Fatty Acid Synthesis and Beta-Oxidation

Fatty Acid Synthesis (Lipogenesis)

• Location: Occurs in the cytoplasm of hepatocytes and adipocytes; requires NADPH from the pentose phosphate pathway and malic enzyme
• Substrate: Acetyl-CoA is the building block; pyruvate carboxylase converts pyruvate to oxaloacetate, which is converted to acetyl-CoA in mitochondria, then transported via the citrate-malate shuttle into cytoplasm
• Key enzyme: Acetyl-CoA carboxylase (ACC) catalyzes the first committed step: acetyl-CoA + CO₂ + ATP → malonyl-CoA + ADP + Pi (rate-limiting step)
• Malonyl-CoA function: Substrate for fatty acid synthase (FAS); also acts as a CPT-I inhibitor, preventing fatty acid import into mitochondria during fed state
• Fatty acid synthase (FAS): A large multifunctional enzyme complex that sequentially adds 2-carbon units from malonyl-CoA to growing fatty acid chain; produces palmitate (16:0) as the primary product; requires 2 NADPH per 2-carbon addition
• Regulation: Activated by insulin (phosphorylates and inactivates ACC inhibitor AMP-kinase); inhibited by glucagon, epinephrine, and AMP; allosterically activated by citrate (signals abundant acetyl-CoA); allosterically inhibited by long-chain fatty acyl-CoAs (negative feedback)
• Post-synthesis modifications: Palmitate is elongated to stearate (18:0) and desaturated to oleate (18:1) by microsomal enzymes; esterified to glycerol-3-phosphate to form triglycerides

Beta-Oxidation (Fatty Acid Catabolism)

• Location: Primarily mitochondrial matrix (most fatty acids); also peroxisomes for very long-chain fatty acids (>20 carbons) and branched-chain fatty acids
• Activation step: Occurs in cytoplasm; fatty acyl-CoA synthetase converts free fatty acid to fatty acyl-CoA (requires 2 ATP equivalents)
• Transport into mitochondria: Carnitine palmitoyltransferase I (CPT-I) in outer mitochondrial membrane transfers acyl group to carnitine; carnitine-acyl translocase in inner membrane exchanges carnitine for CoA; CPT-II on inner membrane regenerates acyl-CoA in matrix
• CPT-I regulation: Inhibited by malonyl-CoA (prevents futile cycling during fed state); this ensures fatty acid synthesis and oxidation don't occur simultaneously
• Beta-oxidation cycle (repeats until 2-carbon units remain):

1. 1.Oxidation: Fatty acyl-CoA dehydrogenase (FADH₂-generating step, inhibited by acetyl-CoA)
2. 2.Hydration: Enoyl-CoA hydratase
3. 3.Oxidation: 3-ketoacyl-CoA dehydrogenase (NAD⁺-dependent; inhibited by NADH and acetyl-CoA)
4. 4.Thiolysis: Thiophorase cleaves acetyl-CoA; regenerates fatty acyl-CoA (2 carbons shorter)

• Products: Each complete cycle generates 1 NADH, 1 FADH₂, and 1 acetyl-CoA; complete oxidation of one palmitate (16:0) yields 8 acetyl-CoA, 7 NADH, 7 FADH₂ ≈ 129 ATP
• Ketone body formation: Excess acetyl-CoA from beta-oxidation exceeds capacity of TCA cycle → converted to ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone) in liver mitochondria, exported for use by extrahepatic tissues
• Regulation: Activated by glucagon and epinephrine; inhibited by high NADH/NAD⁺ and acetyl-CoA/CoA ratios (high energy state); AMP-kinase activation increases fatty acid oxidation
• Special cases: Odd-chain fatty acids yield propionyl-CoA (3-carbon unit) from final cycle → converted to succinyl-CoA (gluconeogenesis substrate); alpha-oxidation handles very long-chain fatty acids before beta-oxidation; omega-oxidation in peroxisomes handles some unsaturated fatty acids

Fatty Acid Synthesis Disorders (Rare)

• Lipodystrophy syndromes: Defective adipose tissue with impaired lipogenesis → severe insulin resistance, hepatic steatosis, hypertriglyceridemia, and early metabolic syndrome despite low body fat
• Metabolic syndrome from excessive lipogenesis: Central obesity, fatty liver (NAFLD), hypertriglyceridemia, insulin resistance despite adequate caloric intake

Beta-Oxidation Disorders (Mitochondrial Fatty Acid Oxidation Defects)

