FA Synthesis

Where does fatty acid synthesis take place (organelle/compartment), and in which tissues is it highest?

In the cytosol; highest in liver and adipose tissue (though most cells can synthesize FAs to some extent).

What's the key 3-carbon intermediate in fatty acid synthesis, and why is this pathway described as energy-expensive?

Malonyl-CoA. The pathway requires 14 reducing equivalents (NADPH) and 7 ATP to build one palmitate molecule — making it highly regulated due to this energy cost.

Write the overall summary equation for palmitate synthesis from acetyl-CoA.

8 acetyl-CoA + 14 NADPH + 7 ATP + 14H+ → Palmitate (C16) + 14 NADP+ + 8 CoA + 7 ADP + 7 Pi + 6H2O

What enzyme converts acetyl-CoA to malonyl-CoA, and what cofactor does it require?

Acetyl-CoA carboxylase (ACC), which requires biotin as a cofactor.

Describe the three domains of ACC and how the reaction moves between them.

1) Biotin carboxylase domain — activates bicarbonate (HCO3-) using ATP; 2) biotin carrier protein domain — biotin (attached to a lysine residue) acts as a flexible swinging arm carrying the CO2 group; 3) transcarboxylase domain — transfers the carboxyl group from biotin onto acetyl-CoA, forming malonyl-CoA.

What is the full reaction catalyzed by ACC?

Acetyl-CoA + HCO3- + ATP → Malonyl-CoA + ADP + Pi

Name the six enzymatic domains of the fatty acid synthase complex and what each one does (in the order chain-building uses them).

MAT (malonyl/acetyl-CoA-ACP transferase) — loads acetyl and malonyl groups onto ACP; KS (β-ketoacyl-ACP synthase) — condensation; KR (β-ketoacyl-ACP reductase) — first reduction (keto→hydroxyl); DH (β-hydroxyacyl-ACP dehydratase) — dehydration; ER (enoyl-ACP reductase) — second reduction (saturates the double bond); ACP (acyl carrier protein) — the flexible arm that shuttles the growing chain between domains.

What functional group on ACP physically carries the growing fatty acid chain between the enzyme domains?

A thiol (–SH) group on the flexible ACP arm (analogous to the –SH on the KS domain that temporarily holds the acyl group during condensation).

What are the two loading steps that "activate" the FAS complex before chain-building begins?

1) Acetyl-CoA is loaded onto the synthase's thiol group at the KS domain; 2) MAT transfers a malonyl group from malonyl-CoA onto the ACP thiol group — so acetyl and malonyl groups are loaded one at a time onto ACP/KS.

What happens during the condensation step (Step 1), and which domain catalyzes it?

The KS domain catalyzes decarboxylation of the malonyl group (releasing CO2) and condenses it with the acetyl group, forming a 4-carbon β-ketoacyl chain attached to ACP.

What happens during the first reduction step (Step 2), which domain performs it, and what cofactor is used?

KR (β-ketoacyl-ACP reductase) reduces the β-keto group to a hydroxyl group, using NADPH (which becomes NADP+).

What happens during the dehydration step (Step 3), and which domain performs it?

DH (β-hydroxyacyl-ACP dehydratase) removes water — eliminating a hydrogen from the α-carbon and the hydroxyl from the β-carbon — forming a trans α,β double bond (enoyl-ACP intermediate).

What happens during the second reduction step (Step 4), which domain performs it, and what cofactor is used?

ER (enoyl-ACP reductase) reduces the double bond using NADPH, producing a fully saturated acyl-ACP (NADPH → NADP+).

After one full 4-step cycle, how many carbons has the chain gained, and what happens next?

2 carbons (from the malonyl-CoA donor, after losing one as CO2 during condensation). The chain is then transferred back to the KS domain, and a new malonyl-CoA is loaded onto ACP to begin the next cycle.

How many total cycles are needed to build the 16-carbon palmitate, and what removes it from the enzyme at the end?

7 cycles total. The finished 16-carbon chain is released by hydrolysis, catalyzed by thioesterase.

Write the reaction for converting 7 acetyl-CoA to 7 malonyl-CoA (the ACC step, repeated).

7 acetyl-CoA + 7 CO2 + 7 ATP → 7 malonyl-CoA + 7 ADP + 7 Pi

Write the overall net reaction combining 1 acetyl-CoA (starter) + 7 malonyl-CoA + 14 NADPH into palmitate.

Acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14H+ → Palmitate + 7 CO2 + 8 CoA + 14 NADP+ + 6 H2O

Combining both the ACC step and FAS steps, what's the full net equation starting from 8 acetyl-CoA?

8 acetyl-CoA + 7 ATP + 14 NADPH + 14H+ → Palmitate + 8 CoA + 7 ADP + 7 Pi + 14 NADP+ + 6 H2O

Acetyl-CoA is made in the mitochondrial matrix, but fatty acid synthesis happens in the cytosol. How does the cell get acetyl-CoA units across the membrane?

Via the citrate shuttle: acetyl-CoA + oxaloacetate → citrate (in the matrix) → citrate crosses the inner mitochondrial membrane via a citrate transporter → in the cytosol, citrate lyase splits citrate back into acetyl-CoA + oxaloacetate.

After citrate lyase regenerates cytosolic oxaloacetate, what happens to it, and why does this matter for fatty acid synthesis?

Oxaloacetate is reduced to malate (producing NADH), then malate is converted to pyruvate by malic enzyme — this step generates NADPH, which is used directly in fatty acid synthesis. Malate and pyruvate are then transported back into the mitochondria to complete the shuttle.

Besides the malic enzyme step, what's another major cytosolic source of NADPH for fatty acid synthesis?

