Pyruvate to oxaloacetate

The pyruvate carboxylase reaction is activated by Mg + and, through mass action, by an increase in either the [ATP]/[ADP] or the [pyruvate]/[oxaloacetate] ratio. It is virtually inactive in the absence of acetyl-CoA, an allosteric activator. The enzyme is allosteiically inhibited by glutamate, since oxaloacetate formed in excess would flood the TCA cycle and result in a buildup of a-ketoglutarate and glutamate. Pyruvate carboxylase 

CHAPTER 15 Carbohydrate Metabolism II Gluconeogenesis, Glycogen Synthesis and Breakdown, and Alternative Pathways 

By increasing substrate availability through stimulation of mitochondrial respiration. This decreases the intramitochondrial concentration of H and increases the rate of pyruvate transport. The [ATP]/[ADP] ratio also rises. 

By increasing the rate of fatty acid oxidation, which results in an increase in the mitochondrial concentration of acetyl-CoA, an allosteric activator of pyruvate carboxylase. 

By decreasing the mitoehondrial concentration of glutamate, an inhibitor of pyruvate carboxylase, through stimulation of the TCA cycle (secondary to the increase in mitochondrial acetyl-CoA) and the aspartate shuttle (secondary to the increase in cytosolic PEPCK induced by glucagon). 

---

Theoretically, a fall in concentration of oxaloacetate, particularly within the mitochondria, could impair the ability of the citric acid cycle to metabolize acetyl-CoA and divert fatty acid oxidation toward ketogenesis. Such a fall may occur because of an increase in the [NADH]/[NAD+] ratio caused by increased P-oxida-tion affecting the equilibrium between oxaloacetate and malate and decreasing the concentration of oxaloacetate. However, pyruvate carboxylase, which catalyzes the conversion of pyruvate to oxaloacetate, is activated by acetyl-CoA. Consequently, when there are significant amounts of acetyl-CoA, there should be sufficient oxaloacetate to initiate the condensing reaction of the citric acid cycle. 

Figure 5.13 Conversion of pyruvate to oxaloacetate and then to phosphoenolpyruvate.

Pyruvate carboxylase (also called PC) is an enzyme that converts pyruvate to oxaloacetate (shown as oxaloacetic acid in the citric acid cycle diagram). Pyruvate carboxylase deficiency is a genetic disorder that is characterized by insufficient quantities of pyruvate carboxylate in the body. How do you think this disorder affects the citric acid cycle Use print and electronic resources to research pyruvate carboxylase deficiency. Find out what its symptoms are, and how it affects the body at the molecular level. Also find out what percent of the population is affected, and how the deficiency can be relieved. Present your findings as an informative pamphlet. If possible, conduct an e-mail interview with an expert on the disorder. 

The degradation of most amino acids is anaplerotic, because it produces either intermediates of the cycle or pyruvate glucogenic amino acids see p. 180). Gluconeogenesis is in fact largely sustained by the degradation of amino acids. A particularly important anaplerotic step in animal metabolism leads from pyruvate to oxaloacetic acid. This ATP-dependent reaction is catalyzed by pyruvate... 

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

Pyruvate is first transported from the cytosol into mitochondria or is generated from alanine within mitochondria by transamination, in which the a-amino group is removed from alanine (leaving pyruvate) and added to an a-keto carboxylic acid (transamination reactions are discussed in detail in Chapter 18). Then pyruvate carboxylase, a mitochondrial enzyme that requires the coenzyme biotin, converts the pyruvate to oxaloacetate (Fig. 14-17) ... 

Acetyl-CoA is a critical regulator of the fate of pyruvate it allosterically inhibits pyruvate dehydrogenase and stimulates pyruvate carboxylase (see Fig. 15-20). In these ways acetyl-CoA prevents it own further production from pyruvate while stimulating the conversion of pyruvate to oxaloacetate, the first step in gluconeo-genesis. 

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). 

Since the system responds to the concentration of acetyl-CoA and thus indirectly to the concentration of oxaloacetate, it adjusts automatically to maintain a functional concentration of oxaloacetate regardless of whether the intermediate removed from the cycle is citrate, a-ketoglutarate, succinyl-CoA, oxaloacetate itself, or any combination of these. The same effects cause the rate of conversion of pyruvate to oxaloacetate to decrease when the rate of removal of biosynthetic precursors decreases. 

Thus, reversal of the glycolytic step from PEP to pyruvate requires two reactions in gluconeogenesis, pyruvate to oxaloacetate by pyruvate carboxylase and oxaloacetate to PEP by PEP carboxykinase. Given that the conversion of PEP to pyruvate in glycolysis synthesizes ATP, it is not surprising that the overall reversal of this step needs the input of a substantial amount of energy, one ATP for the pyruvate carboxylase step and one GTP for the PEP carboxy kinase step. 

Biotin, an essential water-soluble B-complex vitamin, is the coenzyme for four human carboxylases (Fig. 12-2) These include the three mitochondrial enzymes pyruvate carboxylase, which converts pyruvate to oxaloacetate and is the initial step of gluconeogenesis propionyl-CoA carboxylase, which catabolizes several branched-chain amino acids and odd-chain fatty acids and 3-methylcrotonyl-CoA carboxylase, which is involved in the catabolism of leucine and the principally cytosolic enzyme, acetyl-CoA carboxylase, which is responsible for the... 

