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Define Glycolysis.
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What is the major dietary source of glucose?
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Glycolysis
The Glycoloysis Pathway
| Question | Answer |
|---|---|
| Define Glycolysis. | Glycolysis is one of the principal pathways for ATP generation in the brain and erythrocytes. It can generate ATP in the presence or absence of Oxygen as well as in the presence or absence of Mitochondria. |
| What is the major dietary source of glucose? | Starch |
| What is the fate of glucose once it has entered the blood stream? | Glucose in the bloodstream enters cells via specific transporters (GLUT) by facilitated diffusion. |
| Where are GLUT 1 & 3 found? | Neurons and Brain |
| Where is GLUT 2 found? | Liver |
| Where is GLUT 4 found and what is it responsive to? | Adipose Tissue and Muscle. It is responsive to Insulin. GLUT 4 concentration will increase when blood glucose levels are increased. |
| How many reactions does phase 1 of glycolysis have? | 3 reactions. |
| Describe Glucokinase. | Glucokinase is only found in the liver. It has a very high Km for glucose but is only activated when blood glucose levels are high. |
| Describe Hexokinase. | Hexokinase is found in all tissues except the liver. It has a lower Km for glucose but is active all the time, independent of blood glucose levels. |
| What disorder results from inherited mutations in Glucokinase? | Hereditary Diabetes Mellitus, glucose is unable to enter the liver which results in hypoglycemia. |
| What are the 3 possible fates of Glucose-6-phosphate in the liver? | Glucose-6-phosphate can 1) enter the HMP(Hexosmonophosphate) shunt for synthesis of pentose sugars, 2) continue along the glycolysis pathway, or 3) be turned into glycogen for storage. |
| What determines the fate of Glucose-6-phosphate in the liver? | The fate depends on insulin levels, which change depending on the Fed or Fasted state. |
| In what part of the cell are the glycolytic enzymes located? | In the Cytosol. |
| What are the 3 stages of Glycolysis? | Stage 1 is the energy investment stage, Stage 2 is the cleavage of 6C sugar into 3C intermediates, and Stage 3 is the energy generation phase. |
| Describe Phase 1, the energy investment phase, of glycolysis. | This stage is characterized by two phosphorylation reactions that require one ATP each, and a non-phosphorylation intermediate step in between. |
| What is the first phosphorylation reaction of phase 1 of glycolysis? | Glucose is coverted to Glucose-6-Phosphate by Hexokinase or Glucokinase. This step uses 1 ATP and as such, it is irreversible/regulated. |
| What is the intermediate reaction in phase 1 of glycolysis? | The conversion of Glucose-6-phosphate into Fructose-6-phosphate via Phosphoglucose isomerase. |
| What type of enzyme is Phosphoglucose isomerase? | It is an aldoketoisomerase which converts aldoses to ketoses. |
| What is the second phosphorylation reaction of phase 1 of glycolysis? | Fructose-6-phosphate is converted into Fructose-1,6-bisphosphate via phosphofructokinase 1 (PFK1). This step adds 2 phosphates to Fructose-6-phosphate and requires the use of 1 ATP. As such, it is irreversible/regulated. |
| What is the MOST important step of the whole glycolysis pathway? | The conversion of Fructose-6-phosphate to Fructose-1,6-bisphosphate by the allosteric enzyme Phosphofructokinase 1 (PFK1) is the most important regulated step. |
| Describe the first reaction in phase 2 of glycolysis. | Fructose-1,6-bisphosphate is converted into two trioses via the enzyme Aldolase A. The trioses are Glyceraldehyde-3-phosphate and Dihydroxyacetone phosphate. |
| Describe the second reaction in phase 2 of glycolysis. | Dihydroxyacetone phosphate is coverted into another molecule of Glyceraldehyde-3-phosphate via Triosephosphate isomerase. So, the net gain from phase 2 of glycolysis is 2 molecules of Glyceraldehyde-3-phosphate. |
| Does phase 2 of glycolysis require energy (ATP)? | No, it doesn't. |
| Describe the first reaction of phase 3 of glycolysis. | Glyceraldehyde-3-phosphate is converted into 1,3-bisphosphoglycerate via Glyceraldehyde-3-phophate dehydrogenase. This step results in formation of NADH. This step occurs twice because two molecules of glyceraldehyde-3-phosphate were created in phase 2. |
| What can inhibit the first reaction of phase 3 of glycolysis? | Arsenate. Arsenate can prevent the formation of 1,3-bisphosphoglycerate via Glyceraldehyde-3-phosphate dehydrogenase. |
| How many reactions does phase 2 of glycolysis have? | 2 reactions. |
| Describe the second reaction of phase 3 of glycolysis. | 1,3-bisphosphoglycerate is converted to 3-phosphoglyerate via phosphoglycerate kinase. This step generates 1 ATP via substrate level phosphorylation (without ETC or Mitochondria) and occures twice because of 2 molecules of 1,3-bisphosphoglycerate. |
