Mitochondria pyruvate dehydrogenase

As can be seen in Figure 6.42, pyruvate is very much a focal point in GNG. Normally, once pyruvate has entered a mitochondrion, it is converted into acetyl-CoA by pyruvate dehydrogenase complex, but for GNG the pyruvate is diverted in to oxaloacetate (OAA) by pyruvate carboxylase (see Figure 6.43). 

The tricarboxylic acid cycle not only takes up acetyl CoA from fatty acid degradation, but also supplies the material for the biosynthesis of fatty acids and isoprenoids. Acetyl CoA, which is formed in the matrix space of mitochondria by pyruvate dehydrogenase (see p. 134), is not capable of passing through the inner mitochondrial membrane. The acetyl residue is therefore condensed with oxaloacetate by mitochondrial citrate synthase to form citrate. This then leaves the mitochondria by antiport with malate (right see p. 212). In the cytoplasm, it is cleaved again by ATP-dependent citrate lyase [4] into acetyl-CoA and oxaloacetate. The oxaloacetate formed is reduced by a cytoplasmic malate dehydrogenase to malate [2], which then returns to the mitochondrion via the antiport already mentioned. Alternatively, the malate can be oxidized by malic enzyme" [5], with decarboxylation, to pyruvate. The NADPH+H formed in this process is also used for fatty acid biosynthesis. 

Fatty acids are generated cytoplasmically while acetyl-CoA is made in the mitochondrion by pyruvate dehydrogenase.This implies that a shuttle system must exist to get the acetyl-CoA or its equivalent out of the mitochondrion. The shuttle system operates in the following way Acetyl-CoA is first converted to citrate by citrate synthase in the TCA-cycle reaction. Then citrate is transferred out of the mitochondrion by either of two carriers, driven by the electroos-motic gradient either a citrate/phosphate antiport or a citrate/malate antiport as shown in Figure 2-2. 

Fig. 5.4. Two types of energy metabolism in cestodes. (a) Type 1 homolactate fermentation, (b) Type 2 Malate dismutation. Reaction 3 involves a carboxylation step decarboxylation occurs at 6, 7 and 10. Reducing equivalents are generated at reactions 6 and 7 one reducing equivalent is used at reaction 9. Thus, when the mitochondrial compartment is in redox balance and malate is the sole substrate, twice as much propionate as acetate is produced. Key 1, pyruvate kinase 2, lactate dehydrogenase 3, phosphoenolpyruvate carboxykinase 4, malate dehydrogenase 5, mitochondrial membrane 6 malic enzyme 7, pyruvate dehydrogenase complex 8, fumarase 9, fumarate reductase 10, succinate decarboxylase complex. indicates reactions at which ATP is synthesised from ADP cyt, cytosol mit, mitochondrion. (After Bryant Flockhart, 1986.)...

Answer Pyruvate dehydrogenase is located in the mitochondrion, and glyceraldehyde 3-phosphate dehydrogenase in the cytosol. Because the mitochondrial and cytosolic pools of NAD are separated by the inner mitochondrial membrane, the enzymes do not compete for the same NAD pool. However, reducing equivalents are transferred from one nicotinamide coenzyme pool to the other via shuttle mechanisms (see Problem 21). 

The mitochondrion is boimded by two phospholipid membranes. The outer membrane is freely permeable to molecules, including water, with a molecular weight of up to about 5000. The iimer membrane is rich in membrane-boimd proteins and consists, in terms of membrane area, of 50% phospholipid and 50% protein (Lenaz, 1988). Pyruvate dehydrogenase, a mitochondrial enzyme, is water soluble. The proteins of the respiratory chain, as well as ATP synthase, are aU boimd to the inner mitochondrial membrane. The enzymes of the Krebs cycle are water soluble, with the exception of succinate dehydrogenase. This enzyme is bound to the mitochondrial membrane, where it directly funnels electrons, via FAD, to the respiratory chain. 

In anaerobic conditions, cells can metabolize pyruvate to lactate or to ethanol plus CO2 (in the case of yeast), with the reoxidation of NADH. In aerobic conditions, pyruvate is transported into the mitochondrion, where pyruvate dehydrogenase converts it into acetyl CoA and CO2 (see Figure 8-5). 

The control of fatty-acid oxidation is related to the availability of circulating fatty acids and the activity of palmitoyl carnitine transferase 1. When circulating fatty acids are elevated, considerable fatty-acyl CoA is formed in a number of tissues, including the liver, which is sufficient to inhibit both acetyl CoA carboxylase in the cytosol and, indirectly, pyruvate dehydrogenase in the mitochondrion. Under this condition, neither malonyl CoA nor citrate would accumulate thus, there would be a diminution of fatty-acid synthesis. When large amounts of fatty... 

The control of branched-chain amino acid catabolism lies within the activity of branched-chain a-keto acid dehydrogenase. This enzyme-like pyruvate dehydrogenase can occur in an active nonphosphorylated or an inactive phosphorylated form. The enzyme is phosphorylated by a specific kinase in the mitochondrion, the location of both the kinase and the branched-chain a-keto acid dehydrogenase. The kinase is inhibited by branched-chain a-keto acids thus, when these are in excess, the enzyme will be nonphosphorylated and active, allowing catabolism of the excess keto acids and, therefore, catabolism of excess branched-chain amino acids. The mitochondrion also contains a branched-chain a-keto acid dehydrogenase phosphatase, which returns the phosphorylated enzyme back to the active form. Thus, the major control of branched-chain amino-acid catabolism is the activity of the branched-chain a-keto-acid dehydrogenase, which is controlled by phosphorylation, primarily by the specific kinase, and dephosphorylation. 

Fig. 22.12. Major sites of regulation in the glycolytic pathway. Hexokinase and phos-phofructokinase-1 are the major regulatory enzymes in skeletal muscle. The activity of pyruvate dehydrogenase in the mitochondrion determines whether pyruvate is converted to lactate or to acetyl Co A. The regulation shown for pyruvate kinase only occurs for the liver (L) isoenzyme.

Fig. 31.5. Conversion of pyruvate to phosphoenolpyruvate (PEP). Follow the shaded circled numbers on the diagram, starting with the precursors alanine and lactate. The first step is the conversion of alanine and lactate to pyruvate. Pyruvate then enters the mitochondria and is converted to OAA (circle 2) by pyruvate carboxylase. Pyruvate dehydrogenase has been inactivated by both the NADH and acetyl-CoA generated from fatty acid oxidation, which allows oxaloacetate production for gluconeogenesis. The oxaloacetate formed in the mitochondria is converted to either malate or aspartate to enter the cytoplasm via the malate/aspartate shuttle. Once in the cytoplasm the malate or aspartate is converted back into oxaloacetate (circle 3), and phosphoenolpyruvate carboxykinase will convert it to PEP (circle 4). The white circled numbers are alternate routes for exit of carbon from the mitochondrion using the malate/aspartate shuttle. OAA = oxaloacetate FA = fatty acid TG = triacylglycerol.

Fig. 13.1.1. Schematic overview of mitochondrial oxidative phosphorylation. A part of the mitochondrion is represented, showing the outer mitochondrial membrane (OMM), inner mitochondrial membrane (IMM) and crista (an invagination of the inner membrane). Substrates for oxidation enter the mitochondrion through specific carrier proteins, e.g., the pyruvate transporter, (PyrT). Reducing equivalents from fatty acyl CoA dehydrogenases, pyruvate dehydrogenase and the TCA cycle are delivered to the electron transport chain through NADH, succinate ubiquinol oxidoreductase (SQO), electron transfer flavoprotein (ETF) and its ubiquinol-...

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