Lipid Metabolism

Lipid Metabolism

Part 1 of 4 — Lipid Structures, Digestion & Lipoprotein Transport

The Cast of Lipids

Unsaturation lowers melting point (kinks pack poorly) and increases membrane fluidity; cholesterol moderates fluidity in both directions.

Digestion & Absorption

1. Bile salts (amphipathic cholesterol derivatives, released from gallbladder on CCK signal) emulsify dietary fat into micelles.
2. Pancreatic lipase hydrolyzes TG to 2-monoacylglycerol + 2 free fatty acids.
3. Enterocytes absorb these, re-esterify them to TG, and package them with apoB-48 into chylomicrons.
4. Chylomicrons enter lymph (lacteals) — bypassing the liver's first pass — then blood. Short/medium-chain fatty acids skip this and go straight to the portal vein.

Lipoprotein Logic (Density Rises as Fat Is Unloaded)

• Lipoprotein lipase (LPL) on capillary endothelium (muscle, adipose) hydrolyzes TG in chylomicrons/VLDL; it REQUIRES apoC-II as activator; insulin upregulates adipose LPL.
• LDL receptor endocytosis (recognizing apoB-100) internalizes LDL; receptor deficiency = familial hypercholesterolemia (early atherosclerosis).
• LCAT (activated by apoA-I) esterifies cholesterol on HDL, packing it into the core for the trip back.

Mobilizing Stored Fat

Fasting/epinephrine → PKA phosphorylates and activates hormone-sensitive lipase in adipocytes → TG hydrolyzed → free fatty acids travel on serum albumin; glycerol goes to the liver (gluconeogenesis). Insulin does the opposite (inhibits HSL).

PKA also phosphorylates perilipins, the proteins coating the fat droplet, which gives HSL access to the stored TG.

Lipid Transport & Digestion 🎯

Key Takeaways — Part 1

• Fat = reduced + anhydrous = ~9 kcal/g; cis double bonds kink chains, lowering melting point and raising membrane fluidity
• Digestion: bile salts emulsify → pancreatic lipase → 2-MAG + FFA → re-esterified → chylomicrons (apoB-48) → LYMPH
• Density ladder: chylomicron → VLDL (B-100) → IDL → LDL (cholesterol out) vs. HDL (A-I, cholesterol back; LCAT esterifies)
• LPL needs apoC-II; LDL receptor reads apoB-100 (familial hypercholesterolemia when absent)
• Fasting: PKA → hormone-sensitive lipase ON → FFA on albumin + glycerol to liver; insulin reverses

• Low-yield extras: PKA also phosphorylates perilipins, the droplet-coating proteins, to give HSL access to stored TG

Worked Examples — Transport Problems

Question: Outline the path of a long-chain fatty acid from a meal's triglyceride to ATP in cardiac muscle, naming each carrier and enzyme checkpoint.

Solution:

1. Bile-salt micelle → pancreatic lipase frees the FA → enterocyte re-esterifies to TG → chylomicron (apoB-48) → lymph → blood.
2. Cardiac capillary LPL (activated by apoC-II acquired from HDL) hydrolyzes the TG; the FA enters the cardiomyocyte.
3. Activation to fatty acyl-CoA, carnitine shuttle into mitochondria, beta-oxidation → acetyl-CoA → TCA → ETC → ATP.
4. Note the FA never traveled "free" in plasma from the gut — it moved inside a lipoprotein; only adipose-mobilized FAs ride albumin.

MCAT Strategy: Distinguish the two FFA transport modes: lipoprotein TG (fed state, LPL unloads) vs. albumin-bound FFA (fasting, HSL releases).

Question: A bacterium shifted from 37 to 20 degrees C remodels its membrane lipids. Predict the direction of change in fatty acid saturation and chain length, and explain.

Solution:

1. Cold rigidifies membranes; the cell must INCREASE fluidity to compensate (homeoviscous adaptation).
2. It incorporates MORE unsaturated (cis-kinked) fatty acids and SHORTER chains — both pack less tightly, lowering the transition temperature.
3. The reverse occurs on warming. In animal membranes, cholesterol buffers fluidity: it rigidifies fluid membranes and fluidizes rigid ones.

