Lipid Metabolism

Fatty-Acid Beta-Oxidation

β-oxidation degrades fatty acids to acetyl-CoA in the mitochondrial matrix. Step 1: Activation: fatty acid + CoA + ATP → fatty acyl-CoA + AMP + PPi (by fatty acyl-CoA synthetase on outer mitochondrial membrane; consumes 2 ATP equivalents). Step 2: Transport (carnitine shuttle): long-chain fatty acyl-CoA cannot cross the inner mitochondrial membrane directly. Carnitine palmitoyltransferase I (CPT-I) on the outer membrane transfers the acyl group to carnitine, forming acyl-carnitine. A translocase moves acyl-carnitine across the inner membrane. CPT-II on the inner membrane regenerates fatty acyl-CoA inside the matrix. This is the RATE-LIMITING step of β-oxidation, inhibited by malonyl-CoA. Step 3: β-oxidation spiral: four repeating reactions per cycle, each removing a 2-carbon acetyl-CoA: (a) Oxidation by FAD-dependent acyl-CoA dehydrogenase → trans-Δ2-enoyl-CoA + FADH2, (b) Hydration by enoyl-CoA hydratase → L-β-hydroxyacyl-CoA, (c) Oxidation by NAD+-dependent β-hydroxyacyl-CoA dehydrogenase → β-ketoacyl-CoA + NADH, (d) Thiolysis by thiolase (acyl-CoA acetyltransferase) → acetyl-CoA + fatty acyl-CoA shortened by 2 carbons. Each cycle yields 1 FADH2, 1 NADH, and 1 acetyl-CoA. For palmitate (C16:0): 7 cycles → 8 acetyl-CoA + 7 FADH2 + 7 NADH. Total ATP yield: ~106–108 ATP (8 acetyl-CoA × 10 ATP each in TCA = 80, 7 FADH2 × 1.5 = 10.5, 7 NADH × 2.5 = 17.5, minus 2 ATP for activation = 106). Unsaturated fatty acids require additional isomerase and reductase enzymes. Odd-chain fatty acids produce propionyl-CoA (3C) in the final cycle, which is converted to succinyl-CoA (TCA intermediate) via methylmalonyl-CoA (requires vitamin B12).

Fatty-Acid Synthesis

Fatty acid synthesis (lipogenesis) occurs primarily in the liver and adipose tissue cytoplasm, building palmitate (C16:0) from acetyl-CoA. Step 1: Acetyl-CoA transport from mitochondria: citrate shuttle. Citrate (carrying acetyl-CoA) exits mitochondria, and ATP-citrate lyase cleaves it to acetyl-CoA + oxaloacetate in the cytoplasm. Step 2: Acetyl-CoA carboxylase (ACC) catalyzes the committed step: acetyl-CoA + HCO3⁻ + ATP → malonyl-CoA + ADP + Pi. ACC requires biotin (vitamin B7) as a cofactor. ACC is the key regulatory enzyme: activated by citrate and insulin (dephosphorylation), inhibited by palmitoyl-CoA, glucagon, and epinephrine (phosphorylation via AMPK). Step 3: Fatty acid synthase (FAS) is a multifunctional homodimer that catalyzes the iterative addition of 2-carbon units from malonyl-CoA to a growing acyl chain attached to the acyl carrier protein (ACP) domain. Each cycle: condensation, reduction, dehydration, reduction. The growing chain is released as palmitate (C16:0) by thioesterase. The overall reaction: acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14 H+ → palmitate + 8 CoA + 7 CO2 + 14 NADP+ + 6 H2O. NADPH is primarily supplied by the pentose phosphate pathway and malic enzyme. Further elongation and desaturation occur in the ER (elongases, desaturases). Key contrast: β-oxidation occurs in mitochondria, uses NAD+ and FAD, produces acetyl-CoA. Synthesis occurs in cytoplasm, uses NADPH, consumes malonyl-CoA. Reciprocal regulation via malonyl-CoA inhibiting CPT-I prevents futile cycling.

