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Download Figure 17-3 Degradation of glucose via the glycolytic pathway.
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A stripped down Figure of Glycolysis Fates of pyruvate Other sugars (than glucose) Energetics of glycolysis Gluconeogenesis Regulation of glycolysis/gluconeogenesis Fates of Pyruvate Stage 2: x2 1 oxidation 2 substrate level phos. 11 10 11a. Anaerobic Glycolysis – Reduction of Pyr to Lactate: Lactate DH pyruvate + NADH + H+ lactate + NAD+ - Pyruvate is reduced to lactate to recover NAD+ needed for glycolysis - This is a reversible reaction – several isoenzyme forms of LDH Page 603 Figure 17-24 Reaction mechanism of lactate dehydrogenase. Reduction of pyruvate to lactate: lactate dehydrogenase pyruvate + NADH + H+ lactate + NAD+ •reduced at expense of electrons originally donated by 3-phosphoglyceraldehyde, carried by NADH. Thus, no net oxidation occurs in glycolysis = fermentation; another organic serving as electron acceptor. •lactate, end-product under anaerobic conditions, diffuses thru cell membrane as waste into blood - salvaged by liver and rebuilt to form glucose (gluconeogenesis). This occurs in skeletal muscle during periods of strenuous exertion: Cells use O2 faster than can be supplied by circulatory system; cells begin to function anaerobically, reducing pyruvate to lactate rather than further oxidation. Causes soreness due to decreased pH. Lactate fermentation also important commercially since bacteria capable are responsible for production of cheeses, yogurts, and other foods obtained by fermentation of lactose of milk. Stage 2: x2 1 oxidation 2 substrate level phos. 11 11b. Anaerobic Glycolysis – Reduction of Pyr to Ethanol: Pyr Carb. + ADH (in yeast, not humans) pyruvate Æ acetaldehyde acetaldehyde + NADH + H+ Æ ethanol + NAD+ - Pyruvate is decarboxylated to form CO2 + acetaldehyde (TPP) - Acetaldehyde is reduced to ethanol to recover NAD+ needed for glycolysis Figure 17-25 The two reactions of alcoholic fermentation. Page 604 Figure 17-26 Thiamine pyrophosphate. Thiazole as an electron sink + Page 605 Figure 17-27 Reaction mechanism of pyruvate decarboxylase. Page 606 Figure 17-30 The reaction mechanism of alcohol dehydrogenase involves direct hydride transfer of the pro-R hydrogen of NADH to the re face of acetaldehyde. Reduction of pyruvate to ethanol: alcohol dehydrogenase pyruvate + NADH + H+ ethanol + NAD+ In alcoholic fermentation, pyruvate is first decarboxylated to acetaldehyde that then serves as electron acceptor, giving rise to ethanol. Commercially important in baking and brewing industries Other less common fermentation processes (bacteria) yield propionic acid (swiss cheese); butyrate (rancid butter); acetone, isopropanol. Biochemical Regulation of Glycolysis Energetics of Glycolysis The glycolytic pathway is regulated at all three irreversible steps. The regulation is MOSTLY of a straight forward biochemical nature. We might consider it a sort of “primitive” regulation. The complete discussion, however, will require an integration of systems as part of a hormonal response. (More later). Fig. 17-33 PFK activity vs. [F-6-P]. The PFK tetramer (only dimer shown here) is allosterically regulated. PFK is in an R to T equilibrium. ATP stabilizes T state and ADP or AMP the R state. Other effectors include F2,6BP and citrate. ATP F6P Regulator site: ATP =I ADP/AMP=Stim Page 649 Figure 18-23 Comparison of the relative enzymatic activities of hexokinase and glucokinase over the physiological blood glucose range. Liver contains many insulin Independent GluT2 transporters. When blood glucose is high, and insulin is signaling glucose reduction, sugar enters the liver and glucokinase generates G6P which stimulates glycogen synthesis Other sugars feed into glycolysis Lactase expression can be repressed, depending on environment. This is basis for lactose intolerance. Galactose Galactosemia Gluconeogenesis 1. What is the role of this pathway? Convert 3-C lactate or pyruvate into 6-C glucose 2. What is the difference between glycolysis and gluconeogenesis? Need to by-pass the three irreversible steps 3. Where is the pathway located? Uses enzymes located in the cytosol and mito The ability to synthesize glucose is important to mammals since certain tissues, particularly brain and RBC, are almost solely dependent on glucose as an energy source. In normal humans, under fasting conditions, 80% of glucose is consumed by brain. The glycogen reservoir in liver has only 1/2 day supply for the brain. In periods of dietary glucose deprivation, we must be able to make glucose from other sources. Figure 23-9 The Cori cycle. G-6-P Page 850 G-6-P Gluconeogenesis/ glycolysis Figure 23-7 Pathways of gluconeogenesis and glycolysis. 3 HK 2 PFK Page 848 Note that the ∆G values are given in the direction of gluconeogenesis. 1 PK Bypassing the PK step The energy released from ATP hydrolysis is “stored” in carboxylated intermediate. CO2 release will help drive the next step. Page 846 By-Passing “PK”: Two-phase reaction mechanism of pyruvate carboxylase. By-Passing “PK”: Conversion of pyruvate to oxaloacetate and then to phosphoenolpyruvate. Page 845 Step 2 GTP The PK bypass uses OAA, which must be generated in the mitochondria. OAA cannot be transported out so it must be converted to PEP or malate (or Asp but lets ignore that). Gluconeogenesis requires NADH so reducing equivalents must be generated for that purpose; cytoplasmic [NADH]/[NAD] is very low. Pathways converting lactate, pyruvate, and citric acid cycle intermediates to oxaloacetate can all be used to generate glucose. Page 844 The carbon skeletons of certain amino acids are readily made into OAA (glycogenic amino acids) and so protein can be sacrificed to make glucose. TCA Regulation of glycolysis and gluconeogenesis It is clear that these two paths must be coordinately regulated to avoid wasteful futile cycles. When glycolysis is “on”, gluconeogenesis should be “off” Important regulatory enzymes of opposing pathways are located at those steps where the two pathways are not identical. SUMMARY: Pathways from glucose to pyruvate and pyruvate to glucose are regulated by both the level of respiratory fuels and energy charge. Thus, whenever cell has ample ATP and respiratory fuels such as acetylCoA, citrate, or NADH glycolysis is inhibited and gluconeogenesis promoted. These enzymes are in different cell compartments 3 3 2 2 In liver, PK inhibited by phosphorylation 1 1 Role of F2,6 BP in the Interconversion of F-1,6-BP to F-6-P. Page 649 Figure 18-24 Formation and degradation of β-D-fructose2,6-bisphosphate is catalyzed by PFK-2 and FBPase-2.