MCAT Strategy: Fluidity levers: unsaturation UP = fluid; chain length UP = rigid; cholesterol = buffer in both directions.

Question: A fasting patient's plasma is creamy. Ultracentrifugation shows a thick floating layer; the particles contain apoB-48. A heparin-releasable plasma enzyme assay shows normal lipase protein but no activity, which is restored by adding normal HDL. What is deficient?

Solution:

1. apoB-48 + floating layer after an overnight fast = persistent CHYLOMICRONS (they should clear within hours).
2. LPL protein is present but inactive in the patient's plasma — yet normal HDL restores activity. HDL's relevant donation is apoC-II.
3. Conclusion: apoC-II deficiency; chylomicrons (and VLDL) cannot be unloaded. Adding purified apoC-II would be the confirming experiment.

MCAT Strategy: "Restored by adding X" experiments identify the missing cofactor, not the enzyme; map each apoprotein to its function (B-48 assembly, C-II LPL activation, E remnant uptake, B-100 LDL receptor, A-I LCAT).

Lipid Metabolism

Part 2 of 4 — The Carnitine Shuttle & Beta-Oxidation ATP Arithmetic

Step 0: Activation (Pay the Toll at the Door)

In the cytosol, fatty acyl-CoA synthetase attaches CoA to the fatty acid:

$FA + CoA + ATP \rightarrow fatty\ acyl\text{-}CoA + AMP + PP_i$

ATP goes to AMP and the pyrophosphate is immediately hydrolyzed — so activation costs 2 ATP equivalents, paid once per fatty acid no matter how long the chain.

The Carnitine Shuttle (the Regulated Gate)

Long-chain acyl-CoA cannot cross the inner mitochondrial membrane. The shuttle:

1. CPT-I (outer membrane) swaps CoA for carnitine → acyl-carnitine
2. A translocase carries acyl-carnitine into the matrix
3. CPT-II (inner membrane) regenerates acyl-CoA inside

CPT-I is inhibited by malonyl-CoA — the first committed intermediate of fatty acid SYNTHESIS. This one interaction prevents a futile cycle: a fed cell building fat cannot simultaneously burn it. Carnitine deficiency (or CPT deficiency) presents as muscle weakness and hypoketotic hypoglycemia during fasting.

One Beta-Oxidation Cycle = Oxidize, Hydrate, Oxidize, Cleave

Each round shortens the acyl-CoA by 2 carbons and yields 1 FADH$_2$, 1 NADH, 1 acetyl-CoA:

Memory hook: the chemistry mirrors the TCA sequence succinate → fumarate → malate → oxaloacetate (FAD oxidation, hydration, NAD$^+$ oxidation).

The Exam-Day Arithmetic: Palmitate (16 C)

A C$_{16}$ chain needs 7 cycles (n/2 − 1) producing 8 acetyl-CoA, 7 FADH$_2$, 7 NADH:

• 8 acetyl-CoA × 10 (each spins the TCA cycle: 3 NADH, 1 FADH$_2$, 1 GTP) = 80
• 7 NADH × 2.5 = 17.5
• 7 FADH$_2$ × 1.5 = 10.5
• Subtract 2 ATP equivalents for activation

$80 + 17.5 + 10.5 - 2 = 106\ ATP$

Special Chains

• Odd-chain fatty acids: the final thiolysis leaves propionyl-CoA (3 C) → carboxylated (biotin) to methylmalonyl-CoA → methylmalonyl-CoA mutase (vitamin B$_{12}$) → succinyl-CoA, which enters the TCA cycle. This is the ONLY fatty acid fragment that can feed net gluconeogenesis.
• Unsaturated fatty acids: existing cis double bonds require an isomerase (and, for polyunsaturates, a reductase) and skip the FADH$_2$-generating step at that position — slightly LESS ATP than a saturated chain of equal length.
• Very-long chains (> 20 C): shortened first in peroxisomes, where the initial oxidation hands electrons to O$_2$, making H$_2$O$_2$ (no ATP from that first step).

