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Enzyme Classification and Function

• Question 1: An enzyme that catalyzes an oxidation-reduction reaction is classified as a:

  • B. Oxidoreductase: Enzymes that catalyze the transfer of electrons between molecules, facilitating oxidation and reduction reactions.

• Question 2: Kinases are generally classified as:

  • C. Transferases: Enzymes that catalyze the transfer of a functional group, such as a phosphate group, from one molecule to another.

• Question 3: An enzyme that breaks a bond by adding water is a:

  • A. Hydrolase: Enzymes that catalyze hydrolysis reactions, which involve the breaking of a bond in a molecule by the addition of water.

• Question 4: An enzyme that rearranges atoms within a molecule is a:

  • C. Isomerase: Enzymes that catalyze the conversion of a molecule into one of its isomers (structurally different molecule).

Additional Enzyme Classifications

• Question 5: An enzyme that removes a group to form a double bond is most likely a:

  • B. Lyase: Enzymes that catalyze the addition or removal of groups to form or break double bonds.

• Question 6: An enzyme that joins two molecules together using ATP is a:

  • D. Ligase: Enzymes that catalyze the joining of two molecules in the presence of a cofactor, commonly ATP.

Kinetics and Vmax

• Question 7: Which statement about Km is correct?

  • C. A lower Km means higher enzyme affinity for substrate: Km is the substrate concentration at which the reaction velocity is half of Vmax, indicating higher affinity at lower values.

• Question 8: Vmax is best defined as:

  • B. The maximum rate when enzyme is saturated with substrate: Vmax indicates the rate of reaction when all enzyme active sites are occupied.

• Question 9: When [S] = Km, the reaction velocity is:

  • C. 1/2 Vmax: When substrate concentration equals Km, the rate of reaction is at half the maximum velocity.

Lineweaver-Burk Plots

• Question 10: On a Lineweaver-Burk plot, the y-intercept equals:

  • C. 1/Vmax: It represents the inverse of the maximum velocity of the enzymatic reaction.

• Question 11: On a Lineweaver-Burk plot, the x-intercept equals:

  • B. -1/Km: It relates to the inverse of Michaelis constant, representing the substrate affinity.

• Question 12: The slope of a Lineweaver-Burk plot is:

  • B. Km/Vmax: The slope can be derived from the ratio of the two significant kinetic parameters.

Inhibition Types

• Question 13: A competitive inhibitor typically:

  • A. Binds at the active site: This type of inhibitor resembles the substrate and competes for the active site.

• Question 14: A classic noncompetitive inhibitor typically:

  • C. Decreases Vmax while leaving Km unchanged: Noncompetitive inhibitors bind away from the active site, altering the enzyme's effectiveness without affecting substrate binding.

• Question 15: On a Lineweaver-Burk plot, competitive inhibition is identified by:

  • C. Lines intersecting on the y-axis: Competitive inhibitors increase the slope without altering the y-intercept.

• Question 16: On a Lineweaver-Burk plot, noncompetitive inhibition is identified by:

  • A. Same y-intercept: The slope increases while the x-intercept remains constant.

Glycolysis Overview

• Question 17: Glycolysis occurs in the:

  • C. Cytosol: Glycolysis takes place in the cytoplasmic portion of the cell.

• Question 18: The net yield of glycolysis per glucose is:

  • B. 2 ATP and 2 NADH: Glycolysis breaks down glucose to produce a net gain of 2 ATP (after accounting for 2 used) and 2 NADH molecules.

• Question 19: Which enzyme catalyzes the reaction: glucose + ATP → glucose-6-phosphate + ADP?

  • B. Hexokinase: This enzyme catalyzes the phosphorylation of glucose to form glucose-6-phosphate.

• Question 20: Which enzyme catalyzes the committed step of glycolysis: fructose-6-phosphate + ATP → fructose-1,6-bisphosphate + ADP?

  • C. Phosphofructokinase: Catalyzes key regulatory step of glycolysis commitment to metabolize fructose-6-phosphate.

Enzyme Regulation in Glycolysis

• Question 21: Which glycolysis enzyme is allosterically activated by ADP/AMP and inhibited by ATP/citrate?

  • B. Phosphofructokinase: It's sensitive to the energy state of the cell; activated when energy is low (high ADP/AMP) and inhibited when energy is plentiful (high ATP/citrate).

• Question 22: Which enzyme catalyzes the cleavage of fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate?

  • B. Aldolase: Splits this molecule into two three-carbon components.

• Question 23: Which reaction is catalyzed by aldolase?

  • C. Fructose-1,6-bisphosphate → glyceraldehyde-3-phosphate + dihydroxyacetone phosphate: This reaction is crucial for glycolysis, where aldolase facilitates the partitioning of energy-rich compounds.

• Question 24: Which enzyme converts dihydroxyacetone phosphate into glyceraldehyde-3-phosphate?

  • A. Triose phosphate isomerase: This enzyme interconverts the two isomers of triose phosphates produced in glycolysis.

Glycolytic Reactions Producing NADH

• Question 25: Which glycolytic reaction produces NADH?

  • C. Glyceraldehyde-3-phosphate + Pi + NAD+ → 1,3-bisphosphoglycerate + NADH: This step involves oxidation, producing NADH as a carrier of reducing power.

• Question 26: Glyceraldehyde-3-phosphate dehydrogenase catalyzes which reaction?

