Questions: Fatty Acid Synthesis and Regulation

Question 1

A cell has high AMP levels and low citrate. What happens to fatty acid synthesis, and why?

• A. Synthesis accelerates because AMP signals that the cell needs more energy storage
• B. Synthesis is inhibited because AMP activates AMPK, which phosphorylates and inactivates acetyl-CoA carboxylase
• C. Synthesis is unaffected — AMP only regulates glycolysis
• D. Synthesis accelerates because low citrate removes feedback inhibition from fatty acid synthase

Answer: (B)

High AMP signals energy deficit (low ATP:AMP ratio), activating AMP-activated protein kinase (AMPK). AMPK phosphorylates acetyl-CoA carboxylase (ACC), inactivating it and blocking malonyl-CoA production — the first committed step of fatty acid synthesis. Low citrate compounds the inhibition: citrate is the allosteric activator of ACC, and without it, the committed step is further suppressed. The cell would be foolish to spend energy building fat when it is starved for ATP. This is a textbook example of the cell reading its energy state through ACC.

Question 2

Which of the following correctly identifies a key difference between fatty acid synthesis and β-oxidation?

• A. Synthesis uses NADPH as the electron donor; β-oxidation produces NADH and FADH₂
• B. Both pathways use the same enzymes but run in opposite directions
• C. Synthesis occurs in mitochondria; β-oxidation occurs in the cytoplasm
• D. Synthesis produces acetyl-CoA; β-oxidation consumes acetyl-CoA

Answer: (A)

Fatty acid synthesis is explicitly not the reverse of β-oxidation. It uses NADPH (from the pentose phosphate pathway and malic enzyme) as the reducing agent, while β-oxidation produces NADH and FADH₂ as it oxidizes fatty acids. The pathways also use different enzymes, different subcellular compartments (synthesis in cytoplasm, β-oxidation in mitochondria), and different acyl carriers. This separation allows the cell to regulate them independently — a crucial design principle since running both simultaneously in the same compartment would waste energy.

Question 3

The CO₂ added by acetyl-CoA carboxylase is incorporated into the final palmitate product.

• T. True
• F. False

Answer: False

This is a classic misconception. Acetyl-CoA carboxylase adds CO₂ to acetyl-CoA to form malonyl-CoA — but this CO₂ is immediately released in the next step when malonyl-CoA condenses with the growing chain on fatty acid synthase. The CO₂ is not incorporated into the final fatty acid; it serves as a thermodynamic 'handle' that makes the condensation reaction energetically favorable (the decarboxylation drives the reaction forward). Palmitate contains only the carbons from acetyl-CoA units.

Question 4

Fatty acid synthesis is regulated by hormonal signals: insulin promotes synthesis while glucagon suppresses it.

• T. True
• F. False

Answer: True

Insulin, secreted when blood glucose is high, stimulates lipogenesis by activating ACC phosphatase, which dephosphorylates and activates ACC — promoting malonyl-CoA production and fatty acid synthesis. Glucagon (and epinephrine), secreted during energy deficit or stress, promotes ACC phosphorylation via PKA, inactivating the enzyme and suppressing synthesis. This hormonal axis ensures fat synthesis is linked to nutritional state: excess glucose that cannot all be stored as glycogen is channeled into fat, a process coordinated by pancreatic hormones.

Question 5

Why does fatty acid synthesis require a separate 'activation' step that adds and immediately removes a CO₂ group, rather than simply condensing two acetyl-CoA molecules directly?

Model answer: Direct condensation of two acetyl-CoA molecules (a Claisen condensation) is thermodynamically unfavorable — the equilibrium strongly favors the reverse reaction. By first carboxylating acetyl-CoA to malonyl-CoA (using ATP), the subsequent condensation step gains driving force from the simultaneous decarboxylation of malonyl-CoA. The CO₂ is added and immediately released, but the energy cost of its addition is what makes the overall condensation favorable. This is a common biochemical strategy: thermodynamically uphill reactions are driven by coupling to ATP hydrolysis or decarboxylation.

The CO₂ trick is analogous to the biotin-dependent carboxylation strategy used in other biosynthetic pathways (e.g., pyruvate carboxylase). The cell spends one ATP to add the CO₂ via ACC, then 'cashes in' the energy stored in the C–COO⁻ bond to drive the condensation on fatty acid synthase. Without this step, the chain-elongation chemistry would not proceed spontaneously, and the entire pathway would be thermodynamically blocked.