Beta-Oxidation & Energetics 🎯

Key Takeaways — Part 2

• Activation costs 2 ATP equivalents (ATP → AMP + PP$_i$), once per fatty acid
• Carnitine shuttle (CPT-I → translocase → CPT-II) imports long chains; malonyl-CoA inhibits CPT-I (no futile cycle)
• Each cycle: FADH$_2$ + NADH + acetyl-CoA; chain shrinks by 2 C
• Palmitate: 7 cycles → 8 acetyl-CoA, 7 FADH$_2$, 7 NADH → 80 + 10.5 + 17.5 − 2 = 106 ATP
• Odd chains → propionyl-CoA → (biotin, then B12) → succinyl-CoA → the one glucogenic fat fragment
• Unsaturated chains: isomerase/reductase needed, slightly less ATP; very-long chains start in peroxisomes

• Low-yield extras: in acyl-CoA dehydrogenase defects such as MCAD, backed-up fatty acids spill into omega-oxidation, so urinary dicarboxylic acids rise

Worked Examples — Oxidation Arithmetic

Question: Calculate the net ATP from complete oxidation of capric acid (C10:0), using 2.5/NADH and 1.5/FADH$_2$.

Solution:

1. Cycles = 10/2 − 1 = 4; acetyl-CoA = 10/2 = 5.
2. 5 acetyl-CoA × 10 = 50; 4 NADH × 2.5 = 10; 4 FADH$_2$ × 1.5 = 6.
3. Subtotal 66; subtract 2 for activation → 64 ATP.

MCAT Strategy: Two formulas cover every saturated even chain: cycles = n/2 − 1, acetyl-CoA = n/2. Never forget the −2 activation cost — wrong answers on the exam are built from exactly that omission.

Question: An infant has seizures after long gaps between feedings. Labs during an episode: low glucose, LOW ketones, normal carnitine. Where is the lesion, and why does glucose fall so far?

Solution:

1. Fasting + hypoketotic hypoglycemia = the fat-burning pathway is broken (normal fasting produces ABUNDANT ketones).
2. Normal carnitine points past the shuttle: an acyl-CoA dehydrogenase defect (classically MCAD).
3. Glucose crashes because muscle and liver, unable to burn fat, keep consuming glucose, and gluconeogenesis stalls without beta-oxidation-derived acetyl-CoA to activate pyruvate carboxylase and supply ATP.

MCAT Strategy: Tie the observations to dependencies: ketogenesis and gluconeogenesis BOTH lean on beta-oxidation during fasting.

• Example 2 aside: a supporting lab clue in MCAD deficiency is elevated urinary dicarboxylic acids, because the backed-up fatty acids are diverted to backup omega-oxidation in the ER.

Question: Oleate (18:1, cis-9) and stearate (18:0) are both fully oxidized. Which yields more ATP, and by roughly how much?

Solution:

1. Both are 18 C: 8 cycles, 9 acetyl-CoA.
2. Oleate's pre-existing double bond means the acyl-CoA dehydrogenase step is SKIPPED for that round (an isomerase repositions the bond instead) — one fewer FADH$_2$.
3. Stearate wins by ~1.5 ATP (stearate ≈ 120, oleate ≈ 118.5).

MCAT Strategy: Existing unsaturation = electrons already removed = less energy left to harvest. Directionally, unsaturated chains always yield slightly less than the saturated chain of equal length.

Lipid Metabolism

Part 3 of 4 — Ketogenesis & Ketone Body Metabolism

Why Ketones Exist

In prolonged fasting, hepatic beta-oxidation floods the mitochondria with acetyl-CoA while oxaloacetate is being siphoned off for gluconeogenesis. Acetyl-CoA that cannot enter the TCA cycle is repackaged into small, water-soluble, exportable fuels: ketone bodies. They need no albumin, no lipoprotein, and no carnitine shuttle — heart, muscle, and (crucially) the fasting brain burn them directly. After several days of starvation the brain gets most of its energy from ketones, sparing muscle protein that would otherwise be sacrificed for gluconeogenesis.