  • A. Glyceraldehyde-3-phosphate + Pi + NAD+ → 1,3-bisphosphoglycerate + NADH: This is an essential step in energy extraction from glucose.

• Question 27: Which enzyme catalyzes the substrate-level phosphorylation reaction: 1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP?

  • A. Phosphoglycerate kinase: This enzyme facilitates ATP synthesis from the high-energy intermediates.

• Question 28: Enolase catalyzes which reaction?

  • C. 2-phosphoglycerate → phosphoenolpyruvate + H2O: This is a dehydration reaction that forms a compound with a high-energy phosphate bond.

Additional Glycolysis Enzyme Questions

• Question 29: Pyruvate kinase catalyzes which reaction?

  • B. Phosphoenolpyruvate + ADP → pyruvate + ATP: This is the final step in glycolysis, yielding ATP from ADP.

• Question 30: Which set contains the three irreversible regulatory reactions of glycolysis?

  • B. Glucose → glucose-6-phosphate; fructose-6-phosphate → fructose-1,6-bisphosphate; phosphoenolpyruvate → pyruvate: These steps are highly regulated and typically not reversed.

• Question 31: Gluconeogenesis occurs mainly in the:

  • C. Liver: This organ primarily carries out gluconeogenesis to maintain blood glucose levels.

• Question 32: Gluconeogenesis is best described as:

  • B. A pathway that generates glucose from noncarbohydrate precursors: This process is crucial during fasting or intense exercise.

Key Enzymes and Energetics in Gluconeogenesis

• Question 33: The conversion of pyruvate to phosphoenolpyruvate in gluconeogenesis requires:

  • C. Pyruvate carboxylase and PEP carboxykinase: Two enzymes required for this conversion step, each acting sequentially in gluconeogenesis.

• Question 34: Which enzyme bypasses phosphofructokinase in gluconeogenesis?

  • B. Fructose-bisphosphatase: This enzyme converts fructose-1,6-bisphosphate back to fructose-6-phosphate.

• Question 35: Which enzyme catalyzes the final step of gluconeogenesis?

  • C. Glucose-6-phosphatase: It converts glucose-6-phosphate into free glucose, completing the gluconeogenesis pathway.

• Question 36: The energy cost of gluconeogenesis is:

  • B. 4 ATP, 2 GTP, 2 NADH: The overall energetic investment needed to synthesize glucose from pyruvate.

Glycogen Metabolism

• Question 37: Glycogen contains which glycosidic linkages?

  • C. α(1→4) with α(1→6) branch points: Glycogen structure allows for rapid mobilization of glucose when energy is needed.

• Question 38: The enzyme that breaks glycogen to release glucose-1-phosphate is:

  • B. Glycogen phosphorylase: It cleaves the phosphorolytic linkage at the non-reducing ends of glycogen.

• Question 39: In liver glycogenolysis, glucose-6-phosphate can be converted to free glucose by:

  • C. Glucose-6-phosphatase: This enzyme is crucial in facilitating the release of glucose from the liver.

• Question 40: Which statement correctly matches diabetes type with its cause?

  • B. Type I: autoimmune beta-cell destruction; Type II: cells do not respond properly to insulin: These conditions illustrate the mechanisms underlying diabetes mellitus.

Short Answer Questions

• Question 41: A Lineweaver-Burk plot shows that with inhibitor present, the y-intercept stays the same, the slope increases, and the lines intersect on the y-axis. What type of inhibition is this, and why?

  • Competitive inhibition: The inhibitor binds at the active site, leading to an increase in Km while Vmax remains unchanged, causing intersection on the y-axis.

• Question 42: A Lineweaver-Burk plot shows that with inhibitor present, the x-intercept stays the same, while the y-intercept increases. What type of inhibition is this, and why?

  • Noncompetitive inhibition: The inhibitor binds to an allosteric site, decreasing Vmax without affecting Km, which is why the x-intercept remains unchanged.

• Question 43: Name the three irreversible enzymes of glycolysis and give one reason why they are important.

  • Enzymes: Hexokinase, Phosphofructokinase, Pyruvate Kinase.

  • Importance: They serve as major regulatory points in glycolysis and must be bypassed during gluconeogenesis.

• Question 44: Name the bypass enzymes/reactions of gluconeogenesis that replace the irreversible steps of glycolysis.

  • Bypass Enzymes/Reactions:

  • Pyruvate → oxaloacetate by pyruvate carboxylase

  • Oxaloacetate → phosphoenolpyruvate by PEP carboxykinase

  • Fructose-1,6-bisphosphate → fructose-6-phosphate by fructose-bisphosphatase

  • Glucose-6-phosphate → glucose by glucose-6-phosphatase

• Question 45: Explain the difference between glycogenesis and glycogenolysis. Include one key enzyme for each.

  • Glycogenesis: The synthesis of glycogen from glucose; key enzyme is glycogen synthase.

  • Glycogenolysis: The breakdown of glycogen to release glucose-1-phosphate; key enzyme is glycogen phosphorylase.

• Question 46: Briefly explain the Cori cycle and why it is important.

  • Cori Cycle: The metabolic pathway in which lactate produced during glycolysis in muscles is transported to the liver, where it is converted back to glucose via gluconeogenesis. This cycle is important as it helps recycle lactate and maintain blood glucose levels during intense exercise or fasting.