* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download pyruvate
Lipid signaling wikipedia , lookup
Biochemical cascade wikipedia , lookup
Mitochondrial replacement therapy wikipedia , lookup
Basal metabolic rate wikipedia , lookup
Electron transport chain wikipedia , lookup
Metalloprotein wikipedia , lookup
Mitochondrion wikipedia , lookup
Adenosine triphosphate wikipedia , lookup
Photosynthesis wikipedia , lookup
Biosynthesis wikipedia , lookup
Evolution of metal ions in biological systems wikipedia , lookup
Microbial metabolism wikipedia , lookup
Photosynthetic reaction centre wikipedia , lookup
Nicotinamide adenine dinucleotide wikipedia , lookup
Fatty acid synthesis wikipedia , lookup
Lactate dehydrogenase wikipedia , lookup
Oxidative phosphorylation wikipedia , lookup
Amino acid synthesis wikipedia , lookup
Glyceroneogenesis wikipedia , lookup
Fatty acid metabolism wikipedia , lookup
Biochemistry wikipedia , lookup
NADH:ubiquinone oxidoreductase (H+-translocating) wikipedia , lookup
CONTROL, REGULATION AND ACTIVITY OF Pdh and beyond. The two products of the Pdh complex, NADH and acetyl-CoA, are negative allosteric effectors on Pdh-a, the non-phosphorylated, active form of Pdh. These effectors (see Figure 13 Lecture ppt) reduce the affinity of the enzyme for pyruvate, thus limiting the flow of carbon through the Pdh complex. In addition, NADH and acetyl-CoA are powerful positive effectors on Pdh kinase, the enzyme that inactivates Pdh by converting it to the phosphorylated Pdh-b form. Since NADH and acetyl-CoA accumulate when the cell energy charge is high, it is not surprising that high ATP levels also up-regulate Pdh kinase activity, reinforcing down-regulation of Pdh activity in energy-rich cells. Note, however, that pyruvate is a potent negative effector on Pdh kinase, with the result that when pyruvate levels rise, Pdh-a will be favored even with high levels of NADH and acetyl-CoA. Concentrations of pyruvate, which maintain Pdh in the active form (Pdh-a) are sufficiently high so that, in energy-rich cells, the allosterically down-regulated, high Km form of Pdh is nonetheless capable of converting pyruvate to acetyl-CoA. With large amounts of pyruvate in cells having high energy charge and high NADH, pyruvate carbon will be directed to the 2 main storage forms of carbon--glycogen via gluconeogenesis and fat production via fatty acid synthesis---where acetyl-CoA is the principal carbon donor. Although the regulation of Pdh-b phosphatase is not well understood, it is quite likely regulated to maximize pyruvate oxidation under energy-poor conditions and to minimize Pdh activity under energy-rich conditions. The enzyme complex then is the gate keeper for the entrance into the TCA cycle and can be considered one of the rate-limiting steps ultimately of aerobic respiration. Factors regulating the activity of pyruvate dehydrogenase, (Pdh). • Pdh activity is regulated by its state of phosphorylation, being most active in the dephosphorylated state. • Phosphorylation of Pdh is catalyzed by a specific Pdh kinase. The activity of the kinase is enhanced when cellular energy charge is high which is reflected by an increase in the level of ATP, NADH and acetyl-CoA. Conversely, an increase in pyruvate strongly inhibits Pdh kinase. • Additional negative effectors of Pdh kinase are ADP, NAD+ and CoASH, the levels of which increase when energy levels fall. 1 • The regulation of Pdh phosphatase is not completely understood but it is known that Mg2+ and Ca+ activate the enzyme. • In adipose tissue insulin increases Pdh activity and in cardiac muscle Pdh activity is increased by catecholamines. PEPCK Human liver cells contain almost equal amounts of mitochondrial and cytosolic PEPCK (this is unique amongst mammalian cells) so this second reaction can occur in either cellular compartment. For gluconeogenesis to proceed, the OAA produced by PC needs to be transported to the cytosol. However, no transport mechanism exist for its' direct transfer and OAA will not freely diffuse. Mitochondrial OAA can become cytosolic