Step A, the conversion of pyruvate to phosphoenolpyruvate, is accomplished by a circuitous process commencing with pyruvate entering the mitochondrion, which for gluconeogenesis to occur must be in a high-energy state. Under these conditions, the mitochondrial enzyme pyruvate carboxylase catalyzes the conversion of pyruvate to oxaloacetate-. 

The cycle oxidizes acetyl-CoA, and to perform this task, it must convert acetyl-CoA to citrate. For this to be achieved, oxaloacetate must be available. If the removal of intermediates results in a decrease in the amount of oxaloacetate for this purpose, acetyl-CoA cannot be removed and will accumulate. This will inhibit the pyruvate dehydrogenase complex and activate pyruvate carboxylase, leading to the conversion of pyruvate to oxaloacetate. This product is now available to condense with the acetyl-CoA to produce citrate, which will restore the status quo. Reactions like that of pyruvate carboxylase that provide molecules for the replacement of intermediates of the citric acid cycle are known as anaplerotic reactions (Greek, meaning to fill up ana = up + plerotikos from pleroun = to make full ). 

For the conversion of pyruvate to oxaloacetate and the formation of citrate in the mitochondrion, see Chap. 12. Acetyl-CoA for fatty acid synthesis is converted to malonyl-CoA this reaction is catalyzed by acetyl-CoA carboxylase. Seven molecules of acetyl-CoA are converted to malonyl-CoA for the synthesis of one molecule of palmitic acid. 

The metabolic flux distributions around the intermediate pyruvate for different strains and environmental conditions are summarised in Fig. 12. This part of the metabolism has been shown to be an important node for the interconversion between glycolytic C3 metabolites and C4 metabolites of the tricarboxylic acid (TCA) cycle. The different anaplerotic reactions are of special importance for the production of recombinant proteins as they provide precursors, such as oxaloace-tate, for amino acid biosynthesis. Due to that, the flux distribution is noticeably affected by both the cultivation conditions and the carbon source used which indicates flexible adaptation to the environmental situation. The flux from pyruvate to oxaloacetate through the reaction catalysed by pyruvate carboxylase was found to be the main anaplerotic pathway in B. megaterium. 

Pyruvate Carboxylase Pyruvate carboxylase catalyzes the car-boxylation of pyruvate to oxaloacetate - both the first committed step of gluconeogenesis from pyruvate and also an important anaplerotic reaction, permitting repletion of tricarboxylic acid cycle intermediates and hence fatty acid synthesis. The mammalian enzyme is activated aUosterically by acetyl CoA, which accumulates when there is a need for increased activity of pyruvate carboxylase to synthesize oxaloacetate to permit increased citric acid cycle activity or for gluconeogenesis (Attwood, 1995 Jitrapakdee and Wallace, 1999). 

Pyruvate carboxylase converts pyruvate to oxaloacetate in the mitochondrion. Oxaloacetate is converted to malate or aspartate, which travels to the cytosol and is reconverted to oxaloacetate. 

B. The synthesis of fatty acids from glucose occurs in the cytosol, except for the mitochondrial reactions in which pyruvate is converted to citrate. Biotin is required for the conversion of pyruvate to oxaloacetate, which combines with acetyl CoA to form citrate. Biotin is also required by acetyl CoA carboxylase. Pantothenic acid is covalently bound to the fatty acid synthase complex as part of a phosphopantetheinyl residue. The growing fatty acid chain is attached to this residue during the sequence of reactions that produce palmitic acid. NADPH, produced by the malic enzyme as well as by the pentose phosphate pathway, provides reducing equivalents. Citrate, not isocitrate, is a key regulatory compound. 

Other biotin-dependent enzymes include propionyl-CoA carboxylase and pyruvate carboxylase (Chapter 15). The latter, like acetyl-CoA carboxylase, is subject to allosteric regulation. Pyruvate carboxylase, a mitochondrial enzyme, is activated by acetyl-CoA and converts pyruvate to oxaloacetate which, in turn, is converted to glucose via... 

The answer is b. (Murray, pp 627-661. Scriver, pp 3897-3964. Sack, pp 121-138. Wilson, pp 287-3202) Pyruvate carboxylase catalyzes the conversion of pyruvate to oxaloacetate in gluconeogenesis ... 

The four carbon intermediates of the TCA cycle may be replenished or increased by metabolites of the glucogenic amino acids entering at a-ketoglutarate, succinyl-CoA, or oxaloacetate. In addition, the pool can also be increased by the carboxylation of pyruvate to oxaloacetate catalyzed by pyruvate carboxylase. 

Synthesis of PEP. PEP synthesis from pyruvate requires two enzymes pyruvate carboxylase and PEP carboxykinase. Pyruvate carboxylase, found within mitochondria, converts pyruvate to oxaloacetate (OAA) ... 

Pyruvate carboxylase deficiency is a usually fatal disease caused by a missing or defective enzyme that converts pyruvate to oxaloacetate. It is characterized by varying degrees of mental retardation and disturbances in several metabolic QUESTION 9.6... 

Pyruvate carboxylase converts pyruvate to oxaloacetate. If the enzyme is inactive, concentrations of pyruvate in the system rise and pyruvate is converted by NADH to lactate. Excess lactate is then excreted in the urine. 

---