Ketone Bodies

Ketone bodies (acetoacetate, β-hydroxybutyrate, acetone) are produced in liver mitochondria during prolonged fasting, starvation, or untreated diabetes when excess acetyl-CoA from β-oxidation exceeds TCA cycle capacity. The TCA cycle slows because oxaloacetate is depleted—it is diverted to gluconeogenesis to maintain blood glucose for the brain. The accumulated acetyl-CoA is converted to ketone bodies via HMG-CoA synthase (mitochondrial isozyme) and HMG-CoA lyase. Ketone bodies are exported from the liver and reconverted to acetyl-CoA in extrahepatic tissues (brain, heart, skeletal muscle) that possess the enzyme β-ketoacyl-CoA transferase (thiophorase). The liver LACKS thiophorase and thus cannot use ketone bodies—this ensures unidirectional flow from liver (producer) to periphery (consumer). During prolonged fasting, the brain adapts to use ketone bodies for ~60–70% of its energy, sparing glucose for erythrocytes (which lack mitochondria). Ketogenesis is regulated by the glucagon/insulin ratio: high glucagon (fasting) activates lipolysis in adipose tissue, providing free fatty acids to the liver, and promotes the ketogenic pathway. Pathological ketosis: in untreated type 1 diabetes, insulin deficiency causes uncontrolled lipolysis and ketogenesis, leading to diabetic ketoacidosis (DKA)—ketone bodies (acetoacetate and β-hydroxybutyrate are acids) lower blood pH, causing metabolic acidosis. The fruity odor of acetone on the breath is a clinical sign.

Cholesterol Synthesis

Cholesterol synthesis occurs primarily in the liver cytoplasm, using acetyl-CoA as the carbon source. The pathway: 3 acetyl-CoA → HMG-CoA (via thiolase and HMG-CoA synthase, cytoplasmic isozyme). HMG-CoA reductase catalyzes the committed, rate-limiting step: HMG-CoA + 2 NADPH → mevalonate + CoA + 2 NADP+. This enzyme is the target of statin drugs (competitive inhibitors resembling mevalonate). Mevalonate undergoes phosphorylation and decarboxylation to isopentenyl pyrophosphate (IPP, 5C isoprene unit). IPP isomerizes to dimethylallyl pyrophosphate. Condensation of isoprene units builds geranyl pyrophosphate (10C), farnesyl pyrophosphate (15C), and then squalene (30C, via squalene synthase). Squalene cyclizes to lanosterol (the first sterol), which is converted through ~19 additional steps to cholesterol (27C). Regulation: HMG-CoA reductase is regulated at multiple levels: (1) Transcriptional control by SREBP (sterol regulatory element-binding protein): when cholesterol is low, SREBP is cleaved and translocates to the nucleus, activating transcription of HMG-CoA reductase and LDL receptor genes. (2) Proteolytic degradation: high cholesterol accelerates HMG-CoA reductase degradation. (3) Phosphorylation by AMPK (inhibits, during low energy) and dephosphorylation by phosphatase (activates). Statin drugs competitively inhibit HMG-CoA reductase, lowering endogenous cholesterol synthesis and upregulating LDL receptors (increasing LDL clearance from blood).

Lipoprotein Transport

Lipids are transported in plasma as lipoproteins because they are insoluble in water. Exogenous pathway (dietary lipids): intestinal enterocytes package dietary triacylglycerols and cholesterol into chylomicrons (containing apo B-48). Chylomicrons enter lymphatics → blood. Lipoprotein lipase (LPL) on capillary endothelium (activated by apo C-II on chylomicrons) hydrolyzes triacylglycerols to free fatty acids and glycerol. Chylomicron remnants (depleted of TAG, enriched in cholesterol) are taken up by the liver via apo E-mediated endocytosis. Endogenous pathway: the liver packages endogenous triacylglycerols and cholesterol into VLDL (containing apo B-100). VLDL undergoes LPL-mediated TAG hydrolysis in peripheral tissues, progressively becoming IDL and then LDL (cholesterol-rich). LDL delivers cholesterol to peripheral tissues via LDL receptor-mediated endocytosis. High LDL is atherogenic: LDL particles can become oxidized and taken up by macrophages in the arterial wall via scavenger receptors, forming foam cells—the earliest lesion of atherosclerosis. Reverse cholesterol transport: HDL (containing apo A-I) picks up excess cholesterol from peripheral tissues (via ABCA1 transporter) and returns it to the liver (via SR-B1 receptor) for excretion in bile. HDL also has anti-inflammatory and antioxidant properties. CETP (cholesteryl ester transfer protein) exchanges cholesteryl esters from HDL for triacylglycerols from VLDL/LDL—an important intersection of the pathways. Familial hypercholesterolemia: defective or absent LDL receptors → severely elevated LDL cholesterol → premature atherosclerosis.

