Pyruvate cycl

Combining experiments and computations, Schreiner s research has always exhibited a symbiotic character. His research in carbenes exemplifies this interplay. Despite a great deal of previous work on carbenes, Schreiner discovered that the chalcogene carbenes were not well known. Knowing that the simplest example, hydroxymethylene, had not been made or characterized, he was sure that its preparation would be difficult. How does nature make carbenes he asked. The pyruvate cycle—CO2 extrusion from alpha-ketocarboxylic acids. She figured it all out So, the group tried oxalic acid and Bam Extrusion gave us dihydroxycarbene smoothly The surprise was it was so simple. But it was not all to be so simple. 

The reactions of fatty acid synthesis all take place in the cytosol, but acetyl-CoA is made in the mitochondria and can t cross the inner membrane. The Pyruvate-Malate Cycle (Citrate-Pyruvate Cycle) is used to take acetyl- groups to the cytosol while simultaneously providing a source of NADPH from NADH, and thus, coupling fatty acid synthesis to Glycolysis (Fig. 10.7). Note that the acetyl-CoA is first joined to oxaloacetate to make citrate which is readily transported out of the mitochondria using a co-transporter. The citrate is then cleaved to acetyl-CoA and oxaloacetate, a process requiring ATP to make it favourable (recall the condensation was spontaneous). Acetyl-CoA for fatty acid synthesis is now available in the cytosol, but oxaloacetate must be regenerated for the mitosol. 

R 505 D. Seebach, A.K. Beck and DJ. Bierbaum, The World of (3- and y-Peptides Comprised of Homologated Proteinogenic Amino Acids and Other Components , Chem. Bio divers., 2004,1,1111 R 506 D.S. Sem Chemical Proteomics from a Nuclear Magnetic Resonance Spectroscopy Perspective , Expert Rev.Proteom., 2004,1,165 R 507 A.D. Sherry, F.M.H. Jeffrey and C.R. Malloy, Analytical Solutions for C Isotopomer Analysis of Complex Metabolic Conditions Substrate Oxidation, Multiple Pyruvate Cycles, and Gluconeogenesis , Me-tab.Eng., 2004,6,12... 

Krebs Cycle The oxidative process in respiration by which pyruvate (via acetyl coenzyme A) is completely decarboxylated to COj. The pathway yields 15 moles of ATP (150,000 calories). 

The TCA Cycle—A Brief Summary The Bridging Step Oxidadve Decarboxylation of Pyruvate Entry into the Cycle The Citrate Syntha.se Reaction The Lsomerizadon of Citrate by Aconita.se... 

FIGURE 20.1 Pyruvate produced hi glycolysis is oxidized in the tricarboxylic acid (TCA) cycle. Electrons liberated in this oxidation flow through the electron transport chain and drive the synthesis of ATP in oxidative phosphorylation. In eukaryotic cells, this overall process occurs in mitochondria. 

In 1937 Krebs found that citrate could be formed in muscle suspensions if oxaloacetate and either pyruvate or acetate were added. He saw that he now had a cycle, not a simple pathway, and that addition of any of the intermediates could generate all of the others. The existence of a cycle, together with the entry of pyruvate into the cycle in the synthesis of citrate, provided a clear explanation for the accelerating properties of succinate, fumarate, and malate. If all these intermediates led to oxaloacetate, which combined with pyruvate from glycolysis, they could stimulate the oxidation of many substances besides themselves. (Kreb s conceptual leap to a cycle was not his first. Together with medical student Kurt Henseleit, he had already elucidated the details of the urea cycle in 1932.) The complete tricarboxylic acid (Krebs) cycle, as it is now understood, is shown in Figure 20.4. 

Pyruvate produced by glycolysis is a significant source of acetyl-CoA for the TCA cycle. Because, in eukaryotic ceils, glycolysis occurs in the cytoplasm, whereas the TCA cycle reactions and ail subsequent steps of aerobic metabolism take place in the mitochondria, pyruvate must first enter the mitochondria to enter the TCA cycle. The oxidative decarboxylation of pyruvate to acetyl-CoA,... 

Glucose metabolized via glycolysis produces two molecules of pyruvate and thus two molecules of acetyl-CoA, which can enter the TCA cycle. Combining glycolysis and the TCA cycle gives the net reaction shown ... 

FIGURE 20.23 Export of citrate from mitochondria and cytosolic breakdown produces oxaloacetate and acetyl-CoA. Oxaloacetate is recycled to malate or pyruvate, which re-enters the mitochondria. This cycle provides acetyl-CoA for fatty acid synthesis in the cytosol. 

