Chapter 16: Glycolysis Flashcards
What is the purpose of phosphorylating glucose in cytosol?
to trap glucose in the cell and to destabilize glucose and facilitate the next series of metabolic steps
Which two 3-carbon molecules are generated by the cleavage of fructose-1,6-bisphosphate?
glyceraldehyde-3-phosphate and dihydroxyacetone phosphate
What reaction is catalyzed by aldolase?
reversible cleavage of F-1,6-BP to DHAP and GAP
What is the function of glyceraldehyde 3-phosphate dehydrogenase?
oxidation by NAD+ and the formation of acyl phosphate
What is substrate-level phosphorylation?
ATP synthesis when the phosphate donor is a substrate with high-phosphoryl-transfer potential
What are the primary metabolic fates of pyruvate?
ethanol, lactate, and acetyl CoA
Fructose can enter glycolysis at two distinct points, depending on the tissue. How is fructose metabolized in adipose tissue?
Fructose is converted to fructose 6-phosphate.
How are the glycolytic enzymes regulated?
through transcriptional control, through reversible phosphorylation and through allosteric control
Which of the following must be regenerated for glycolysis to proceed?
NAD+
What type of organism cannot survive in the presence of oxygen?
obligate anaerobes
Which of the following intermediates is needed for the conversion of galactose into glucose by reacting with galactose 1-phosphate?
UDP-glucose
Define: Hexokinase
Phosphorylates glucose
Define: Phosphoglucose isomerase
Converts glucose 6-phosphate into fructose 6-phosphate
Define: Phosphofructokinase
Forms fructose 1,6-bisphosphate
Define: Aldolase
Cleaves fructose 1,6-bisphosphate
Define: Triose phosphate isomerase
Catalyzes the interconversion of three-carbon isomers
Define: Glyceraldehyde 3-phosphate dehydrogenase
Generates the first high- phosphoryl-transfer- potential compound that is not ATP
Define: Phosphoglycerate kinase
Generates the first molecule of ATP
Define: Enolase
Generates the second
high-phosphoryl- transfer-potential compound that is not ATP
Define: Pyruvate Kinase
Generates the second
molecule of ATP
Lactic acid fermentation and alcoholic fermentation are oxidation-reduction reactions. Identify the ultimate electron donor and electron acceptor.
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.
Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule?
Glucose 6-phosphate
3 ATP
Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule?
Dihydroxyacetone phosphate
2 ATP
Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule?
Glyceraldehyde 3-phosphate
2 ATP
Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule?
Fructose
2 ATP
Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule?
Sucrose
4 ATP
Why is it advantageous for the liver to have both hexokinase and glucokinase to phosphorylate glucose?
Glucokinase enables the liver to remove glucose from the blood when hexokinase is saturated, ensuring that glucose is captured for later use.
The interconversion of DHAP and GAP greatly favors the formation of DHAP at equilibrium. Yet the conversion of DHAP by triose phosphate isomerase proceeds readily. Why?
The GAP formed is immediately removed by subsequent reactions, resulting in the conversion of DHAP into GAP by the enzyme
The recommended daily allowance for vitamin niacin is 15 mg per day. How would glycolysis be affected by niacin deficiency?
The conversion of glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate would be impaired. Glycolysis would be less effective.
Why can’t the reactions of the glycolytic pathway simply be run in reverse to synthesize glucose?
The free energy for the reverse of glycolysis is +90 kJ mol^-1 (+22 kcal mol^-1), far too endergonic to take place.
What are the principle fates of pyruvate generated in glycolysis?
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.
What reactions of glycolysis are irreversible under intracellular conditions?
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.
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-biphosphate.
Explain why depleting the substrate of PFK to form fructose 2,6-bisphosphate is not a wasteful use of substrate.
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.
Glucose 6-phosphate:
Inhibits hexokinase
Low ATP/AMP ratio:
Stimulates phosphofructokinase
Citrate:
Inhibits phosphofructokinase in the liver
Low pH:
Inhibits phosphofructokinase
Fructose 2,6-bisphosphate;
Stimulates pyruvate kinase
Insulin:
Causes the insertion of GLUT4 into cell membranes
Has a high KM for glucose:
Glucokinase
Transporter specific to liver and pancreas:
GLUT2
High ATP/AMP ratio:
Inhibits phosphofructokinase
Sucrose is commonly used to preserve fruits. Why is glucose not suitable for preserving foods?
Glucose is reactive because its open-chain form contains an aldehyde group.
In other words, glucose is a reducing sugar.
In the liver, fructose can be converted into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate without passing through the phosphofructokinase-regulated reaction.
Show the reactions that make this conversion possible.
Why might ingesting high levels of fructose have deleterious physiological effects?
Fructose +ATP –> fructose 1-phosphate + ADP; Fructokinase
Fructose 1-phosphate –> dihydroxyacetone phosphate + glyceraldehyde; Fructose 1-phosphate aldolase
Glyceraldehyde +ATP –> Glyceraldehyde 3-Phosphate + ADP; Triose kinase
The primary controlling step of glycolysis catalyzed by phosphofructokinase is bypassed by the preceding reactions. Glycolysis will proceed in an unregulated fashion.
