Chapter 16- Glycolysis – Flashcards
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Just right. Suggest some possible reasons why glucose is fuel used by all organisms.
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Glucose is formed under prebiotic conditions. It is the most stable hexose sugar and consequently, has a low tendency, relative to other monosaccharides, to nonenzymatically react with proteins.
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Hexokinase
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Generates the second molecule of ATP
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Phosphoglucose isomerase
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Converts glucose 6-phosphate into fructose 6-phosphate
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Phosphofructokinase
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Forms fructose 1,6-bisphosphate
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Aldolase
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Cleaves fructose 1,6-bisphosphate
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Triose phosphate isomerase
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Catalyzes the interconversion of
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Glyceraldehyde 3-phosphate dehydrogenase
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Generates the first highphosphoryl- transferpotential compound that is not ATP
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Phosphoglycerate kinase
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Generates the first molecule of ATP
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Phosphoglycerate mutase
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Converts 3-phosphoglycerate into 2-phosphoglycerate
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Enolase
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Generates the second high-phosphoryl-transfer- potential compound that is not ATP
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Pyruvate kinase
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Generates the second molecule of ATP
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Who takes? Who gives? Lactic acid fermentation and alcoholic fermentation are oxidation-reduction reactions. Identify the ultimate electron do
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In both cases, the electron donor is glyceraldehyde 3-phosphate. In lactic acid fermentation, the electron acceptor is pyruvate, converting it into lactate. In alcoholic fermentation, acetaldehyde is the electron acceptor, forming ethanol.
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ATP yield. Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule? (a) Glucose 6-phosphate (b) Dihydroxyacetone phosphate (c) Glyceraldehyde 3-phosphate (d) Fructose (e) Sucrose
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(a) 3 ATP; (b) 2 ATP; (c) 2 ATP; (d) 2 ATP; (e) 4 ATP
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Enzyme redundancy? Why is it advantageous for the liver to have both hexokinase and glucokinase to phosphorylate glucose?
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Glucokinase enables the liver to remove glucose from the blood when hexokinase is saturated, ensuring that glucose is captured for later use.
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Enzyme properties. In the liver and the pancreas, hexokinase and glucokinase phosphorylate glucose. Glucokinase is active only when the blood concentration of glucose is high. How might glucokinase differ kinetically from hexokinase so as to function only at high glucose levels?
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Glucokinase has a higher KM value, which allows this enzyme to become more active at high glucose concentrations, conditions that saturate hexokinase.
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Required isomerization. Why is the isomerization of glucose 6-phosphate to fructose 6-phosphate an important step in glycolysis? How is the conversion of the fructose isomer back into the glucose isomer prevented?
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Glucose cannot be cleaved into two three-carbon fragments, whereas fructose can, and three-carbon molecules are metabolized in the second stage of glycolysis. The conversion of fructose 6-phosphate into fructose 1,6-bisphosphate prevents the glucose isomer from being re-formed.
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Magic? The interconverison of DHAP and GAP greatly favors the formation of DHAP at equilibrium. Yet the conversion of DHAP by triose phosphate isomerase proceeds readily. Why?
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The GAP formed is immediately removed by subsequent reactions, resulting in the conversion of DHAP into GAP by the enzyme
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Between two extremes. What is the role of a thioester in the formation of ATP in glycolysis?
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A thioester couples the oxidation of glyceraldehyde 3-phosphate to 3-phosphoglycerate with the formation of 1,3-bisphosphoglycerate. 1,3-Bisphosphoglycerate can subsequently power the formation of ATP.
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Corporate sponsors. Some of the early research on glycolysis was supported by the brewing industry. Why would the brewing industry be interested in glycolysis?
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Glycolysis is a component of alcoholic fermentation, the pathway that produces alcohol for beer and wine. The belief was that understanding the biochemical basis of alcohol production might lead to a more efficient means of producing
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Recommended daily allowance. The recommended daily allowance for the vitamin niacin is 15 mg per day. How would glycolysis be affected by niacin deficiency?
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The conversion of glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate would be impaired. Glycolysis would be less effective.
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Running in reverse. Why can't the reactions of the glycolytic pathway simply be run in reverse to synthesize glucose?
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The DG°′ for the reverse of glycolysis is +90 kJ mol−1 (+22 kcal mol−1), far too endergonic to take place.
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Destiny. What are the principle fates of pyruvate generated in glycolysis?
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Pyruvate can be metabolized to ethanol in alcoholic fermentation, to lactate in lactic acid fermentation, or be completely oxidized to CO2 and H2O in cellular respiration.
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Road blocks. What reactions of glycolysis are irreversible under intracellular conditions?
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The conversion of glucose into glucose 6-phosphate by hexokinase; the conversion of fructose 6-phosphate into fructose 1,6-bisphosphate by phosphofructokinase; the formation of pyruvate from phosphoenolpyruvate by pyruvate kinase
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No pickling. Why is it in the muscle's best interest to export lactic acid into the blood during intense exercise?
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Lactic acid is a strong acid (problem 2.15). If it remained in the cell, the pH of the cell would fall, which could lead to the denaturation of muscle protein and result in muscle damage.
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Diverted resources. Phosphofructokinase converts fructose 6-phosphate to fructose 1,6-bisphosphate, the committed step on the pathway that synthesizes ATP. However, some fructose 6-phosphate is converted into fructose 2,6-bisphosphate. Explain why depleting the substrate of PFK to form fructose 2,6-bisphosphate is not a wasteful use of substrate.
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Recall from our discussion of enzyme kinetics in Chapter 7 that substrates are usually present in much higher concentration than their enzymes. Consequently, converting a small amount of substrate into a potent activator of PFK will lead to a rapid increase in the rate of ATP synthesis.
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Glucose 6-phosphate
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Inhibits hexokinase
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Low ATP/AMP ratio
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Stimulates phosphofructokinase
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Citrate
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Inhibits phosphofructokinase in the liver
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Low pH
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Inhibits phosphofructokinase in muscle
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Fructose 1,6-bisphosphate
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Stimulates pyruvate kinase
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Fructose 2,6- bisphosphate
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Stimulates phosphofructokinase in the liver
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Insulin
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Causes the insertion of GLUT4 into cell membranes
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Has a high KM for glucose
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Glucokinase
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Transporter specific to liver and pancreas
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GLUT2
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High ATP/AMP ratio
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Inhibits phosphofructokinase
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Not MTV. A ligand-gated ion channel and a voltagegated ion channel play keys roles in the secretion of insulin by the pancreas. What are the channels, and what is their function in insulin secretion?
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A potassium channel, a ligand-gated channel, is inhibited by binding ATP. This inhibition alters the voltage across the plasma membrane, which activates a calcium channel, a voltage-gated channel, allowing an influx of calcium ions. The calcium ions stimulate the fusion of insulin-containing granules with the plasma membrane, resulting in the secretion of insulin.
