Which molecule, essential for the transport of long-chain fatty acids into mitochondria, is synthesized from lysine?

Which molecule, essential for the transport of long-chain fatty acids into mitochondria, is synthesized from lysine?

• A. Acetyl-CoA
• B. Carnitine
• C. NAD+
• D. FAD

Answer: (B)

The key idea is that long-chain fatty acids need a specialized carrier to enter the mitochondria, and the molecule that does this is carnitine. Fatty acids are first activated to acyl-CoA in the cytosol. Then carnitine palmitoyltransferase I on the outer mitochondrial membrane transfers the acyl group to carnitine, forming acylcarnitine. This acylcarnitine is shuttled across the inner membrane by a translocase, and once inside the matrix, carnitine palmitoyltransferase II transfers the acyl group back to CoA to reform acyl-CoA for beta-oxidation. Carnitine is made in the liver and kidney from the essential amino acid lysine (with help from methionine for methyl groups). The synthesis begins with lysine residues in proteins being converted to trimethyllysine, which is further processed to gamma-butyrobetaine and finally hydroxylated to carnitine, with vitamin C as a cofactor in the last step. This connection explains why adequate lysine is important for maintaining the carnitine pool and efficient fatty acid oxidation. Acetyl-CoA is the end product of beta-oxidation and a substrate for the TCA cycle, not a transporter. NAD+ and FAD are electron carriers involved in oxidation-reduction steps, not carriers that move fatty acids into mitochondria.

The key idea is that long-chain fatty acids need a specialized carrier to enter the mitochondria, and the molecule that does this is carnitine. Fatty acids are first activated to acyl-CoA in the cytosol. Then carnitine palmitoyltransferase I on the outer mitochondrial membrane transfers the acyl group to carnitine, forming acylcarnitine. This acylcarnitine is shuttled across the inner membrane by a translocase, and once inside the matrix, carnitine palmitoyltransferase II transfers the acyl group back to CoA to reform acyl-CoA for beta-oxidation.

Carnitine is made in the liver and kidney from the essential amino acid lysine (with help from methionine for methyl groups). The synthesis begins with lysine residues in proteins being converted to trimethyllysine, which is further processed to gamma-butyrobetaine and finally hydroxylated to carnitine, with vitamin C as a cofactor in the last step. This connection explains why adequate lysine is important for maintaining the carnitine pool and efficient fatty acid oxidation.

Acetyl-CoA is the end product of beta-oxidation and a substrate for the TCA cycle, not a transporter. NAD+ and FAD are electron carriers involved in oxidation-reduction steps, not carriers that move fatty acids into mitochondria.