| Is 1,3-bisphosphoglycerate a low energy compound? | No, 1,3-bisphosphoglycerate is a high energy compound. |
| When is the first ATP generated in the glycolysis pathway? | In reaction 2 of phase 3. |
| Describe the third reaction of phase 3 of glycolysis. | 3-phosphoglycerate is converted to 2-phosphoglycerate via phosphoglycerate mutase. This moves the phosphate from carbon 3 to carbon 2. This occurs twice because of 2 molecules of 3-phosphoglycerate. |
| Are the first 4 reactions in phase 3 of glycolysis reversible. | Yes, all 3 reactions of phase 3 of glycolysis are reversible. |
| Describe the fourth reaction of phase 3 of glycolysis. | 2-phosphoglycerate is converted to phosphoenol pyruvate by the enzyme Enolase. This reactions creates 1 ATP via substrate level phophorylation (without ETC or mitochondria). It occurs twice because of 2 molecules of 2-phosphoglycerate. |
| How many reactions are their in phase 3 of glycolysis? | There are 5 reactions, 4 reversible and 1 irreversible. |
| What compound inhibits the fourth reaction of phase 3 of glycolysis? | Floride. |
| Is Phosphoenol pyruvate a high energy compound? | Yes, phosphoenol pyruvate is a high energy compound. |
| Describe the fifth reaction of phase 3 of glycolysis. | Phosphoenol pyruvate is converted to pyruvate via pyruvate kinase. This creates 1 molcule of ATP via substrate level phosphorylation (with ETC or mitochondria). This step is irreversible and occurs twice because of 2 molecules of phosphoenol pyruvate. |
| Which step of phase 3 of glycolysis is irreversible? | The fifth step of phase 3 is irreversible, it is the conversion of phosphoenol pyruvate to pyruvate. |
| How many molecules of pyruvate are generated from one molecule of glucose via glycolysis? | 2 molecules of pyruvate are formed. |
| How many carbons does pyruvate have? | Pyruvate has 3 carbons. |
| What is the net gain of ATP created from anaerobic glycolysis? | 2 ATP are formed by the conversion of glucose to pyruvates via anaerobic glycolysis. Technically 4 ATP are formed but 2 are needed to fuel the cycle so the net gain is only 2 ATP. |
| What are the three possible fates of pyruvate? | Aerobically, pyruvate is converted into Acetyl CoA via pyruvate dehydrogenase. Anaerobically, pyruvate is converted into Lactate via lactate dehydrogenase. Pyruvate can also be converted to Oxaloacetate during gluconeogenesis in the liver. |
| What happens to Acetyl CoA? | It enters the TCA cycle. |
| What happens to Lactate? | It goes to the liver where it enters the Cori cycle. |
| What happens to NADH when Pyruvate is converted to Lactate? | The NADH created via glyceraldehyde-3-phosphate dehydrogenase is oxidized to form NAD+ which allows glycolysis to proceed in the absence of mitochondria. |
| Is the NADH/NAD+ reaction reversible? | Yes, the direction of the reaction depends on the NADH/NAD+ ratio in the cell. |
| How is Lactate (Lactic Acid) related to exercise? | During exercise, skeletal white muscle is actively contracting to form lactic acid. The buildup of lactic acid causes the pH to drop, creating painful muscle cramps. |
| What happens when there is a high NADH/NAD+ ratio? | This condition typically occurs during exercise when white skeletal muscle is actively contracting. The conversion of Pyruvate to Lactate is favored. |
| Does Cardiac muscle have a low or high NADH/NAD+ ratio? | Cardiac muscle has a low NADH/NAD+ ratio and is predominantly aerobic due to high concentrations of mitochondria. The glycolysis that occurs anaerobically creates lactate, which cardiac mitochondria covert back then to Acetyl CoA to enter the TCA cycle. |
| Describe the Cori Cycle. | Glucose is converted to 2 molecules of pyruvate via glycolysis, then to 2 molecules of Lactate. Lactate is then transported to the liver to be converted back to pyruvate and then to glucose (with addition of 6 ATP) to return to RBCs/skeletal muscle. |
| Is Lactate broken down within RBCs and skeletal muscle? | No, it must be transported to the liver/hepatocytes. |
| Which enzymes mediate the three irreversible reactions in glycolysis and which is most important? | The three irreversible steps of glycolysis are 1) glucokinase/hexokinase, 2) Phosphofructokinase1 (PFK1 – MOST important), and 3) Pyruvate kinase. |
| What are the enzymes responsible for the two substrate level phosphorylation reactions in glycolysis? | Phosphoglycerate kinase and pyruvate kinase |
| Describe aerobic glycolysis. | In aerobic glycolysis, glucose is converted to 2 molecules of pyruvate, forming 2 molecules of NADH in the process. Each NADH yields 3 ATP each, for a total combined contribution of 6 ATP. |