In a sort of reciprocal arrangement, the cell also feeds many intermediates back into the TCA cycle from other reactions. Since such reactions replenish the TCA cycle intermediates, Hans Kornberg proposed that they be called anaplerotie reactions (literally, the filling up reactions). Thus, PEP carboxylase and pyruvate carboxylase synthesize oxaloacetate from pyruvate (Figure 20.24). 

Pyruvate carboxylase is the most important of the anaplerotie reactions. It exists in the mitochondria of animal cells but not in plants, and it provides a direct link between glycolysis and the TCA cycle. The enzyme is tetrameric and contains covalently bound biotin and an Mg site on each subunit. (It is examined in greater detail in our discussion of gluconeogenesis in Chapter 23.) Pyruvate carboxylase has an absolute allosteric requirement for acetyl-CoA. Thus, when acetyl-CoA levels exceed the oxaloacetate supply, allosteric activation of pyruvate carboxylase by acetyl-CoA raises oxaloacetate levels, so that the excess acetyl-CoA can enter the TCA cycle. 

The catabolism of amino acids provides pyruvate, acetyl-CoA, oxaloacetate, fumarate, a-ketoglutarate, and succinate, ail of which may be oxidized by the TCA cycle. In this way, proteins may serve as excellent sources of nutrient energy, as seen in Chapter 26. 

Oxidation of 2 molecules of glyceraldehyde-3-phosphate yields 2 NADH Pyruvate conversion to acetyl-CoA (mitochondria) 2 NADH Citric acid cycle (mitochondria) 2 molecules of GTP from 2 molecules of succinyl-CoA + 2 + 2... 

Eormadon of PEP by pyruvate Pi dikinase reini-dates the cycle. The CO9 liberated in the bundle slieadi cell is used to syndiesize hexose by die convendonal rubisco-Calvin cycle series of reacdons. 

COMPARTMENTALIZED PYRUVATE CARBOXYLASE DEPENDS ON METABOLITE CONVERSION AND TRANSPORT The second interesting feature of pyruvate carboxylase is that it is found only in the matrix of the mitochondria. By contrast, the next enzyme in the gluconeogenic pathway, PEP carboxykinase, may be localized in the cytosol or in the mitochondria or both. For example, rabbit liver PEP carboxykinase is predominantly mitochondrial, whereas the rat liver enzyme is strictly cytosolic. In human liver, PEP carboxykinase is found both in the cytosol and in the mitochondria. Pyruvate is transported into the mitochondrial matrix, where it can be converted to acetyl-CoA (for use in the TCA cycle) and then to citrate (for fatty acid synthesis see Figure 25.1). /Uternatively, it may be converted directly to 0/ A by pyruvate carboxylase and used in glu-... 

Acetyl-CoA is a potent allosteric effector of glycolysis and gluconeogenesis. It allosterically inhibits pyruvate kinase (as noted in Chapter 19) and activates pyruvate carboxylase. Because it also allosterically inhibits pyruvate dehydrogenase (the enzymatic link between glycolysis and the TCA cycle), the cellular fate of pyruvate is strongly dependent on acetyl-CoA levels. A rise in... 

BOTH NADPH AND ATP ARE NEEDED BY THE CELL, BUT RmOSE-5-P IS NOT Under some conditions, both NADPH and ATP must be provided in the cell. This can be accomplished in a series of reactions similar to case 3, if the fructose-6-P and glyceraldehyde-3-P produced in this way proceed through glycolysis to produce ATP and pyruvate, which itself can yield even more ATP by continuing on to the TCA cycle (Figure 23.40). The net reaction for this alternative is... 

Succinyl-CoA derived from propionyl-CoA can enter the TCA cycle. Oxidation of succinate to oxaloacetate provides a substrate for glucose synthesis. Thus, although the acetate units produced in /3-oxidation cannot be utilized in glu-coneogenesis by animals, the occasional propionate produced from oxidation of odd-carbon fatty acids can be used for sugar synthesis. Alternatively, succinate introduced to the TCA cycle from odd-carbon fatty acid oxidation may be oxidized to COg. However, all of the 4-carbon intermediates in the TCA cycle are regenerated in the cycle and thus should be viewed as catalytic species. Net consumption of succinyl-CoA thus does not occur directly in the TCA cycle. Rather, the succinyl-CoA generated from /3-oxidation of odd-carbon fatty acids must be converted to pyruvate and then to acetyl-CoA (which is completely oxidized in the TCA cycle). To follow this latter route, succinyl-CoA entering the TCA cycle must be first converted to malate in the usual way, and then transported from the mitochondrial matrix to the cytosol, where it is oxida-... 

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