LECTURE SLIDES.pptx for glycolysis of sugar

These discribes the process of glycolysis

The rate limiting state

Fate of pyruvate

And energy expenditure

1. Glycolysis

2. Outline Definition Relevance Stoichiometry Energetics Regulation Clinical significance Nutritional requirements

3. Definition

4. RELEVANCE Energy Production : Glycolysis is the primary pathway for the breakdown of glucose to produce energy in the form of ATP. This is especially important in cells that lack mitochondria, such as red blood cells, and in conditions where oxygen is limited (anaerobic conditions). Intermediates for Other Pathways : Glycolysis provides intermediates for other metabolic pathways. For instance, glucose-6-phosphate can enter the pentose phosphate pathway to produce NADPH and ribose-5-phosphate, and pyruvate, the end product of glycolysis, can be further metabolized in the citric acid cycle or used in anabolic processes.

5. Stochiometry The breakdown of glucose molecule is brought about by sequential reaction of 10 enzymes which can be divided into two phases: Phase1: Preparatory Phase (energy requiring phase) This phase is also called glucose activation phase. This comprises the first five reactions, which convert Glucose (6 carbon) to two molecules of Glyceraldehyde-3-Phosphate (3 carbon). These reactions consume 2 ATPs per glucose. Phase 2: Payoff Phase (energy yielding phase) This phase is also made up of 5 reaction steps. In this stage, a series of changes convert glyceraldehyde 3-phosphate to pyruvate.

6. PREPARATORY PHASE

7. Step 1: Phosphorylation of glucose Glucose is phosphorylated to glucose-6-phosphate. This reaction is catalysed by the specific enzyme  glucokinase (hexokinase D)  in liver cells and by non specific enzyme  hexokinase  in liver and extrahepatic tissue. The enzyme splits the ATP into ADP, and the phosphate group is added onto the glucose. This is irreversible regulatory reaction step of glycolysis. Mg2+ acts as cofactor.

8. DIFFERENCES BETWEEN HEXOKINASE AND GLUCOKINASE CHARACTERISTICS HEXOKINASE GLUCOKINASE Occurrence In all tissues In the liver and pancreas Specificity Acts on hexoses (Glucose fructose and mannose) Specific to glucose. Affinity for substrate High affinity Low affinity Km Value Low Km (0.1mM) High Km (10mM) Regulation Inhibited by its product (glucose-6- phosphate) Not inhibited by glucose-6-phosphate Function Blood glucose can be utilized by cells even at low concentration. Utilized to produce glycogen when blood glucose level is high

9. STEP 2: ISOMERIZATIO OF GLUCOSE-6-PHOSPHATE Glucose-6-phosphate is isomerised to Fructose-6- phosphate by the enzyme  Phosphohexose Isomerase . It involves Aldose-Ketose Isomerism. It is freely reversible reaction.

10. STEP 3: SECOND PHOSPHORYLATION Fructose-6-phosphate is further phosphorylated to Fructose 1,6-bisphosphate. One ATP is utilized. The enzyme  phosphofructokinase-1   catalyses the transfer of a phosphate group from ATP to fructose-6-phosphate. This step is second irreversible step in glycolysis. Rate limiting commited step of glycolysis.

11. STEP 4: CLEAVAGE OF F-1,6-BISP Enzyme  aldolase  splits 6-carbon Fructose 1,6-bisphosphate into two 3-carbon compounds, namely, Glyceraldehyde-3-phosphate and Dihydroxy acetone phosphate (DHAP). Aldolase is a Lyase. This reaction is reversible.

12. Step 5: Isomerization of Dihydroxyacetone Phosphate As GAP is on the direct pathway of glycolysis, whereas DHAP is not. DHAP is isomerized to Glyceraldehyde 3-phosphate by the enzyme  phosphotriose isomerase . Hence two molecules of glyceraldehyde 3-phosphate are formed from one molecule of glucose. This inter conversion is reversible.

13. PAYOFF PHASE

14. Step 6: Dehydrogenation of Glyceraldehyde-3-Phosphate Glyceraldehyde-3-phosphate is oxidised to a high energy compound 1,3-bisphosphoglycerate by enzyme  glyceraldehyde-3-phosphate dehydrogenase . It is a NAD dependent reversible reaction which generates NADH.

15. Step 7: Conversion of 1,3-Bisphosphoglycerate to 3-Phosphoglycerate The enzyme  phosphoglycerate kinase  transfers the high-energy phosphoryl group from the carboxyl group of 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate. This is only kinase reaction in glycolysis, which is reversible. This step generates ATP at substrate level phosphorylation.

16. Step 8: Inter-Molecular Shift of Phosphate Group

17. Step 9: Dehydration of 2-Phosphoglycerate

18. Step 10: Conversion of Phosphoenolpyruvate to Pyruvate Pyruvate kinase  catalyzes the conversion of phosphenol pyruvate to pyruvate. This is the second step in glycolysis that generates ATP at substrate level phosphorylation. This is irreversible reaction.

19. ENERGETICS Step  No. Enzyme Source No. of ATP 1 Hexokinase – -1 3 Phosphofructokinase – -1 6 Glyceraldehyde-3-phosphate dehydrogenase NADH (+2.5)x2=5 7 Phosphoglycerate Kinase ATP (+1)x2=2 10 Pyruvate Kinase ATP (+1)x2=2 Net Energy Yield 9-2= 7 ATPs

20. Regulation

21. Rate-Limiting Step The rate-limiting step is the slowest step in a metabolic pathway. It acts as a bottleneck, controlling the overall rate of the pathway. Regulation : This step is heavily regulated by various mechanisms, including allosteric regulation, covalent modification, and changes in gene expression. Enzymes catalyzing rate-limiting steps are often targets for feedback inhibition by the end products of the pathway. Reversibility : The rate-limiting step is not necessarily irreversible. It can be either reversible or irreversible, but its regulation is crucial for controlling the pathway's flow. Example : In glycolysis, the enzymes phosphofructokinase-1 (PFK-1), hexokinase and pyruvate kinase catalyzes the rate-limiting steps Committed Step The committed step is the first irreversible step in a metabolic pathway that commits the substrate to proceed through the entire pathway to the end product. Once this step occurs, the substrate is destined for a specific metabolic fate. Regulation : The committed step is also a key regulatory point in the pathway, but it specifically ensures that substrates do not enter the pathway unless the cell needs the final products. This step prevents wasteful use of resources. Irreversibility : By definition, the committed step is irreversible. This irreversibility ensures that once the pathway is initiated, it proceeds to completion. Example : In glycolysis, the committed step is often considered to be the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, catalyzed by PFK-1, because this step commits glucose to complete the glycolytic pathway.

22. CLINICAL SIGNIFICANCE Fluoride inhibits enolase. This property can be used when it is required to prevent glycolysis in blood prior to the estimation of blood glucose in medical laboratories.

23. Nutritional requirements Magnesium, manganese (kinases) and zinc (aldolase) are important minerals needed for the pathway to occur.

24. Fate of pyruvate The fate of pyruvate depends on (a) the type of organism (b) environmental conditions (presence or absence of oxygen)

25. Fate of pyruvate

26. Energy Yield in Anaerobic Glycolysis Step No. Enzyme Source No. of ATP 1 Hexokinase – -1 3 Phosphofructokinase – -1 7 Phosphoglycerate Kinase ATP (+1)x2=2 10 Pyruvate Kinase ATP (+1)x2=2 Net Energy Yield 4-2= 2 ATPs