decarboxylations often drive reactions otherwise highly endergonic. This metabolic motif is used in the citric acid cycle (Section IS.x.x), the pentose phosphate pathway (Section 17.x.x), and fatty acid synthesis (Section 22.x.x). 

The CO2 that was added to pyruvate to form oxaloacetate is released in the reaction catalyzed by phosphoenolpyruvate carboxykinase (PEPCK), which generates PEP (Fig. 31.7A). For this reaction, GTP provides a source of energy as well as the phosphate group of PEP. Pyruvate carboxylase is found in mitochondria. In various species, PEPCK is located either in the cytosol or in mitochondria, or it is distributed between these two compartments. In humans, the enzyme is distributed about equally in each compartment. 

Anaplerotic reactions refer to C3-carboxylation and C4-decarboxylation around the phosphoenolpyruvate-pyruvate-oxaloacetate node, which interconnect the TCA cycle with glycolysis. These reactions result in direct oxaloacetate formation or depletion. Carboxylation of phosphoenolpyruvate catalyzed by phosphoenolpyruvate carboxylase and that of pyruvate by pyruvate carboxylase contribute to its formation. Accordingly, decarboxylation of oxaloacetate catalyzed by phosphoenolpyruvate carboxykinase and oxaloacetate decarboxylase form phosphoenolpyruvate and... 

For the production of glutamic acid, anaplerotic reactions are important to supply oxaloacetate to the TC A cycle for the latter to proceed. C. glutamicum possesses the following two anaplerotic reactions, catalyzed by phosphoenolpyruvate carboxylase (PPC) and pyruvate carboxylase (PC) [63,64] ... 

The fundamental basis of photosynthetic carbon metabolism is the incorporation of carbon dioxide by ribulose-bisphosphate carboxylase (rubisco). This leads to the synthesis of three-carbon sugars which are either exported from the chloroplast or metabolized to regenerate the acceptor ribulose bisphosphate. Rubisco is a bifunctional enzyme in that, in parallel to carboxylation, it catalyzes an oxygenation reaction that leads to phospho-glycolate. This is the starting point for photorespiratory metabolism, which will be discussed below (Section 1.6.2). In C4 plants, the conventional C3 pattern of the photosynthetic carbon reduction Calvin cycle is confined to the bundle sheath cells. The surrounding mesophyll cells act as an ancillary carbon dioxide pump, fixing carbon dioxide via phosphoenolpyruvate carboxylase into C4 acids. These are transported to the bundle sheath for decarboxylation.In this way, photorespiration is limited because of the elevated carbon dioxide levels. 

FIGURE 20.24 Phosphoenolpyruvate (PEP) carboxylase, pyrnvate carboxylase, and malic enzyme catalyze anaplerotlc reactions, replenishing TCA cycle Intermediates. 

The Jirst indirect route in glucose synthesis involves the formation of phosphoenolpyruvate from pyruvate without the intervention of pyruvate kinase. This route is catalyzed by two enzymes. At first, pyruvate is converted into oxaloacetate. This reaction occurs in the mitochondria as the pyruvate molecules enter them, and is catalyzed by pyruvate carboxylase according to the scheme... 

Table 16-2 shows the most common anaplerotic reactions, all of which, in various tissues and organisms, convert either pyruvate or phosphoenolpyruvate to ox-aloacetate or malate. The most important anaplerotic reaction in mammalian liver and kidney is the reversible carboxylation of pyruvate by C02 to form oxaloacetate, catalyzed by pyruvate carboxylase. When the citric acid cycle is deficient in oxaloacetate or any other intermediates, pyruvate is carboxylated to produce more oxaloacetate. The enzymatic addition of a carboxyl group to pyruvate requires energy, which is supplied by ATP—the free energy required to attach a carboxyl group to pyruvate is about equal to the free energy available from ATP. 

Today the metabolic network of the central metabolism of C. glutamicum involving glycolysis, pentose phosphate pathway (PPP), TCA cycle as well as anaplerotic and gluconeogenetic reactions is well known (Fig. 1). Different enzymes are involved in the interconversion of carbon between TCA cycle (malate/oxaloacetate) and glycolysis (pyruvate/phosphoenolpyruvate). For anaplerotic replenishment of the TCA cycle, C. glutamicum exhibits pyruvate carboxylase [20] and phosphoenol-pyruvate (PEP) carboxylase as carboxylating enzymes. Malic enzyme [21] and PEP carboxykinase [22,23] catalyze decarboxylation reactions from the TCA cycle... 

The first reaction is catalyzed by pyruvate carboxylase and the second by phosphoenolpyruvate carboxykinase. The sum of these reactions is ... 

This last pair of reactions is complicated by the fact that p)rruvate carboxylase is found in the mitochondria, whereas phosphoenolp)mivate carboxykinase is found in the cytoplasm. As we will see in Chapters 22 and 23, mitochondria are organelles in which the final oxidation of food molecules occurs and large amounts of ATP are produced. A complicated shuttle system transports the oxaloacetate produced in the mitochondria through the two mitochondrial membranes and into the cytoplasm. There, phosphoenolp)mivate carboxykinase catalyzes its conversion to phosphoenolpyruvate. 

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