12 - Fatty Acid Mobilization, Oxidation, and Synthesis

What happens to fatty acids in the fed state?

Insulin promotes fatty acid synthesis and storage.

What happens to fatty acids during fasting?

Insulin decreases, while catecholamines can stimulate adipose lipolysis. Fatty acids are released and oxidized for energy.

Does fatty acid oxidation only begin after glycogen is depleted?

No. Fatty acid oxidation occurs every day. During an overnight fast, the liver can break down glycogen and oxidize fatty acids at the same time.

What are the main roles of the liver, adipose tissue, skeletal muscle, and heart in fatty acid metabolism?

• Liver: Makes fatty acids when fed, oxidizes them during fasting, and exports TAG in VLDL.

• Adipose tissue: Stores TAG and releases fatty acids.

• Skeletal muscle: Uses fatty acids as fuel.

• Heart: Relies heavily on fatty acids for energy.

What are the main roles of the liver in fatty acid metabolism?

Makes fatty acids when fed, oxidizes them during fasting, and exports TAG in VLDL.

What is adipose tissue’s role in fatty acid metabolism?

Stores energy as TAG and releases fatty acids during lipolysis.

What is skeletal muscle’s role in fatty acid metabolism?

Uses fatty acids as a major fuel for energy.

What is the heart’s role in fatty acid metabolism?

Relies heavily on fatty acids as a major source of energy.

Which tissues cannot significantly use long-chain fatty acids for energy?

Red blood cells cannot perform β-oxidation, and the brain uses little long-chain fatty acid.

What does adipose lipolysis do?

Lipases break stored triacylglycerols (TAGs) into three fatty acids and one glycerol.

Which hormones stimulate or inhibit adipose lipolysis?

• Epinephrine and norepinephrine: Stimulate lipolysis.

• Insulin: Inhibits lipolysis.

• Glucagon: Is not a major direct regulator of human adipose lipolysis.

How are free fatty acids transported through the blood?

They circulate largely bound to albumin

Why does glycerol travel to the liver after lipolysis?

Adipose tissue has little glycerol kinase, so the liver processes glycerol more effectively.

How does the liver use glycerol to make glucose? (pathway)

glycerol → glycerol-3-phosphate → DHAP → gluconeogenesis → glucose.

What enzyme activates a free fatty acid, and what product does it make?

Acyl-CoA synthetase converts a free fatty acid into fatty acyl-CoA.

How much energy does fatty acid activation cost?

It uses ATP → AMP + PPᵢ, costing two ATP equivalents. Hydrolysis of PPᵢ makes the reaction essentially irreversible.

How are fatty acids classified by chain length?

• Short-chain: Fewer than 6 carbons.

• Medium-chain: 6–12 carbons.

• Long-chain: 13–21 carbons.

What are examples of medium- and long-chain fatty acids?

MCT oil commonly contains C8 and C10 fatty acids. Palmitate is a long-chain fatty acid with 16 carbons (C16).

Which fatty acids require the carnitine shuttle to enter mitochondria?

Long-chain fatty acyl groups.

What does CPT-I do in the carnitine shuttle?

CPT-I, located on the outer mitochondrial membrane, transfers the fatty acyl group from CoA to carnitine, forming acylcarnitine.

What does the carnitine-acylcarnitine translocase do?

It transports acylcarnitine into the mitochondrial matrix in exchange for free carnitine.

What does CPT-II do?

CPT-II transfers the acyl group from carnitine back to CoA, rebuilding fatty acyl-CoA in the mitochondrial matrix.

Does carnitine transport intact fatty acyl-CoA into the mitochondrial matrix?

No. Carnitine carries the acyl group, not intact acyl-CoA. Carnitine is recycled after transport.

Do short- and medium-chain fatty acids require the carnitine shuttle?

No. They can enter mitochondria without the shuttle and are activated in the matrix. They still require the appropriate β-oxidation enzymes.

What causes primary systemic carnitine deficiency?

Defective cellular uptake of carnitine, which impairs long-chain fatty acid oxidation.

What are the major signs of carnitine deficiency?

Muscle weakness, cardiomyopathy, and fasting hypoketotic hypoglycemia.

What can cause secondary carnitine depletion?

Valproate use and dialysis.

What is levocarnitine used for?

It replaces carnitine in patients who need it. Routine supplementation is not required for every patient taking valproate.

Where does β-oxidation occur?

Mainly in the mitochondrial matrix.

What are the four repeated reactions of β-oxidation, in order?

1. Oxidation.

2. Hydration.

3. Oxidation.

4. Thiolysis.

What happens during the first oxidation step of β-oxidation?

Acyl-CoA dehydrogenase oxidizes fatty acyl-CoA using enzyme-bound FAD, producing FADH₂.

How do electrons from the first β-oxidation reaction enter the electron transport chain?

Electrons pass through electron transfer flavoprotein (ETF) to coenzyme Q.

What happens during the hydration step of β-oxidation?

Water is added across the double bond formed during the first oxidation.

What happens during the second oxidation step of β-oxidation?

NAD⁺ is reduced to NADH.

What happens during thiolysis?

The fatty acyl chain is split, releasing acetyl-CoA and a fatty acyl-CoA that is two carbons shorter.

Does β-oxidation directly produce ATP?

No. It produces NADH, FADH₂, and acetyl-CoA, which support ATP production through other pathways.

How many β-oxidation cycles are needed to completely break down palmitate (C16:0)?

Seven cycles.

What are the products of complete β-oxidation of palmitate?

Eight acetyl-CoA, seven NADH, and seven FADH₂

How much ATP-equivalent energy comes from the seven NADH and seven FADH₂ produced from palmitate?

