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CARBOHYDRATES, GLYCOLYSIS, GLUCONEOGENESIS
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- 5. • Glycolysis isthe metabolic pathway that converts glucose C6H12O6, into pyruvic acid, CH3COCOOH. • The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). • Metabolic pathway that does not require oxygen. The wide occurrence of glycolysis in other species indicates that it is an ancient metabolic pathway. • In most organisms, glycolysis occurs in the liquid part of cells, the cytosol. • The most common type of glycolysis is the Embden–Meyerhof–Parnas (EMP) pathway, which was discovered by Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas. • It also refers to other pathways, such as the Entner–Doudoroff pathway and various heterofermentative and homofermentative pathways.
- 6. Investment phase –wherein ATP is consumed
- 7. Yield phase –wherein more ATP is produced than originally consumed. It is characterised by a net gain of the energy-rich molecules ATP and NADH. Since glucose leads to two triose sugars in the preparatory phase, each reaction in the pay-off phase occurs twice per glucose molecule. This yields 2 NADH molecules and 4 ATP molecules, leading to a net gain of 2 NADH molecules and 2 ATP molecules from the glycolytic pathway per glucose.
- 8. The first stepis phosphorylation of glucose by a family of enzymes called hexokinases to form glucose 6- phosphate (G6P). This reaction consumes ATP, but it acts to keep the glucose concentration low, promoting continuous transport of glucose into the cell through the plasma membrane transporters. In addition, it blocks the glucose from leaking out – the cell lacks transporters for G6P, and free diffusion out of the cell is prevented due to the charged nature of G6P. Glucose may alternatively be formed from the phosphorolysis or hydrolysis of intracellular starch or glycogen. Cofactors: Mg2+
- 9. G6P is thenrearranged into fructose 6-phosphate (F6P) by glucose phosphate isomerase. Fructose can also enter the glycolytic pathway by phosphorylation at this point. The change in structure is an isomerization, in which the G6P has been converted to F6P. The reaction requires an enzyme, phosphoglucose isomerase, to proceed. This reaction is freely reversible under normal cell conditions.
- 10. The energy expenditureof another ATP in this step is justified in 2 ways: The glycolytic process (up to this step) becomes irreversible, and the energy supplied destabilizes the molecule. Because the reaction catalyzed by phosphofructokinase 1 (PFK-1) is coupled to the hydrolysis of ATP (an energetically favorable step) it is, in essence, irreversible, and a different pathway must be used to do the reverse conversion during gluconeogenesis. This makes the reaction a key regulatory point. This is also the rate-limiting step. Cofactors: Mg2+
- 11. Destabilizing the moleculein the previous reaction allows the hexose ring to be split by aldolase into two triose sugars: dihydroxyacetone phosphate (a ketose), and glyceraldehyde 3-phosphate (an aldose).
- 12. Triosephosphate isomerase rapidlyinterconverts dihydroxyacetone phosphate with glyceraldehyde 3-phosphate (GADP) that proceeds further into glycolysis. This is advantageous, as it directs dihydroxyacetone phosphate down the same pathway as glyceraldehyde 3-phosphate, simplifying regulation.
- 13. The aldehyde groupsof the triose sugars are oxidized, and inorganic phosphate is added to them, forming 1,3-bisphosphoglycerate. The hydrogen is used to reduce two molecules of NAD+, a hydrogen carrier, to give NADH + H+ for each triose.
- 14. This step isthe enzymatic transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP by phosphoglycerate kinase, forming ATP and 3-phosphoglycerate. At this step, glycolysis has reached the break- even point: 2 molecules of ATP were consumed, and 2 new molecules have now been synthesized. This step, one of the two substrate-level phosphorylation steps, requires ADP; thus, when the cell has plenty of ATP (and little ADP), this reaction does not occur. Because ATP decays relatively quickly when it is not metabolized, this is an important regulatory point in the glycolytic pathway.
- 15. Phosphoglycerate mutase isomerizes3-phosphoglycerate into 2-phosphoglycerate.
- 16. Enolase next converts2-phosphoglycerate to phosphoenolpyruvate. Cofactors: 2 Mg2+, one "conformational" ion to coordinate with the carboxylate group of the substrate, and one "catalytic" ion that participates in the dehydration.
