carnitine shuttle

Long-chain fatty acids (>12 carbons) cannot cross the inner mitochondrial membrane directly, unlike short- and medium-chain fatty acids. The carnitine shuttle solves this: (1) Fatty acyl-CoA is converted to fatty acyl-carnitine by CPT-I on the outer mitochondrial membrane; (2) carnitine-acylcarnitine translocase transports fatty acyl-carnitine into the mitochondrial matrix in exchange for free carnitine; (3) CPT-II on the inner membrane reconverts fatty acyl-carnitine back to fatty acyl-CoA, releasing free carnitine to be recycled. The fatty acyl-CoA then enters β-oxidation to generate acetyl-CoA, NADH, and FADH2 for the TCA cycle and oxidative phosphorylation.

Regulation: CPT-I is inhibited by malonyl-CoA, the first intermediate in fatty acid synthesis (produced by acetyl-CoA carboxylase). This reciprocal regulation ensures that fatty acid synthesis and oxidation do not occur simultaneously—in the fed state (high insulin), malonyl-CoA is high, inhibiting fatty acid oxidation and favoring synthesis; in the fasted state (high glucagon/low insulin), malonyl-CoA is low, permitting CPT-I activity and fatty acid oxidation.

Clinical significance: - CPT-II deficiency: The most common inherited disorder of lipid metabolism causing recurrent myoglobinuria, often triggered by prolonged exercise, fasting, or illness. Presents with muscle pain, weakness, and rhabdomyolysis, typically in adolescents/adults. Milder than CPT-I deficiency. - CPT-I deficiency: Presents in infancy with hypoketotic hypoglycemia, hepatomegaly, and can be life-threatening during fasting since the liver cannot switch to fatty acid oxidation. - Primary carnitine deficiency: Impaired carnitine transport into cells causes accumulation of fatty acids, leading to hypoketotic hypoglycemia, cardiomyopathy, skeletal myopathy, and hepatic dysfunction. - Secondary carnitine deficiency: Can occur with certain organic acidemias or with valproic acid use (which depletes carnitine). - Diagnostic clue on boards: Fasting-induced hypoketotic hypoglycemia with elevated free fatty acids suggests a defect in fatty acid oxidation/carnitine shuttle, whereas ketotic hypoglycemia suggests other causes.