The pentose phosphate pathway.

¹⁴C-labeled malonyl-CoA (labeled at carbon #2) and unlabeled acetyl-CoA are combined with FAS to make palmitate. Which carbons of palmitate end up labeled?

All the even-numbered carbons will be labeled, except for carbon #16. Reasoning: acetyl-CoA (unlabeled) provides the starter 2-carbon unit (carbons 15-16, the omega end), and every subsequent 2-carbon extension comes from malonyl-CoA's labeled carbon after losing the other carbon as CO2 during condensation — so the labeled carbon from each malonyl-CoA becomes the new carbon closest to the growing chain each cycle, landing on even-numbered positions except the very last (terminal methyl, C16) which came from the unlabeled acetyl-CoA starter.

How does palmitoyl-CoA (a β-oxidation product) inhibit fatty acid synthesis?

It allosterically inhibits acetyl-CoA carboxylase (ACC) — feedback inhibition preventing synthesis when there's already plenty of fatty acid product available.

What activates acetyl-CoA carboxylase (ACC) allosterically?

Citrate.

How does malonyl-CoA (a fatty acid synthesis intermediate) inhibit β-oxidation?

It allosterically inhibits carnitine acyltransferase I (CPT1/CAT1), blocking fatty acyl-CoA from entering the mitochondria for oxidation — ensuring synthesis and oxidation don't run simultaneously.

Trace the insulin signaling pathway from receptor binding to its ultimate effect on ACC and HSL.

Insulin binds its receptor → receptor autophosphorylation → phosphorylates IRS1 → recruits PI3K, converting PIP2 to PIP3 → PIP3 recruits AKT to the membrane, activating it → AKT activates phosphodiesterase (PDE) → PDE degrades cAMP to AMP → reduced PKA activity → phosphoprotein phosphatases dominate → dephosphorylate (and thereby activate) ACC and dephosphorylate (and thereby inactivate) HSL.

Under insulin signaling, what's the net effect on ACC and HSL activity, and what does this accomplish metabolically?

ACC is activated (dephosphorylated) → increased fatty acid synthesis. HSL is inactivated (dephosphorylated) → decreased fat mobilization. Net effect: increased fat storage, decreased gluconeogenesis, increased glycolysis.

Trace the glucagon/GPCR signaling pathway from ligand binding to PKA activation.

Glucagon binds its GPCR → heterotrimeric G protein's α subunit exchanges GDP for GTP and dissociates from βγ → GTP-bound α subunit activates adenylyl cyclase → converts ATP to cAMP → cAMP binds PKA regulatory subunits, releasing active catalytic subunits → PKA phosphorylates target proteins including ACC and HSL.

Under glucagon signaling, what's the net effect on ACC and HSL activity (via phosphorylation), and what does this accomplish metabolically?

ACC is inhibited (phosphorylated) → decreased fatty acid synthesis. HSL is activated (phosphorylated) → increased mobilization of stored fat. Net effect: increased gluconeogenesis, decreased fat synthesis, increased fat breakdown.

Besides ACC and HSL, what other enzyme is affected during insulin signaling, and how?

Pyruvate dehydrogenase (the supplier of acetyl-CoA) is activated under insulin/fed conditions, supporting increased fatty acid synthesis.

In a low-carb state, what happens to glucagon/insulin levels, and what's the downstream effect on adipocytes?

Glucagon increases, insulin decreases → fatty acids are released from adipocytes (increased lipolysis).

In the liver during a low-carb state, why does acetyl-CoA accumulate even as fatty acid oxidation increases?

Oxaloacetate is increasingly diverted to gluconeogenesis, so less is available to combine with acetyl-CoA in the TCA cycle → citrate synthesis decreases → the TCA cycle slows → acetyl-CoA accumulates despite being generated at a high rate from fatty acid oxidation.

Why does fatty acid transport into the mitochondria increase during a low-carb state?

Since ACC/FA synthesis activity decreases, malonyl-CoA levels drop — and since malonyl-CoA normally inhibits CPT1 (carnitine acyltransferase I), less inhibition means more CPT1 activity and more fatty acid transport into the mitochondria for oxidation.

What's the ultimate consequence of high acetyl-CoA accumulation in the liver during prolonged low-carb states, and what's the danger?

Excess acetyl-CoA is converted into ketone bodies, which increase in the blood and are used by the brain — but if they accumulate excessively (plasma and urine), this can lead to ketoacidosis.

In a high-carb state, what happens to insulin and GLUT4, and what's the effect on adipocytes?

Insulin increases → GLUT4 transporters increase glucose uptake → fatty acid release from adipocyte triacylglycerols is inhibited (lipolysis suppressed).

Why does citrate accumulate during a high-carb/fed state, and what does elevated citrate do?

Excess carbohydrate beyond storage capacity is oxidized, increasing ATP and citrate concentrations. Citrate then allosterically activates ACC (which is also dephosphorylated in this state), driving up malonyl-CoA levels and fatty acid biosynthesis.

How does citrate lyase activity change in the fed state, and why does this matter?

Citrate lyase activity increases, increasing the transport of acetyl-CoA equivalents (via citrate) from mitochondria to cytosol — supplying more raw material for fatty acid synthesis.

Where does the extra NADPH for fatty acid synthesis come from during a high-carb state?

Malic enzyme (acting on the oxaloacetate/malate generated from the citrate shuttle) and the pentose phosphate pathway.

Why does fatty acid transport into the mitochondria decrease in the fed state?

Rising malonyl-CoA levels inhibit carnitine acyltransferase I (CPT1), blocking fatty acids from entering the mitochondria — consistent with the cell prioritizing fat synthesis and storage over oxidation when fuel is abundant.