The Pathway (Liver Mitochondria Only)

1. 2 acetyl-CoA → acetoacetyl-CoA (thiolase, running in reverse)
2. Acetoacetyl-CoA + acetyl-CoA → HMG-CoA (HMG-CoA synthase, the mitochondrial isoform — the committed, rate-limiting step)
3. HMG-CoA → acetoacetate + acetyl-CoA (HMG-CoA lyase)
4. Acetoacetate + NADH → beta-hydroxybutyrate (ratio reflects the mitochondrial NADH/NAD$^+$ ratio) or spontaneously decarboxylates to acetone (exhaled — fruity breath)

Note the location contrast: mitochondrial HMG-CoA is for ketones; cytosolic HMG-CoA (via the cytosolic synthase and HMG-CoA reductase) is for cholesterol. Same intermediate name, different compartment, different fate.

Using Ketones (Everywhere EXCEPT the Liver)

Peripheral tissues reverse the process: beta-hydroxybutyrate → acetoacetate (regenerating NADH) → activated by succinyl-CoA:acetoacetate-CoA transferase (thiophorase/SCOT) → acetoacetyl-CoA → 2 acetyl-CoA → TCA.

The liver lacks thiophorase. That single missing enzyme is why the producer cannot consume its own product — ketones are a strictly altruistic export. Red blood cells also cannot use ketones (no mitochondria at all).

When Ketogenesis Goes Too Far

• Diabetic ketoacidosis (DKA): with no insulin, hormone-sensitive lipase and ketogenesis run unopposed; ketoacids (pKa ~4) dissociate and overwhelm bicarbonate buffering → anion-gap metabolic acidosis, Kussmaul breathing, fruity acetone breath. Typical of type 1 diabetes.
• Prolonged fasting/starvation: ketosis is milder because some insulin remains to restrain lipolysis.
• Alcoholic ketoacidosis: ethanol metabolism raises NADH/NAD$^+$, pushing the ketone pool toward beta-hydroxybutyrate.

• The standard nitroprusside dipstick detects acetoacetate (weakly acetone), not beta-hydroxybutyrate, so it can UNDERESTIMATE ketosis when NADH is high.

Ketone Body Metabolism 🎯

Key Takeaways — Part 3

• Ketogenesis = liver mitochondria repackaging excess acetyl-CoA when OAA is drained by gluconeogenesis
• Pathway: 2 acetyl-CoA → acetoacetyl-CoA → HMG-CoA (mito HMG-CoA synthase, rate-limiting) → acetoacetate → beta-hydroxybutyrate/acetone
• Liver lacks thiophorase → cannot burn its own ketones; RBCs cannot use them either (no mitochondria)
• Fasting brain runs largely on ketones after ~3 days, sparing muscle protein
• DKA: unopposed lipolysis + ketogenesis → anion-gap acidosis; high NADH (alcohol) shifts pool to beta-hydroxybutyrate
• Compartment trap: mitochondrial HMG-CoA → ketones; cytosolic HMG-CoA → cholesterol

• Low-yield extras: nitroprusside dipsticks detect acetoacetate, not beta-hydroxybutyrate, so they underestimate high-NADH (alcoholic) ketosis

Worked Examples — Fasting Physiology

Question: Rank the body's dominant fuel sources for the brain at 4 hours, 2 days, and 2 weeks without food.

Solution:

1. 4 hours: dietary glucose is gone; glycogenolysis maintains blood glucose — brain burns glucose.
2. 2 days: glycogen is exhausted (~24 h); gluconeogenesis (from amino acids, glycerol, lactate) feeds the brain glucose; ketogenesis is ramping.
3. 2 weeks: ketone bodies supply roughly two-thirds of brain energy; gluconeogenesis slows, dramatically reducing muscle protein breakdown.

MCAT Strategy: The purpose of the ketone switch is protein sparing. A question asking why urinary nitrogen excretion FALLS in late starvation is really asking about brain ketone adaptation.