• Carnitine deficiency (Primary): Muscle weakness, cardiomyopathy, hypoglycemia in infants; presents as "floppy baby syndrome"
• CPT-I deficiency (most common): Hypoketotic hypoglycemia with seizures in infants during fasting; normal ketone production fails; hepatomegaly and elevated liver enzymes
• CPT-II deficiency: Three phenotypes:

1. 1.Infantile form (severe): Presents <1 week with hypoketotic hypoglycemia, hepatomegaly, cardiomyopathy, muscle hypotonia; high mortality
2. 2.Hepatic form (intermediate): Childhood onset with recurrent hypoglycemic episodes and hepatomegaly triggered by illness/fasting
3. 3.Myopathic form (mildest): Adult-onset muscle pain, weakness, and rhabdomyolysis triggered by prolonged exercise or fasting; normal infant period** distinguishes from infantile form

• Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency (most common FAOD): Hypoketotic hypoglycemia (typically <6 hours fasting), lethargy, hepatomegaly, and developmental delay in infancy; risk of sudden unexplained infant death; dicarboxylic aciduria on urine organic acids
• Long-chain acyl-CoA dehydrogenase (LCAD) deficiency: Hypoglycemia, hypoketotic state, hepatomegaly, cardiomyopathy, muscle weakness; can present with sudden infant death or peripartum cardiomyopathy
• Trifunctional protein (TFP) deficiency: Neonatal presentation with hypoglycemia, hepatomegaly, cardiomyopathy; progressive myopathy and peripheral neuropathy; retinal dystrophy with visual loss (distinguishing feature)
• 3-ketoacyl-CoA thiolase deficiency: Hypoketotic hypoglycemia, developmental delay, metabolic acidosis; accumulation of propionyl-CoA leads to secondary propionic acidemia

General Clinical Pearls

• Hypoketotic hypoglycemia is a hallmark presentation of beta-oxidation disorders (inadequate glucose production AND failed ketone generation = severe metabolic derangement)
• Fasting, illness, or increased metabolic demand (exercise, infection, fever) precipitate acute decompensation

The regulatory hinge examiners love

• Malonyl-CoA inhibits CPT-I: the single most tested fact — the committed product of synthesis blocks fatty acid entry into mitochondria, so lipogenesis and beta-oxidation can never run simultaneously. Insulin (via active, dephosphorylated acetyl-CoA carboxylase) raises malonyl-CoA; glucagon/epinephrine and AMPK lower it.
• Biotin is the cofactor for acetyl-CoA carboxylase (as for pyruvate carboxylase and propionyl-CoA carboxylase) — a stem describing biotin deficiency should point you to carboxylases, not dehydrogenases.
• NADPH sources: pentose phosphate pathway (G6PD) plus malic enzyme; citrate shuttle moves acetyl-CoA out of mitochondria.

Fatty acid oxidation defects

• Hypoketotic hypoglycemia after a fast or febrile illness is the buzzword. Best next steps: bedside glucose, then plasma acylcarnitine profile plus urine organic acids — and give IV dextrose to abort lipolysis before results return; do not simply feed and observe.
• MCAD deficiency is the most common FAO defect, features medium-chain (C8) acylcarnitine elevation and dicarboxylic aciduria, and is included on the HHS Recommended Uniform Screening Panel with ACMG ACT-sheet follow-up. Chronic management is avoidance of prolonged fasting with a sick-day carbohydrate plan.
• Common distractor: MCAD is often dressed up as Reye syndrome or sudden infant death — the absent ketones with low glucose is the tell. Also, medium-chain triglyceride oil, useful in long-chain defects, is inappropriate in MCAD.
• Carnitine: primary carnitine (OCTN2/SLC22A5) deficiency causes cardiomyopathy, weakness, and hypoketotic hypoglycemia and responds to L-carnitine; suspect secondary deficiency with valproate or hemodialysis. Carnitine is not blanket therapy for every FAO defect.
• CPT-II, myopathic form: adult rhabdomyolysis after prolonged exercise or fasting. Contrast with McArdle disease — brief, high-intensity exertion, second-wind phenomenon, no hypoglycemia.
• Even-chain fatty acids cannot make glucose; only the propionyl-CoA → succinyl-CoA carbons from odd-chain fatty acids (and glycerol) are gluconeogenic. This is the classic trap in fasting-physiology stems.
• Peroxisomal disease (VLCFA accumulation in Zellweger; phytanic acid via alpha-oxidation in Refsum) presents neurologically, not as fasting hypoglycemia.