Ketone Bodies and fatty acid biosynthesis

What hormone is released when blood glucose (BG) is low?

Glucagon

What happens after glucagon is released in adipose tissue?

Glucagon binds its receptor on the adipose cell membrane.

How does glucagon signaling activate PKA?

It stimulates adenylate cyclase via a G-protein, producing cAMP, which activates PKA.

Which enzymes/proteins does PKA phosphorylate in lipid droplets?

Hormone-sensitive lipase (HSL) and perilipin.

What is the role of perilipin once phosphorylated?

It allows hormone sensitive lipase (HSL) access to the lipid droplet surface.

What does HSL do once activated?

Hydrolyzes triacylglycerols (TAG) into free fatty acids (FFA).

How are free fatty acids transported in the blood?

They leave the adipocyte and bind to serum albumin.

How do free fatty acids enter myocytes?

They are released from albumin and transported into the cell by a specific FA transporter.

What happens to fatty acids inside the myocyte?

They are oxidized to CO₂, and the oxidation energy is conserved as ATP.

Where does fatty acid (FA) synthesis occur in the cell?

in the cytoplasm

What is ACC and why is it important?

ACC is the first enzyme in fatty acid synthesis; it converts acetyl-CoA to malonyl-CoA

How does insulin affect acetyl-CoA carboxylase (ACC)?

Insulin activates phosphatases → dephosphorylate ACC → ACC becomes active.

What effect does high carbohydrate intake have on β-oxidation?

High carbs → ↑ FA synthesis → ↓ β-oxidation.

What molecule inhibits carnitine acyltransferase I (CAT I)?

Malonyl-CoA.

What is the role of CAT I (carnitine acyltransferase I)?

It transports fatty acyl-CoA into the mitochondria for β-oxidation.

Why does malonyl-CoA inhibit CAT I?

To prevent newly synthesized fatty acids from being immediately oxidized.

How does glucagon affect ACC?

Glucagon activates PKA → phosphorylates ACC → ACC becomes inactive → ↓ malonyl-CoA.

What happens to β-oxidation when malonyl-CoA levels fall?

CAT I inhibition is relieved → FA transport into mitochondria increases → β-oxidation increases.

Where does β-oxidation occur in the cell?

In the mitochondria.

What are the main products of β-oxidation?

Acetyl-CoA + NADH (and FADH₂).

In what organ does β-oxidation occur during starvation?

the liver

Why is oxaloacetate diverted during starvation?

It is used for gluconeogenesis to prioritize fueling the brain.

What happens to the TCA cycle when oxaloacetate is diverted?

The TCA cycle slows due to lack of oxaloacetate.

What happens to ATP production and acetyl-CoA levels during starvation-induced β-oxidation in the liver?

ATP production decreases because acetyl-CoA cannot enter the TCA cycle (due to low oxaloacetate), and acetyl-CoA builds up as β-oxidation continues.

What happens to excess acetyl-CoA in the liver during starvation, and why is this important?

Excess acetyl-CoA is redirected into ketone body synthesis, providing an alternative fuel source for extrahepatic tissues.

Is the last step of β-oxidation reversible?

yes

What are the three ketone bodies, and which one is exhaled?

Acetone (exhaled), acetoacetate, and D-β-hydroxybutyrate (the last two are acidic).

Why does the brain use ketone bodies during starvation?

Fatty acids cannot cross the blood-brain barrier, but acetoacetate and β-hydroxybutyrate can.

What drives ketone body synthesis from acetyl-CoA in the liver?

Excess acetyl-CoA from β-oxidation reverses the final β-oxidation step to make acetoacetyl-CoA, which is then converted to HMG-CoA and acetoacetate.

Where is HMG-CoA lyase found, and why is that important?

It is found only in the liver mitochondrial matrix, allowing ketone synthesis in liver mitochondria (not cytosol).

Is the conversion of acetoacetate to acetone reversible?

no

What does acetoacetate decarboxylase require?

TPP

Why does β-oxidation continue during ketone body formation?

Ketone synthesis releases CoA, allowing β-oxidation to keep generating fatty acids and acetyl-CoA.

What happens hormonally in uncontrolled diabetes?

Lack of insulin + high glucagon → high cAMP → activation of HSL → ↑ lipolysis → ↑ β-oxidation → ↑ acetyl-CoA.

Why can uncontrolled diabetes lead to ketoacidosis?

Excess acetyl-CoA causes overproduction of ketone bodies, lowering blood pH.

What is the main purpose of fatty acid (FA) biosynthesis?

To store excess fuel as triacylglycerols (TAG).

What reducing power is required for FA synthesis, and where does it occur?

Requires NADPH; occurs in the cytosol (not mitochondria).

What are the three functional regions (domains) of acetyl-CoA carboxylase (ACC)?

Biotin carrier protein, biotin carboxylase, and transcarboxylase.