Question: A type 1 diabetic presents with glucose 480 mg/dL, pH 7.12, low bicarbonate, and an elevated anion gap. Explain each abnormality from hormone signaling downward.

Solution:

1. No insulin + high glucagon/catecholamines → HSL fully active → massive FFA flux to the liver.
2. Beta-oxidation outstrips the TCA cycle; ketogenesis pours out acetoacetate and beta-hydroxybutyrate.
3. These ketoacids dissociate at blood pH (pKa ~4), consuming HCO$_3^-$: anion-gap metabolic acidosis; hyperventilation (Kussmaul) is respiratory compensation.
4. Glucose is high because tissues cannot take it up (GLUT4 needs insulin) while the liver simultaneously overproduces it.

MCAT Strategy: DKA is the single most integrated lipid-metabolism vignette: hormone signaling, lipolysis, ketogenesis, and acid-base in one patient.

Question: Give two biochemical reasons the liver converts acetyl-CoA into ketone bodies rather than exporting acetyl-CoA itself.

Solution:

1. No transporter: CoA thioesters are large, charged, and membrane-impermeant; there is no plasma-membrane carrier for acetyl-CoA, and cells guard their CoA pool.
2. Solubility and delivery: acetoacetate and beta-hydroxybutyrate are small organic acids, freely soluble in plasma, crossing the blood-brain barrier on monocarboxylate transporters — no albumin or lipoprotein required.
3. Bonus: converting 2 acetyl-CoA into one 4-carbon ketone also frees CoA for continued beta-oxidation in the hepatocyte.

MCAT Strategy: When asked why a shuttle or conversion exists, check membrane permeability first — the answer is usually a transport problem.

Lipid Metabolism

Part 4 of 4 — Fatty Acid Synthesis, Cholesterol & MCAT Integration

Synthesis Is Not Oxidation Run Backward

The Fed-State Assembly Line

1. Citrate shuttle: mitochondrial acetyl-CoA cannot cross the membrane, so it condenses with OAA to citrate, exits, and is re-cleaved by cytosolic ATP-citrate lyase. (High citrate ALSO allosterically signals abundance — it activates the next enzyme and inhibits PFK-1.)
2. Acetyl-CoA carboxylase (ACC) — biotin, uses CO$_2$ and ATP — makes malonyl-CoA. This is the rate-limiting step: activated by citrate and insulin (dephosphorylation), inhibited by phosphorylation (by AMPK, and in the glucagon/epinephrine-signaled fasting state) and by palmitoyl-CoA (product feedback).
3. Fatty acid synthase (FAS) repeats condensation-reduction-dehydration-reduction, consuming NADPH (from the pentose phosphate pathway and malic enzyme), releasing one CO$_2$ per malonyl-CoA added, until palmitate (16:0) is released. Stoichiometry: 8 acetyl-CoA, 7 ATP, 14 NADPH.

Elongation (ER/mitochondria) and desaturation (ER; humans cannot desaturate beyond C9 — hence essential linoleic and alpha-linolenic acids) customize the product. Recall the shuttle payoff from Part 2: malonyl-CoA inhibits CPT-I, so synthesis automatically switches off oxidation.

Cholesterol Synthesis in One Breath

Cytosolic acetyl-CoA → acetoacetyl-CoA → HMG-CoA → (HMG-CoA reductase + 2 NADPH) → mevalonate → ... → squalene → cholesterol.

HMG-CoA reductase (smooth ER) is the rate-limiting enzyme: inhibited competitively by statins, repressed transcriptionally by cholesterol itself, phosphorylated OFF by AMPK, activated by insulin. Cholesterol's fates: membranes, bile acids (the major disposal route), steroid hormones, vitamin D. When LDL delivers cholesterol to a cell, the cell downregulates both HMG-CoA reductase AND LDL-receptor synthesis — the feedback loop statin therapy exploits (less internal synthesis → more LDL receptors → lower plasma LDL).

