Carboxylation of pyruvate

Fig. 16. Ferredoxin (Fd)/ferredoxin-NADP+-reductase (FNR) mediated enzymatic carboxylation of pyruvic acid to form malic acid catalyzed by the NADPH-dependent malic enzyme (ME)...

The conversion of pyruvate into phosphoenolpyravate, which bypasses the pyruvate kinase reaction, requires two separate reactions carboxylation of pyruvate to... 

Carboxylation of pyruvate to oxaloacetate occurs in the mitochondria (Figure 6-7). 

Biotin (vitamin B ) is widespread in foods and is also synthesized by intestinal bacteria. It is a coenzyme for the carboxylation of pyruvate, acetyl-coenzyme-A (CoA), propionyl CoA, and /1-methyl-crotonyl CoA and is involved in fatty acid formation and in energy release from carbohydrates. In humans deficiencies only occur in patients with an abnormal gut flora and manifests itself as exfoliative dermatitis and alopecia. 

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. 

Pyruvate carboxylase has four identical subunits, each containing a molecule of biotin covalently attached through an amide linkage to the -amino group of a specific Lys residue in the enzyme active site. Carboxylation of pyruvate proceeds in two steps (Fig. 16-16) first, a carboxyl group derived from HCO3 is attached to biotin,... 

Carboxylation of pyruvate to oxaloacetate (OAA) by pyruvate carboxylase is a biotin-dependent reaction (see Figure 8.24). This reaction is important because it replenishes the citric acid cycle intermediates, and provides substrate for gluconeogenesis (see p. 116). 

Biotin acts as a carboxyl group carrier in a series of carboxylation reactions, a function originally suggested by the fact that aspartate partially replaces biotin in promoting the growth of the yeast Torula cremonis. Aspartate was known to arise by transamination from oxaloacetate, which in turn could be formed by carboxylation of pyruvate. Subsequent studies showed that biotin was needed for an enzymatic ATP-dependent reaction of pyruvate with bicarbonate ion to form oxaloacetate (Eq. 14-3). This is a (3 carboxylation coupled to the hydrolysis of ATP. 

The first step in the conversion of pyruvate to PEP entails the carboxylation of pyruvate to form oxaloacetate. This reaction is catalyzed by pyruvate carboxylase. As in... 

The carboxylation of pyruvate supplies a significant portion of the thermodynamic push for the next step in the sequence. This is because the free energy change for decarboxylation of /3-keto carboxylic acids such as oxaloacetate is large and negative. The oxaloacetate formed from pyruvate by carboxylation is converted to phosphoenolpyruvate in a reaction catalyzed by phosphoenolpyruvate carboxyki-nase. In many species, including mammals, this reaction involves a GTP-to-GDP conversion. 

The biocatalyzed photosystem of NADPH regeneration (Sect. 6.1) has been coupled to the biocatalyzed carboxylation of pyruvic acid and a-ketoglutaric acid... 

Regulation of Pyruvate Carboxylase The carboxylation of pyruvate by pyruvate carboxylase occurs at a very low rate unless acetyl-CoA, a positive allosteric modulator, is present. If you have just eaten a meal rich in fatty acids (triacylglycerols) but low in carbohydrates (glucose), how does this regulatory property shut down the oxidation of glucose to C02 and H20 but increase the oxidation of acetyl-CoA derived from fatty acids ... 

Schulman, M., Ghambeer, R. K., Ljungdahl, L. G., and Wood, H. G., 1973, Total synthesis of acetate from C02. VII. Evidence with Clostridium thermoaceticum that die carboxyl of acetate is derived from the carboxyl of pyruvate by transcarboxylation and not by fixation of CO2, J. Biol. Chem. 248 6255n626L... 

The first step in gluconeogenesis is the carboxylation of pyruvate to form oxaloacetate at the expense of a molecule of ATP. Then, oxaloacetate is decarboxylated and phosphorylated to yield phosphoenolpyruvate, at the expense of the high... 

The carboxylation of pyruvate takes place in three stages ... 

Figure 16.28. Compartmental Cooperation. Oxaloacetate utilized in the cytosol for gluconeogenesis is formed in the mitochondrial matrix by carboxylation of pyruvate. Oxaloacetate leaves the mitochondrion by a specific transport system (not shovm) in the form of malate, -which is reoxidized to oxaloacetate in the cytosol.

How is oxaloacetate replenished Mammals lack the enzymes for the net conversion of acetyl CoA into oxaloacetate or any other citric acid cycle intermediate. Rather, oxaloacetate is formed by the carboxylation of pyruvate, in a reaction catalyzed by the biotin-dependent enzyme pyruvate carboxylase. 

A third fate of pyruvate is its carboxylation to oxaloacetate inside mitochondria, the first step in gluconeogenesis. This reaction and the subsequent conversion of oxaloacetate into phosphoenolpyruvate bypass an irreversible step of glycolysis and hence enable glucose to be synthesized from pyruvate. The carboxylation of pyruvate is also important for replenishing intermediates of the citric acid cycle. Acetyl CoA activates pyruvate carboxylase, enhancing the synthesis of oxaloacetate, when the citric acid cycle is slowed by a paucity of this intermediate. 

The photochemical carboxylation of pyruvic acid by this process is endergonic by about AG° = 11.5 kcal mol and represents a true uphill photosynthetic pathway. The carbon dioxide fixation product can then act as the source substrate for subsequent biocatalyzed transformations. For example, photogenerated malic acid can act as the source substrate for aspartic acid (Figure 35). In this case, malic acid is dehydrated by fumarase (Fum) and the intermediate fumaric acid is aminated in the presence of aspartase (Asp) to give aspartic acid. 

The interaction between Tyr 143 and the carboxylate of pyruvate in subunit 2 has already been mentiond (Section III,D). In subunit 1, however, where pyruvate is absent, Tyr 143 is now hydrogen bonded to an oxygen of one of the heme propionate groups (25). Thus it would appear that this residue is involved in both lactate dehydrogenation and interdomain communication. The possible central role of Tyr 143 will be discussed further in Section VI,D,3. 

Carboxylation. Gluconeogenesis begins with the carboxylation of pyruvate to yield oxaloacetate. As in the third step of fatty acid synthesis (Figure 29.8), the reaction requires ATP and the coenzyme biotin, acting as a carrier of CO2. 

Biotin is involved in the carboxylation of pyruvate (which forms oxaloacetate), acetyl CoA (which forms malonyl CoA), and other compounds. 

Fig. I. Simplified scheme of the TCA cycle and the formation of glutamate from a-ketoglutarate (a-kg). In astrocytes glutamate is amidated to glutamine in GABAergic neurons some of the glutamate is decarboxylated and enters the GABA shunt. In both neurons and astrocytes anaplerosis occurs via carboxylation of pyruvate to malate or oxaloacetate (ox-ac) from which aspartate is formed.

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