- 17. A final substrate-levelphosphorylation now forms a molecule of pyruvate and a molecule of ATP by means of the enzyme pyruvate kinase. This serves as an additional regulatory step, similar to the phosphoglycerate kinase step. Cofactors: Mg2+
- 18. 3 Gross Productsof Glycolysis • 4 ATP • 2 NADH • 2 pyruvate molecules
- 19. Regulation • Allosteric Regulation –ATP • High level of ATP would slow down the pathway • High level of ADP,AMP would mean low level of ATP thus speeds up the pathway – Hexokinase - Glucose‐6‐phosphate formation. The entry point of glucose is the formation of glucose‐6‐phosphate. Hexokinase is feedback‐inhibited by its product, so the phosphorylation of glucose is inhibited if there is a buildup of glucose‐6‐ phosphate. – Phosphofructokinase » energy charge of the cell » is also activated by fructose‐2,6‐ bisphosphate, » is inhibited by citrate. – Pyruvate kinase » ATP inhibits pyruvate kinase, similar to the inhibition of PFK. » Pyruvate kinase is also inhibited by acetyl‐Coenzyme A. » activated by fructose‐1,6‐bisphosphate. (feed-forward activation)
- 20. Fermentation Are catabolic reactionthat occur with no oxidation. Fermentation reacts NADH with an endogenous, organic electron acceptor. Usually this is pyruvate formed from sugar through glycolysis. The reaction produces NAD+ and an organic product, typical examples being ethanol, lactic acid, and hydrogen gas (H2), and often also carbon dioxide. occurs in an anaerobic environment. In the presence of O2, NADH, and pyruvate are used to generate ATP in respiration. This is called oxidative phosphorylation.
- 21. Lactate Fermentation Homolactic fermentation(producing only lactic acid) is the simplest type of fermentation. Pyruvate from glycolysis undergoes a simple redox reaction, forming lactic acid. Overall, one molecule of glucose (or any six-carbon sugar) is converted to two molecules of lactic acid: C6H12O6 → 2CH3CHOHCOOH It occurs in the muscles of animals when they need energy faster than the blood can supply oxygen. It also occurs in some kinds of bacteria (such as lactobacilli) and some fungi. It is the type of bacteria that convert lactose into lactic acid in yogurt, giving it its sour taste. These lactic acid bacteria can carry out either homolactic fermentation, where the end- product is mostly lactic acid, or heterolactic fermentation, where some lactate is further metabolized to ethanol and carbon dioxide(via the phosphoketolase pathway), acetate, or other metabolic products, e.g.: C6H12O6 → CH3CHOHCOOH + C2H5OH + CO2
- 23. Alcohol Fermentation One glucosemolecule is converted into two ethanol molecules and two carbon dioxide molecules. It is used to make bread dough rise: the carbon dioxide forms bubbles, expanding the dough into a foam. The ethanol is the intoxicating agent in alcoholic beverages such as wine, beer and liquor. Fermentation of feedstocks, including sugarcane, corn, and sugar beets, produces ethanol that is added to gasoline. In some species of fish, including goldfish and carp, it provides energy when oxygen is scarce.
- 24. Before fermentation, aglucose molecule breaks down into two pyruvate molecules (Glycolysis). The energy from this exothermic reaction is used to bind inorganic phosphates to ADP, which converts it to ATP, and convert NAD+ to NADH. The pyruvates break down into two acetaldehyde molecules and give off two carbon dioxide molecules as waste products. The acetaldehyde is reduced into ethanol using the energy and hydrogen from NADH, and the NADH is oxidized into NAD+ so that the cycle may repeat. The reaction is catalyzed by the enzymes pyruvate decarboxylase and alcohol dehydrogenase.
- 25. Pentose Phosphate Pathway •Alternative pathway for glycose oxidation • Provides the cell the Energy in the form of reducing power for biosynthesis • Produces NADPH • Occurs in the cytosol of the cell. glucose-6-phosphate + 2NADP+ + H2O → ribulose-5-phosphate + 2NADPH + CO2
- 26. • The pentosephosphate pathway provides several molecules that are important in biosynthesis. • It is important in liver and mammary glands, and the adrenal cortex. • It also produces ribose which is important for biosynthesis of nucleotides. • is most active in tissues involved in cholesterol and fatty acid biosynthesis.
- 28. Gluconeogenesis • Production ofglucose. • An anabolic pathway occur in the liver. • Reverse reaction of Glycolysis except for step 1, 3, and 10 are irreversible. • GTP is the source of phosphate group.
- 30. Precursors for Gluconeogenesis •Gluconeogenic precursors – Glycerol – Lactate – α-Ketoacids – Acetyl Co A and precursors of Acetyl Co A.
- 31. Regulation • Glucagon – Allostericeffectors Inhibited by high level of ATP Activates fructose1,6-biphosphate Inhibits phosphofructokinase • Substrate availability • Allosteric Activation of Acetyl Co A • Stimulated by high level of AMP, ADP and P
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- 40. Glycosyl alpha 4:6 transferase Amylo(alpha1:4alpha1:6) transglycosylase
- 41. Summary • Glycolysis • Hexosemonophosphate p / pentose pp • Gluconeogenesis • Glycogenolosis • Glycogen synthesis • fermentation