Hormonal Regulation of Lipid Metabolism

Lipid metabolism is tightly regulated by the insulin/glucagon ratio and catecholamines. Fed state (high insulin): insulin activates lipoprotein lipase (LPL) in adipose tissue, promoting fatty acid uptake and storage. Insulin activates acetyl-CoA carboxylase (ACC) via dephosphorylation, stimulating malonyl-CoA production and fatty acid synthesis. Malonyl-CoA inhibits CPT-I, blocking β-oxidation. Insulin also activates HMG-CoA reductase (cholesterol synthesis) and LDL receptor expression. Fasted state (high glucagon, low insulin): glucagon and epinephrine activate hormone-sensitive lipase (HSL) in adipose tissue via cAMP/PKA cascade → lipolysis releases free fatty acids and glycerol. In the liver: glucagon phosphorylates and INACTIVATES ACC (via AMPK), reducing malonyl-CoA levels, which relieves inhibition of CPT-I, allowing β-oxidation to proceed. The resulting acetyl-CoA is used for ketogenesis (if oxaloacetate is depleted) or TCA cycle. Glucagon also inhibits HMG-CoA reductase via phosphorylation. Exercise/stress (epinephrine): epinephrine activates HSL in adipose tissue (same cAMP/PKA cascade as glucagon) and muscle lipoprotein lipase, mobilizing fatty acids for muscle β-oxidation. AMPK (AMP-activated protein kinase) is the cellular energy sensor: when AMP/ATP ratio is high (low energy), AMPK phosphorylates and inactivates ACC (reducing malonyl-CoA, promoting β-oxidation) and HMG-CoA reductase (reducing cholesterol synthesis). PPAR-α (peroxisome proliferator-activated receptor alpha) is a nuclear receptor activated by fatty acids that upregulates genes for β-oxidation enzymes. Fibrate drugs (PPAR-α agonists) lower triacylglycerols by increasing β-oxidation.

How it works

Lipid metabolism follows two opposing tracks that never run simultaneously: in the fed state, insulin drives fatty acid and cholesterol synthesis in the liver and storage in adipose; in the fasted state, glucagon and epinephrine drive lipolysis and β-oxidation. The molecular switch is malonyl-CoA: when it is high (fed state, ACC active), it inhibits CPT-I and blocks β-oxidation, channeling fatty acids toward storage. When malonyl-CoA is low (fasted state, ACC phosphorylated/inactive), CPT-I is active, importing fatty acids into mitochondria for β-oxidation. Ketone body production is an overflow pathway: when β-oxidation produces more acetyl-CoA than the TCA cycle can process (due to oxaloacetate depletion for gluconeogenesis), the liver diverts acetyl-CoA to ketone bodies for export. Cholesterol metabolism is regulated primarily at HMG-CoA reductase, with statins exploiting this to lower LDL cholesterol. The MCAT tests the logic of these reciprocal regulatory mechanisms: which enzyme is phosphorylated in the fasted state, which pathway is active, and what happens when one pathway is blocked.

How it works

Lipid metabolism follows two opposing tracks that never run simultaneously: in the fed state, insulin drives fatty acid and cholesterol synthesis in the liver and storage in adipose; in the fasted state, glucagon and epinephrine drive lipolysis and β-oxidation. The molecular switch is malonyl-CoA: when it is high (fed state, ACC active), it inhibits CPT-I and blocks β-oxidation, channeling fatty acids toward storage. When malonyl-CoA is low (fasted state, ACC phosphorylated/inactive), CPT-I is active, importing fatty acids into mitochondria for β-oxidation. Ketone body production is an overflow pathway: when β-oxidation produces more acetyl-CoA than the TCA cycle can process (due to oxaloacetate depletion for gluconeogenesis), the liver diverts acetyl-CoA to ketone bodies for export. Cholesterol metabolism is regulated primarily at HMG-CoA reductase, with statins exploiting this to lower LDL cholesterol. The MCAT tests the logic of these reciprocal regulatory mechanisms: which enzyme is phosphorylated in the fasted state, which pathway is active, and what happens when one pathway is blocked.