| What is the net gain of ATP from aerobic glycolysis? | 10 ATP are created (4 from glucose conversion to pyruvates in glycolysis and 6 from NADH) but 2 are needed to fuel the cycle so the net gain is only 8 ATP. |
| Where is NADH converted to form ATP? | NADH is transported by the malate-aspartate shuttle or the glycerol-phosphate shuttle to the mitochondria to be converted into 3 molecules of ATP. |
| When is ATP used during glycolysis? | The first step of phase 1 of glycolysis using glucokinase/hexokinase, and the third step of phase 1, using Phosphofructokinase 1 (PFK1). |
| Why is fluoride added to tubes that will be used for estimation of blood glucose in the lab? | If fluoride is not added, there would be a reduction in glucose due to its utilization by RBCs/WBCs which would result in an erroneously low value for blood glucose. |
| What is the significance of glycolysis in adipose tissue? | In fat tissue, glyceraldehyde-3-phosphate provides the glycerol component that is necessary for the formation of triacylglycerides. |
| What is the significance of glycolysis in Red Blood Cells? | Not only is glycolysis the major source of energy for RBCs, it also forms 2,3-bisphosphoglycerate (2,3-BPG) which affects Hemoglobin’s affinity for oxygen. |
| How does 2,3-BPG affect Hemoglobin’s affinity of oxygen? | 2,3-BPG binds to the Beta chains of hemoglobin and helps to unload oxygen at the tissue level. 2,3-BPG formation is increased when a person moves to high altitudes because facilitation of oxygen unloading is necessary. |
| Describe Hemolytic Anemia. | RBC’s rely solely upon glycolysis for ATP. ATP is needed to maintain the electrolyte gradient (operated by Na+/K+ Pump) to maintain cell membrane. Defective glycolysis causes ATP deficiency which ruins the electrolyte gradient and membrane causing lysis. |
| Deficiency of which enzymes results in hemolytic anemia? | Glucose-6-phosphate deficiency is most common, but it can also result from pyruvate kinase deficieny. Altered kinetic properties of the enzymes can also be a cause of hemolytic anemia. |
| What happens to 2,3-BPG levels in pyruvate kinase deficient RBCs? | 2,3-BPG levels are INCREASED. |
| What happens to 2,3-BPG levels in hexokinase deficient RBCs? | 2,3-BPG levels are DECREASED. |
| Which Tissues/Cells DEPEND on glycolysis? | Erythrocytes (for ATP and 2,3-BPG), Brains (requires glycolysis during the fasting state), Actively contracting skeletal muscle (for glycolytic fiber function), Retina/Lens (for ATP), and Tumor Cells (for ATP). |
| What is Lactic Acidosis? | Lactic Acidosis is an example of metabolic acidosis which occurs when pH is decreased, pCO2 is decreased, and HCO3- is decreased (in the compensated stage). |
| What are the 6 conditions under which Lactic Acidosis is observed? | 1) Increased conversion of pyruvate to lactate (increased NADH/NAD+), 2) strenuous exercise, 3) inherited pyruvate dehydrogenase deficiency(Leigh’s Disease), 4) Thiamine deficiency (low pyr. dehyd. activity) 5) Gluconeogenesis defect, and 6) blood loss. |
| Describe the ‘Warburg Effect’. | Cancer cells rely on glycolysis as the main source of ATP. |
| Describe metabolic changes in cancer. | Hypoxia in a tumor causes an activation of HIF-1 which stimulates angiogenesis to the tumor and metabolic changes that results in an increase in glycolytic enzyme concentration in the tumor cells. |
| How is Warburg Effect visualized and how can it be exploited for cancer therapy? | A glucose analog called Fluorodeoxyglucose (FDG) is used for PET. Tumor cells preferentially utilize FDG causing an uptake of FDG by the tumor allowing them to be visualized on the scan. Glycolysis inhibitors are being explored as therapeutic agents. |
| Name two stimulators of glycolysis. | 1) Increased AMP levels (due to contraction of muscles, an energy deficient state) and 2) Increased levels of Fructose-2,6-bisphosphate (due to high levels of blood glucose). |
| Name two inhibitors of glycolysis. | 1) Increase in ATP levels (due to energy excess in the resting state) and 2) Decreased Fructose-2,6-bisphophate levels (due to low blood glucose levels |
| Which step of glycolysis is inhibited by Arsenate? | Glyceraldehyde-3-Phosphate to 1,3 Bisphosphoglycerate via Glyceraldehyde-3-Phosphate dehydrogenase |
| Which step of glycolysis is inhibited by Fluoride? | 2-Phosphoglycerate to Phosphoenolpyruvate via Enolase |
| At which steps is ATP used? | Glucose to G-6-P via glucokinase/hexokinase, F-6-P to F1,6BP via PFK-1 |
| At which steps is ATP made? | 1,3BPG to 3-phosphoglycerate via phosphoglycerate kinase, Phosphoenolpyruvate to pyruvate via pyruvate kinase |
| At which step is NAD+ used and NADH made? | Glyceraldehyde-3-phosphate to 1,3 bisphosphoglycerate via glyceraldeyde-3-phosphate dehydrogenase |
Created by:
sprater16