Medium MCAT Metabolism Practice Questions

Concept Explanation

Metabolism consists of the highly regulated chemical reactions within a cell that manage energy resources through catabolic pathways, which break down molecules to release energy, and anabolic pathways, which consume energy to build complex molecules. On the MCAT, understanding metabolism requires a deep dive into the major biochemical pathways: glycolysis, the citric acid cycle (Krebs cycle), oxidative phosphorylation, gluconeogenesis, glycogen metabolism, and fatty acid oxidation. Central to these processes are electron carriers like $\ \text{NAD}^+$ and $\ \text{FAD}$, which facilitate the transfer of energy to produce ATP, the cell's universal energy currency. Mastery of this subject involves recognizing key regulatory enzymes—such as phosphofructokinase-1 (PFK-1) in glycolysis or fructose-1,6-bisphosphatase in gluconeogenesis—and understanding how hormones like insulin and glucagon coordinate these pathways to maintain blood glucose homeostasis. Knowledge of Medium MCAT Redox Practice Questions is often helpful here, as metabolic pathways are essentially series of coupled oxidation-reduction reactions.

Solved Examples

1. Calculate the net ATP yield from the complete oxidation of one molecule of glucose in a eukaryotic cell using the malate-aspartate shuttle.
   1. Glycolysis produces 2 net ATP and 2 NADH.
   2. Pyruvate dehydrogenase complex (PDC) produces 1 NADH per pyruvate (2 NADH per glucose).
   3. The Citric Acid Cycle produces 3 NADH, 1 $\ \text{FADH}_2$, and 1 GTP (equivalent to ATP) per turn. Since there are two turns per glucose, this equals 6 NADH, 2 $\ \text{FADH}_2$, and 2 ATP.
   4. Total carriers: 10 NADH and 2 $\ \text{FADH}_2$.
   5. Using standard MCAT conversion factors (2.5 ATP per NADH and 1.5 ATP per $\ \text{FADH}_2$): $(10 \ \times 2.5) + (2 \ \times 1.5) = 25 + 3 = 28$ ATP from oxidative phosphorylation.
   6. Add substrate-level phosphorylation: 2 (Glycolysis) + 2 (TCA) = 4 ATP.
   7. Total: $28 + 4 = 32$ ATP.
2. Determine the effect of high ATP levels on the activity of Phosphofructokinase-1 (PFK-1).
   1. PFK-1 is the rate-limiting enzyme of glycolysis, converting fructose-6-phosphate to fructose-1,6-bisphosphate.
   2. ATP acts as both a substrate and an allosteric inhibitor for this enzyme.
   3. When ATP levels are high, it binds to a regulatory site, decreasing the enzyme's affinity for its substrate.
   4. This serves as a negative feedback mechanism to slow down energy production when the cell has sufficient energy.
3. Identify the primary source of carbon for gluconeogenesis during a prolonged fast.
   1. After glycogen stores are depleted, the body must synthesize glucose from non-carbohydrate precursors.
   2. Major precursors include lactate (from anaerobic metabolism), glycerol (from triglyceride breakdown), and glucogenic amino acids (from muscle protein proteolysis).
   3. Glucogenic amino acids, particularly alanine, enter the pathway via conversion to pyruvate or citric acid cycle intermediates.

Practice Questions

1. Which of the following enzymes is responsible for the "committed step" of glycolysis and is inhibited by citrate?
2. A patient has a defect in the enzyme glucose-6-phosphatase. Which metabolic process is most directly impaired?
3. In the presence of an uncoupler like 2,4-dinitrophenol (DNP), what happens to the rate of the Citric Acid Cycle and oxygen consumption?

4. How many molecules of NADH and $\ \text{FADH}_2$ are produced specifically by the Beta-oxidation of a 16-carbon saturated fatty acid (Palmitate) to Acetyl-CoA?
5. Which enzyme in the Citric Acid Cycle catalyzes a reaction that directly produces a high-energy nucleoside triphosphate via substrate-level phosphorylation?
6. During strenuous exercise, the build-up of lactate in muscle tissues is eventually processed by the liver. What is the name of this metabolic cycle?
7. What is the net yield of ATP when one molecule of glucose-1-phosphate, derived from glycogen, is processed through glycolysis to pyruvate?
8. Arsenate is a chemical that can replace inorganic phosphate in the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase. What is the likely result of this substitution on the net ATP yield of glycolysis?
9. Which of the following hormones stimulates the activity of the enzyme Fructose-2,6-bisphosphatase in the liver?
10. A researcher finds that a specific cell line lacks the malate-aspartate shuttle but possesses the glycerol-3-phosphate shuttle. Compared to normal cells, these cells will produce how much less ATP per glucose?