The cholesterol sensor is the ER transcription factor SREBP: when ER cholesterol falls, SREBP is released to the nucleus and turns on both the HMG-CoA reductase and LDL-receptor genes.

The Master Integration Table

Synthesis, Cholesterol & Integration 🎯

Key Takeaways — Part 4

• Synthesis vs oxidation: cytosol vs matrix, NADPH vs FAD/NAD$^+$, ACP vs CoA, malonyl-CoA in vs acetyl-CoA out
• Citrate shuttle exports acetyl-CoA; ACC (biotin, rate-limiting) makes malonyl-CoA — insulin/citrate ON, glucagon/AMPK/palmitoyl-CoA OFF
• Palmitate stoichiometry: 8 acetyl-CoA + 7 ATP + 14 NADPH; humans cannot desaturate past C9 → linoleate/linolenate essential
• Cholesterol: HMG-CoA reductase rate-limiting; statins inhibit it → SREBP senses low cholesterol → more LDL receptors → lower plasma LDL
• AMPK phosphorylates ACC and HMG-CoA reductase OFF; malonyl-CoA's fall reopens CPT-I
• Fed = store and build (VLDL out); fasting = HSL, beta-oxidation, ketones

• Low-yield extras: bempedoic acid inhibits ATP-citrate lyase and lowers LDL by statin-like logic

Worked Examples — Integration Problems

Question: A carbon atom enters an adipocyte-bound pathway as glucose during a large carbohydrate meal. Trace it into stored triacylglycerol.

Solution:

1. Glycolysis (liver): glucose → pyruvate; pyruvate dehydrogenase (mitochondrion) → acetyl-CoA.
2. Citrate shuttle → cytosolic acetyl-CoA → ACC → malonyl-CoA → FAS builds palmitate (NADPH from the pentose phosphate pathway).
3. Palmitate esterified to glycerol-3-phosphate → TG, packaged with apoB-100 into VLDL.
4. Adipose capillary LPL (insulin-stimulated, apoC-II-activated) hydrolyzes VLDL TG; the fatty acid enters the adipocyte and is re-esterified for storage.

MCAT Strategy: This one question chains five sub-topics; practice writing the arrow diagram from memory — passages love asking which step a given drug or deficiency would break.

Question: A drug inhibits ATP-citrate lyase in hepatocytes. Predict effects on (a) fatty acid synthesis, (b) cytosolic acetyl-CoA, (c) plasma LDL.

Solution:

1. (a) Cytosolic acetyl-CoA supply falls → less malonyl-CoA → fatty acid synthesis drops.
2. (b) Cytosolic acetyl-CoA falls — and with it, substrate for cholesterol synthesis too.
3. (c) Reduced cholesterol synthesis triggers SREBP-driven LDL-receptor upregulation → plasma LDL falls (the same logic as a statin, one step further upstream).

MCAT Strategy: Both fatty acids AND cholesterol begin from cytosolic acetyl-CoA; any block upstream of it hits both pathways at once.

• Example 2 aside: a real drug works this way. Bempedoic acid inhibits ATP-citrate lyase and lowers plasma LDL.

Question: Compare the dominant direction of lipid traffic in adipose tissue at 1 hour and at 18 hours after a mixed meal, naming the controlling enzymes.

Solution:

1. 1 hour (fed, insulin high): adipose LPL (insulin-induced) unloads chylomicron/VLDL TG into the adipocyte; HSL is dephosphorylated and off; net flux IN.
2. 18 hours (fasted, glucagon/catecholamines up): HSL phosphorylated and on; FFA leave on albumin, glycerol to the liver; hepatic malonyl-CoA is low, so CPT-I admits the FFA the liver receives for beta-oxidation and ketogenesis; net flux OUT.
3. The SAME tissue reverses its lipolysis switch through HSL's phosphorylation state, which flips within minutes without any new HSL being synthesized.

MCAT Strategy: Insulin = dephosphorylation = storage; glucagon/epinephrine = phosphorylation via PKA = mobilization. That single rule answers most fed/fasted regulation questions.