via three pathways, conversion to PEP (as indicated above through the action of the mitochondrial PEPCK), transamination to aspartate or reduction to malate, all of which are transported to the cytosol. If OAA is converted to PEP by mitochondrial PEPCK, it is transported to the cytosol where it is a direct substrate for gluconeogenesis and nothing further is required. Transamination of OAA to aspartate allows the aspartate to be transported to the cytosol where the reverse transamination occurs yielding cytosolic OAA. This transamination reaction requires continuous transport of glutamate into, and a-ketoglutarate out of, the mitochondrion. Therefore, this process is limited by the availability of these other substrates. Either of these latter two reactions will predominate when the substrate for gluconeogenesis is lactate and alanine. Whether mitochondrial decarboxylation or transamination occurs is a function of the availability of PEPCK or transamination intermediates. Mitochondrial OAA can also be reduced to malate in a reversal of the TCA cycle reaction catalyzed by malate dehydrogenase (MDH). The reduction of OAA to malate requires NADH, which will be accumulating in the mitochondrion as the energy charge increases. The increased energy charge will allow cells to carry out the ATP costly process of gluconeogenesis. The resultant malate is transported to the cytosol where it is oxidized to OAA by cytosolic MDH, which requires NAD+ and yields NADH. The reducing agent for the GAPdh reaction in gluconmeogensis. 2 NADH Recycling through the Malate Shuttle The NADH produced during the cytosolic oxidation of malate to OAA. Conversion of pyruvate to PEP requires the action of two mitochondrial enzymes. The first is an ATP-requiring reaction catalyzed by pyruvate carboxylase, (PC). As the name of the enzyme implies, pyruvate is carboxylated to form OAA. The CO2 in this reaction is in the form of bicarbonate (HCO32-). This reaction is an anaplerotic reaction since it can be used to fill-up the TCA cycle. The second enzyme in the conversion of pyruvate to PEP is PEPCK. PEPCK requires GTP in the decarboxylation of OAA to yield PEP. Since PC incorporated CO2 into pyruvate and it is subsequently released in the PEPCK reaction, no net OAA is utilized during the glyceraldehyde-3-phosphate dehydrogenase reaction of glycolysis. The coupling of these two oxidation-reduction reactions is required to keep gluconeogenesis functional when pyruvate is the principal source of carbon atoms. The conversion of OAA to malate predominates when pyruvate (derived from amino acid such as alalnine and lactate catabolism) is the source of carbon atoms for gluconeogenesis. When in the cytoplasm, OAA is converted to PEP by the cytosolic version of PEPCK. Hormonal signals control the level of PEPCK protein as a means to regulate the flux through gluconeogenesis. Isocitrate The isocitrate used in the first reaction of the glyoxylate cycle is restored by the action of three enzymes characteristic of Krebs cycle (malate dehydrogenase, citrate synthase and cis-aconitase) on Lmalate, with the utilization of a second molecule of acetyl-CoA. The glyoxylate cycle thus enables the synthesis of a mole of succinate from two moles of acetate (as acetyl-CoA), being the overall net reaction: 2 acetyl-CoA + 2H2O + NAD+ = succinate + 2CoA + NADH + H+ The glyoxylate cycle replenishes intermediates of the Krebs cycle and conserves carbon that would otherwise be oxidized and lost to biosynthetic pathways, with the final result of a net conversion of fats to carbohydrates. 3 This replenishing function has been termed anaplerotic and probably plays an essential role in growth of microorganisms on fatty acids, in germination of oil-rich seedlings and in development of certain animal embryos. Isocitrate lyase is an enzyme that functions at a branch point of carbon metabolism and diverts isocitrate through a carbonconserving pathway, the glyoxylate cycle, bypassing the two decarboxylative steps of the tricarboxylic acid cycle that convert isocitrate to succinyl-CoA. 4