A stripped down Figure of Glycolysis

Transcription

1 A stripped down Figure of Glycolysis

2 Fates of pyruvate Other sugars (than glucose) Energetics of glycolysis Gluconeogenesis Regulation of glycolysis/gluconeogenesis

3 Fates of Pyruvate

4 Stage 2: x2 1 oxidation 2 substrate level phos a. 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

5 Page 603 Figure Reaction mechanism of lactate dehydrogenase.

6 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.

7 Stage 2: x2 1 oxidation 2 substrate level phos b. Anaerobic Glycolysis Reduction of Pyr to Ethanol: Pyr Carb. + ADH (in yeast, not humans) pyruvate acetaldehyde acetaldehyde + NADH + H + ethanol + NAD+ - Pyruvate is decarboxylated to form CO 2 + acetaldehyde (TPP) - Acetaldehyde is reduced to ethanol to recover NAD + needed for glycolysis

8 Figure The two reactions of alcoholic fermentation. Figure Thiamine pyrophosphate. Page 604

9 Thiazole as an electron sink +

10 Page 605 Figure Reaction mechanism of pyruvate decarboxylase.

11 Figure The reaction mechanism of alcohol dehydrogenase involves direct hydride transfer of the pro-r hydrogen of NADH to the re face of acetaldehyde. Page 606

12 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.

13

14 Biochemical Regulation of Glycolysis

15

16 Energetics of Glycolysis

17 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).

18 Fig PFK activity vs. [F-6-P].

19 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

20 Figure Comparison of the relative enzymatic activities of hexokinase and glucokinase over the physiological blood glucose range. Page 649 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

21 Other sugars feed into glycolysis Lactase expression can be repressed, depending on environment. This is basis for lactose intolerance.

22 Galactose Galactosemia

23 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.

24 Figure 23-9 The Cori cycle. G-6-P G-6-P Page 850

25 Gluconeogenesis/ glycolysis Figure 23-7 Pathways of gluconeogenesis and glycolysis. Note that the G values are given in the direction of gluconeogenesis. 3 HK 2 PFK Page PK

26 Bypassing the PK step The energy released from ATP hydrolysis is stored in carboxylated intermediate. CO 2 release will help drive the next step.

27 Page 846 By-Passing PK : Two-phase reaction mechanism of pyruvate carboxylase.

28 By-Passing PK : Conversion of pyruvate to oxaloacetate and then to phosphoenolpyruvate. Step 2 Page 845 GTP

29 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.

30

31 Pathways converting lactate, pyruvate, and citric acid cycle intermediates to oxaloacetate can all be used to generate glucose. The carbon skeletons of certain amino acids are readily made into OAA (glycogenic amino acids) and so protein can be sacrificed to make glucose. Page 844 TCA

32 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 acetyl- CoA, citrate, or NADH glycolysis is inhibited and gluconeogenesis promoted.

33 These enzymes are in different cell compartments In liver, PK inhibited by phosphorylation 1 1

34 Role of F2,6 BP in the Interconversion of F-1,6-BP to F-6-P.

35 Page 649 Figure Formation and degradation of β-d-fructose- 2,6-bisphosphate is catalyzed by PFK-2 and FBPase-2.