Comparisons

• B/B (β-oxidation): Mitochondrial matrix, carnitine shuttle (CPT-I rate-limiting, inhibited by malonyl-CoA). 7 cycles for palmitate (C16).
• B/B (Fatty acid synthesis): Cytoplasm, ACC (committed step, biotin cofactor). FAS multifunctional enzyme. Uses NADPH.
• B/B (Ketone bodies): Liver mitochondria produce; liver lacks thiophorase so cannot use them. Brain adapts during fasting. DKA in type 1 diabetes.
• B/B (Cholesterol): HMG-CoA reductase = rate-limiting, target of statins. SREBP-SCAP regulates transcription. LDL receptor mediates uptake.
• B/B (Lipoproteins): Chylomicrons (dietary, apo B-48), VLDL (endogenous, apo B-100), LDL (cholesterol delivery), HDL (reverse transport, apo A-I).
• C/P (ATP yield): Palmitate ~106 ATP vs. glucose ~30-32 ATP. Fatty acids are more reduced = more energy per carbon.

Common confusions

• Confusing the location of β-oxidation and fatty acid synthesis. β-oxidation: mitochondrial matrix. Synthesis: cytoplasm. This compartmentalization prevents futile cycling.
• Forgetting that the liver CANNOT use ketone bodies. The liver produces ketone bodies for export but lacks thiophorase. The brain and muscle use them. This is a classic MCAT fact.
• Mixing up HMG-CoA reductase (cholesterol synthesis, cytoplasmic, statin target) and HMG-CoA synthase (ketogenesis, mitochondrial, not statin target). Same intermediate, different pathways, different compartments.
• Thinking malonyl-CoA is ONLY for fatty acid synthesis. Malonyl-CoA also regulates β-oxidation by inhibiting CPT-I. This dual role is frequently tested.
• Forgetting the ATP cost of fatty acid activation. Activation to fatty acyl-CoA costs 2 ATP equivalents (ATP → AMP + PPi). This must be subtracted from the total ATP yield.
• Confusing the essential fatty acids with essential amino acids. Linoleic (ω-6) and α-linolenic (ω-3) are essential fatty acids. Know these vs. the 9 essential amino acids.

Quick review

• β-oxidation: mitochondrial matrix. Each cycle: oxidation (FAD → FADH2), hydration, oxidation (NAD+ → NADH), thiolysis → acetyl-CoA + shortened fatty acyl-CoA.
• Carnitine shuttle: CPT-I (outer membrane, rate-limiting, inhibited by malonyl-CoA) → translocase → CPT-II (inner membrane).
• Palmitate (C16:0): 7 cycles → 8 acetyl-CoA + 7 FADH2 + 7 NADH. Net ~106 ATP.
• Fatty acid synthesis: cytoplasm. ACC (acetyl-CoA → malonyl-CoA, committed step, biotin). FAS builds palmitate from acetyl-CoA + malonyl-CoA + NADPH.
• Malonyl-CoA: substrate for synthesis AND inhibitor of CPT-I (reciprocal regulation—prevents simultaneous synthesis and oxidation).
• Ketone bodies: acetoacetate, β-hydroxybutyrate, acetone. Made in liver mitochondria when acetyl-CoA exceeds TCA capacity. Liver LACKS thiophorase.
• Cholesterol synthesis: rate-limiting enzyme = HMG-CoA reductase (cytoplasmic). Target of statins. Regulated by SREBP, phosphorylation (AMPK), degradation.
• Lipoproteins: Chylomicrons (dietary, apo B-48) → VLDL (endogenous, apo B-100) → IDL → LDL (cholesterol to tissues). HDL (reverse transport, apo A-I).
• Fed state: insulin → activates ACC (fatty acid synthesis), LPL (fat storage), HMG-CoA reductase (cholesterol synthesis).
• Fasted state: glucagon/epinephrine → activates HSL (lipolysis), inactivates ACC (via PKA/AMPK), relieves CPT-I inhibition → β-oxidation.
• DKA: uncontrolled lipolysis and ketogenesis in type 1 diabetes due to insulin deficiency. Metabolic acidosis from acetoacetate and β-hydroxybutyrate.