What reaction does ACC catalyze, and why is it important?

Converts acetyl-CoA (2C) → malonyl-CoA (3C); this is the rate-limiting step of FA synthesis.

What activates and inhibits acetyl-CoA carboxylase?

Activated by: citrate & insulin.

Inhibited by: palmitoyl-CoA, glucagon, epinephrine (via phosphorylation).

How many enzymatic activities does the fatty synthase in a single large polypeptide contain?

7

What are the four repeated steps of long-chain fatty acid (LCFA) assembly?

1. Condensation

2. Reduction (NADPH → NADP⁺)

3. Dehydration

4. Reduction (NADPH → NADP⁺)

Which enzymes catalyze the four steps of LCFA assembly?

• β-Ketoacyl-ACP synthase (condensation)

• β-Ketoacyl-ACP reductase (1st reduction)

• β-Hydroxyacyl-ACP dehydratase (dehydration)

• Enoyl-ACP reductase (2nd reduction)

How does the fatty acid chain grow during palmitate synthesis?

It grows 2 carbons at a time, donated by activated malonate, which loses CO₂ during each elongation step.

What happens to each malonyl-CoA that is added to the growing chain?

Each malonyl-CoA added becomes a carboxyl group in the fatty acid chain after decarboxylation.

Where is the acetyl-CoA located during fatty acid synthesis?

At the far end of the chain (the beginning “primer” unit).

What occurs after each 2-carbon addition during palmitate synthesis?

Sequential reductions convert the growing chain into a fully saturated fatty acid.

What is ACP and what are its functions?

ACP (acyl carrier protein):

• Attachment site for malonyl-CoA and the FA chain

• Contains the prosthetic group 4'-phosphopantetheine

• Terminal –SH group binds acyl intermediates as esterified malonyl or acyl groups

How many malonyl-CoA and acetyl-CoA molecules are required to synthesize palmitate (16:0)?

7 malonyl-CoA + 1 acetyl-CoA

Where can palmitate go after its synthesis, and what happens there?

It can move to the endoplasmic reticulum (ER) for elongation to stearic acid (18:0).

What are the roles of desaturases and elongases?

• Desaturases: introduce double bonds into fatty acids (e.g., by acyl-CoA desaturase).

• Elongases: extend the fatty acid chain by adding 2 carbons at a time.

What is the role of ACP in fatty acid synthase (step one of FA synthesis)?

ACP is the attachment site that holds the growing fatty acid chain and malonyl groups via its –SH group.

What are the step-wise reactions of fatty acid synthase (FAS)?

ACP loads substrates→ ACP provides the –SH attachment site for the growing FA chain.

Acetyl transfer→ Malonyl/acetyl-CoA–ACP transferase moves acetyl-CoA → cysteine –SH of β-ketoacyl-ACP synthase.

Malonyl transfer→ Same enzyme transfers malonyl-CoA → ACP –SH.

Condensation (CO₂ lost)→ β-Ketoacyl-ACP synthase condenses acetyl + malonyl → β-ketoacyl-ACP (+ CO₂ released).

First reduction→ β-Ketoacyl-ACP reductase converts β-ketoacyl-ACP → D-β-hydroxyacyl-ACP (uses NADPH).

Dehydration→ β-Hydroxyacyl-ACP dehydratase removes H₂O, forming trans-Δ²-acyl-ACP.

Second reduction→ Enoyl-ACP reductase reduces double bond → saturated acyl-ACP (uses NADPH).

Chain extension→ Product is 2 carbons longer; cycle repeats until palmitate (16:0) is formed.

After the condensation reaction, where is the fatty acyl chain located?

It becomes bound to the prosthetic group of ACP.

What happens to the acyl chain after the reduction steps?

The acyl chain transfers from ACP → Cys on the enzyme, opening ACP for a new malonyl-CoA.

How do FA synthesis reduction steps compare to β-oxidation?

They are the reverse/opposite of β-oxidation steps.

Why is FA synthesis energetically costly?

You lose energy as heat, so you gain less ATP from FAs than the energy required to synthesize them.

What are the two cellular roles of NADPH/NADP⁺?

• NADPH → NADP⁺: synthetic reactions (via reductases)• NADP⁺ → NADPH: energy generation

What are the main sources of NADPH for fatty acid synthesis?

Malic enzyme (especially in adipocytes)

Pentose phosphate pathway (PPP) in hepatocytes

What are the two major functions of the citrate shuttle?

Moves citrate from mitochondria → cytoplasm

Supplies acetyl-CoA for FA synthesis

How does citrate exit the mitochondria and regenerate acetyl-CoA in the cytosol?

Citrate is exported → ATP-citrate lyase converts it to acetyl-CoA + oxaloacetate in the cytosol.

For each citrate that exits mitochondria, what is produced in the cytosol?

1 acetyl-CoA + 1 NADPH (from the malate → pyruvate reaction via malic enzyme).