Carnitine biosynthesis

Carnitine biosynthesis is a method for the endogenous production of L-carnitine, a molecule that is essential for energy metabolism. In humans and many other animals, L-carnitine is obtained from both diet and by biosynthesis. The carnitine biosynthesis pathway is highly conserved among many eukaryotes and some prokaryotes.

L-Carnitine is biosynthesized from Nε-trimethyllysine. At least four enzymes are involved in the overall biosynthetic pathway. They are Nε-trimethyllysine hydroxylase, 3-hydroxy-Nε-trimethyllysine aldolase, 4-N-trimethylaminobutyraldehyde dehydrogenase and gamma-butyrobetaine dioxygenase.

Nε-Trimethyllysine hydroxylase
The first enzyme of the L-carnitine biosynthetic pathway is Nε-trimethyllysine hydroxylase. Nε-trimethyllysine hydroxylase is a iron(II) and 2-oxoglutarate (2OG) dependent oxygenase. Nε-trimethyllysine hydroxylase catalyses the hydroxylation reaction of Nε-trimethyllysine to 3-hydroxy-Nε-trimethyllysine.

The origin of Nε-trimethyllysine is mainly from lysosomal or proteasomal degradation of methylated proteins. An Nε-trimethyllysine biosynthetic pathway involving Nε-trimethyllysine-Lysine methyltransferase has also been identified, for example, in Neurospora crassa. A similar biosynthetic pathway may also be functional in humans.

3-Hydroxy-Nε-trimethyllysine aldolase
The second enzyme of the L-carnitine biosynthetic pathway pathway is 3-hydroxy-Nε-trimethyllysine aldolase. 3-hydroxy-Nε-trimethyllysine aldolase is a pyridoxal phosphate dependent aldolase, and it catalyses the cleavage of 3-hydroxy-Nε-trimethyllysine into 4-N-trimethylaminobutyraldehyde and glycine.

The true identity of 3-hydroxy-Nε-trimethyllysine aldolase is elusive and the mammalian gene encoding 3-hydroxy-Nε-trimethyllysine aldolase has not been identified. 3-hydroxy-Nε-trimethyllysine aldolase activity has been demonstrated in both L-threonine aldolase and serine hydroxymethyltransferase, although whether this is the main catalytic activity of these enzymes remains to be established.

4-N-Trimethylaminobutyraldehyde dehydrogenase
The third enzyme of L-carnitine biosynthesis is 4-N-trimethylaminobutyraldehyde dehydrogenase. 4-N-trimethylaminobutyraldehyde dehydrogenase is a NAD+ dependent enzyme. 4-N-trimethylaminobutyraldehyde dehydrogenase catalyses the dehydrogenation of 4-N-trimethylaminobutyraldehyde into gamma-butyrobetaine.

Unlike 3-hydroxy-Nε-trimethyllysine aldolase, 4-N-trimethylaminobutyraldehyde dehydrogenase has been identified and purified from many sources including rat and Pseudomonas. However, the human 4-N-trimethylaminobutyraldehyde dehydrogenase has so far not been identified. There is considerable sequence similarity between rat 4-N-trimethylaminobutyraldehyde dehydrogenase and human aldehyde dehydrogenase 9, but the true identity of 4-N-trimethylaminobutyraldehyde dehydrogenase remains to be established.

γ-Butyrobetaine hydroxylase
The final enzyme of the L-carnitine biosynthesis is γ-butyrobetaine hydroxylase. Like Nε-trimethyllysine hydroxylase, γ-butyrobetaine hydroxylase is a 2OG and iron(II) dependent oxygenase. γ-Butyrobetaine hydroxylase catalyse the stereospecific hydroxylation of γ-butyrobetaine to L-carnitine.

γ-Butyrobetaine hydroxylase is the most studied enzyme among the four enzymes in the biosynthetic pathway. It has been purified from many sources, such as Pseudomonas, rat,  cow, guinea pig and human. Recombinant human γ-butyrobetaine hydroxylase has also been produced by Escherichia coli and baculoviruses systems.