CarnitinSystem

CarnitinSystem is a cellular metabolic pathway that enables the transport of long-chain fatty acids from the cytosol into the mitochondrial matrix for beta-oxidation, thereby supporting aerobic energy production. The system involves the carrier molecule carnitine and a set of enzymes known as the carnitine palmitoyltransferases and related transport proteins. The initial step transfers the fatty acyl group from coenzyme A to carnitine via CPT1, forming acylcarnitine. The acylcarnitine is shuttled across the inner mitochondrial membrane by CACT (carnitine-acylcarnitine translocase), then converted back to fatty acyl-CoA by CPT2 within the matrix.

Regulation occurs mainly through malonyl-CoA, which inhibits CPT1 and thus gates fatty acid entry when glucose is plentiful. During fasting or increased energy demand, malonyl-CoA falls and the CarnitinSystem operates more actively. The components are most prominent in liver and skeletal muscle but are present in other tissues as well. The system demonstrates substrate specificity for long-chain fatty acids, while medium-chain fatty acids can enter mitochondria independently of the shuttle to some extent.

Disruption of any component of the CarnitinSystem can cause fatty acid oxidation disorders. CPT1, CPT2, or CACT deficiencies, among others, may present with hypoketotic hypoglycemia, hepatomegaly, muscle weakness, or cardiomyopathy, often in infancy or childhood. Diagnosis commonly relies on acylcarnitine profiling, enzyme activity assays, and genetic testing. Management focuses on avoiding fasting, dietary modification (including possible medium-chain triglyceride supplementation), and, in some cases, carnitine supplementation to address secondary deficiencies or enhance excretion of toxic acyl groups. Newborn screening programs include tests for related disorders.