• 7 NADH × 2.5 ATP = 17.5 ATP.

• 7 FADH₂ × 1.5 ATP = 10.5 ATP.

• Total = 28 ATP equivalents.

How much ATP-equivalent energy comes from the eight acetyl-CoA produced from palmitate?

Eight acetyl-CoA × 10 ATP equivalents each = 80 ATP equivalents.

What is the net ATP yield from complete oxidation of palmitate?

108 gross ATP equivalents − 2 for activation = approximately 106 net ATP equivalents.

Where does most of the ATP from palmitate oxidation come from?

Oxidation of the eight acetyl-CoA through the TCA cycle and electron transport chain.

What does medium-chain acyl-CoA dehydrogenase (MCAD) do?

It catalyzes the first, FAD-dependent oxidation step of β-oxidation for medium-chain fatty acyl-CoA.

What causes hypoketotic hypoglycemia in MCAD deficiency?

Impaired fatty acid oxidation lowers ATP production needed for gluconeogenesis and reduces acetyl-CoA, an activator of pyruvate carboxylase. This reduces glucose production and ketone formation.

When can MCAD deficiency cause symptoms?

During fasting or illness. Symptoms can include vomiting, lethargy, seizures, and hypoglycemia with low ketone levels.

How is MCAD deficiency managed during fasting or illness?

Avoid prolonged fasting, provide carbohydrates during illness, and use IV glucose if needed.

Does MCT oil bypass MCAD deficiency?

No. MCT oil does not bypass MCAD because medium-chain fatty acids still require the appropriate β-oxidation enzymes.

Besides MCAD deficiency, what can cause hypoketotic hypoglycemia?

Excess insulin, such as from insulin or sulfonylurea overdose, suppresses fatty acid release and ketone production.

Can glycerol from TAGs be used to make glucose?

Yes. Glycerol can enter gluconeogenesis through DHAP.

Can even-chain fatty acids produce net glucose in humans?

No. Even-chain fatty acids produce acetyl-CoA, which cannot provide net glucose in humans.

How does fatty acid oxidation support gluconeogenesis if it cannot provide net glucose carbon?

Fatty acid oxidation supplies the ATP needed for gluconeogenesis, while glycerol can supply carbon for glucose production.

Is fatty acid synthesis simply β-oxidation running in reverse?

No. The pathways use different enzymes, occur in different compartments, and use different carriers:

• Synthesis: mainly cytosol, uses NADPH.

• β-oxidation: mainly mitochondrial matrix, uses NAD⁺ and FAD.

Why must acetyl-CoA be transported indirectly out of the mitochondrion for fatty acid synthesis?

Acetyl-CoA cannot cross the inner mitochondrial membrane directly.

How is mitochondrial acetyl-CoA transported to the cytosol?

Acetyl-CoA combines with oxaloacetate to form citrate. Citrate exits the mitochondrion, and ATP-citrate lyase uses ATP and CoA to produce cytosolic acetyl-CoA and oxaloacetate.

What reaction does acetyl-CoA carboxylase (ACC) catalyze?

ACC converts acetyl-CoA + HCO₃⁻ + ATP into malonyl-CoA.

Why is ACC important in fatty acid synthesis?

ACC catalyzes the committed step of fatty acid synthesis by producing malonyl-CoA.

What vitamin does ACC require, and what does the vitamin do?

Biotin (vitamin B7) carries the activated carboxyl group during the reaction.

How can raw egg whites interfere with fatty acid synthesis?

Raw egg-white avidin binds biotin, potentially impairing biotin-dependent ACC activity.

What activates ACC?

Insulin and citrate promote ACC activity.

What happens to ACC when cellular energy is low?

ACC is inhibited, lowering malonyl-CoA and favoring fatty acid oxidation.

What does fatty acid synthase do?

It builds fatty acid chains by adding two-carbon units derived from malonyl-CoA.

Why does malonyl-CoA add two carbons to a growing fatty acid chain even though it contains three carbons?

One carbon leaves as CO₂, so the growing fatty acid gains two carbons per malonyl-CoA.

What reducing agent does fatty acid synthesis use?

NADPH, which supplies reducing power for synthesis.

Where does the NADPH used in fatty acid synthesis come from?

The pentose phosphate pathway and cytosolic malic enzyme.

What are the requirements to synthesize one palmitate molecule?

• 1 acetyl-CoA to start the chain.

• 7 malonyl-CoA, requiring 7 ATP for their formation.

• 14 NADPH.

How does malonyl-CoA regulate fatty acid oxidation?

Malonyl-CoA inhibits CPT-I, reducing the entry of long-chain fatty acyl groups into mitochondria.

Why does malonyl-CoA inhibition of CPT-I make metabolic sense?

When fatty acid synthesis is active, malonyl-CoA prevents long-chain fatty acids from entering mitochondria for oxidation. This avoids strongly activating synthesis and breakdown at the same time.

What happens to malonyl-CoA during fasting?

Malonyl-CoA decreases, relieving CPT-I inhibition and allowing more long-chain fatty acid oxidation.

What are the effects of ACC inhibitors being studied for fatty liver?

They lower malonyl-CoA, decreasing fatty acid synthesis and increasing fatty acid oxidation.

lipolysis

TAG → 3 fatty acids + glycerol

Glucose from lipids

Glycerol can make glucose; even-chain fatty acids cannot produce net glucose

ACC

Makes malonyl-CoA; requires biotin

Malonyl-CoA

Inhibits CPT-I, linking synthesis to suppression of oxidation

fatty acid synthase

Adds 2 carbons per malonyl-CoA and uses NADPH