Answers & Explanations

1. Phosphofructokinase-1 (PFK-1). The committed step of glycolysis is the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. Citrate, an intermediate of the Citric Acid Cycle, signals that the cell has high energy levels and plenty of biosynthetic precursors, thus inhibiting PFK-1 to slow glycolysis.
2. Gluconeogenesis and Glycogenolysis. Glucose-6-phosphatase is found in the lumen of the endoplasmic reticulum of liver cells. It removes the phosphate group from glucose-6-phosphate, allowing free glucose to be released into the bloodstream. This is the final step for both the breakdown of glycogen (glycogenolysis) and the synthesis of glucose (gluconeogenesis).
3. Both increase. Uncouplers allow protons to leak across the inner mitochondrial membrane, bypassing ATP synthase. This destroys the proton gradient. To compensate for the lack of ATP, the Electron Transport Chain (ETC) works harder, consuming more oxygen. Because NADH is being consumed rapidly by the ETC, the Citric Acid Cycle speeds up to provide more electron carriers.
4. 7 NADH and 7 $\ \text{FADH}_2$. For a saturated fatty acid with $n$ carbons, the number of rounds of beta-oxidation is $(n/2) - 1$. For a 16-carbon chain, this is $(16/2) - 1 = 7$ rounds. Each round produces 1 NADH and 1 $\ \text{FADH}_2$. Note that it also produces 8 Acetyl-CoA molecules.
5. Succinyl-CoA synthetase. This enzyme converts Succinyl-CoA to Succinate. The energy released from the thioester bond cleavage is used to phosphorylate GDP to GTP (or in some tissues, ADP to ATP). This is the only step in the TCA cycle that produces a high-energy phosphate bond directly.
6. Cori Cycle. The Cori cycle (or lactic acid cycle) involves the transport of lactate from the muscles to the liver, where it is converted back into glucose via gluconeogenesis. This glucose is then returned to the muscles to be used for energy.
7. 3 ATP. Normally, glycolysis has a net yield of 2 ATP because 2 are invested and 4 are produced. However, glucose-1-phosphate from glycogen is converted to glucose-6-phosphate by phosphoglucomutase without requiring ATP. Therefore, the "investment phase" only uses 1 ATP (at the PFK-1 step), while the "payoff phase" still yields 4 ATP, resulting in a net of 3.
8. Net ATP yield becomes zero. Glyceraldehyde-3-phosphate dehydrogenase normally produces 1,3-bisphosphoglycerate, which then produces ATP in the next step. Arsenate results in a bypass where the high-energy phosphate bond is never formed, meaning the 2 ATP usually gained in the first payoff step are lost. Since 2 ATP were invested, the net becomes 0.
9. Glucagon. Glucagon triggers a signaling cascade that activates Protein Kinase A (PKA). PKA phosphorylates the bifunctional enzyme PFK-2/FBPase-2, activating the phosphatase domain (FBPase-2). This lowers the concentration of Fructose-2,6-bisphosphate, thereby inhibiting glycolysis and stimulating gluconeogenesis.
10. 2 ATP. The malate-aspartate shuttle allows 1 NADH to yield ~2.5 ATP. The glycerol-3-phosphate shuttle transfers electrons to FAD, meaning that cytoplasmic NADH only yields ~1.5 ATP. Since there are 2 cytoplasmic NADH per glucose, the difference is $2 \ \times (2.5 - 1.5) = 2$ ATP. You can learn more about energetic yields in Medium MCAT General Chemistry Practice Questions.

1. Which molecule acts as a potent allosteric activator of PFK-1 in the liver to override ATP inhibition?

• ACitrate
• BFructose-2,6-bisphosphate
• CGlucose-6-phosphate
• DAcetyl-CoA

Frequently Asked Questions

What is the difference between substrate-level phosphorylation and oxidative phosphorylation?

Substrate-level phosphorylation involves the direct transfer of a phosphate group from a high-energy metabolic intermediate to ADP/GDP, occurring in the cytoplasm or mitochondrial matrix. Oxidative phosphorylation uses the energy from an electrochemical proton gradient generated by the electron transport chain to power ATP synthase.

Why does FADH2 yield less ATP than NADH?

NADH donates its electrons to Complex I of the electron transport chain, whereas $\ \text{FADH}_2$ donates electrons to Complex II. Because Complex II does not pump protons across the membrane, $\ \text{FADH}_2$ contributes to a smaller proton gradient, resulting in a lower ATP yield per molecule.

How does insulin regulate glycolysis and gluconeogenesis?

Insulin promotes glycolysis by increasing the expression of key enzymes like glucokinase and PFK-1, and by activating the PFK-2 domain of the bifunctional enzyme. Conversely, it inhibits gluconeogenesis by decreasing the transcription of enzymes such as PEPCK and glucose-6-phosphatase.

What are the three irreversible steps of glycolysis?

The three irreversible steps are catalyzed by hexokinase (or glucokinase), phosphofructokinase-1, and pyruvate kinase. These steps have large negative Gibbs free energy changes and serve as the primary sites of regulation for the pathway. For more on reaction energetics, see Medium MCAT Kinetics Practice Questions.

Which metabolic pathways occur in the mitochondria?

The mitochondria house the Citric Acid Cycle, the Electron Transport Chain, Beta-oxidation of fatty acids, and the initial and final steps of the urea cycle. Additionally, the first step of gluconeogenesis (pyruvate to oxaloacetate) occurs within the mitochondrial matrix.