Carnitine biosynthesis explained

Carnitine biosynthesis is a method for the endogenous production of L-carnitine, a molecule that is essential for energy metabolism.^{[1] [2] [3] [4]} In humans and many other animals, L-carnitine is obtained from both diet and by biosynthesis.^{[5] [6]} The carnitine biosynthesis pathway is highly conserved among many eukaryotes and some prokaryotes.^{[7] [8] [9]}

L-Carnitine is biosynthesized from N^ε-trimethyllysine.^[10] At least four enzymes are involved in the overall biosynthetic pathway. They are N^ε-trimethyllysine hydroxylase, 3-hydroxy-N^ε-trimethyllysine aldolase, 4-N-trimethylaminobutyraldehyde dehydrogenase and γ-butyrobetaine hydroxylase.

N^ε-Trimethyllysine hydroxylase

See main article: Trimethyllysine dioxygenase.

The first enzyme of the L-carnitine biosynthetic pathway is N^ε-trimethyllysine hydroxylase, an iron and 2-oxoglutarate (2OG)-dependent oxygenase that also requires ascorbate.^[11] N^ε-trimethyllysine hydroxylase catalyses the hydroxylation reaction of N^ε-trimethyllysine to 3-hydroxy-N^ε-trimethyllysine.

The current consensus theory about the origin of N^ε-trimethyllysine in mammals is that mammals utilise lysosomal or proteasomal degradation of proteins containing N^ε-trimethyllysine residues as starting point for carnitine biosynthesis.^{[12] [13] [14]} An alternative theory involving endogenous non-peptidyl biosynthesis was also proposed, based on evidence gathered from a study involving feeding normal and undernourished human subjects with the amino acid lysine.^[15] Although N^ε-trimethyllysine biosynthetic pathway involving N^ε-trimethyllysine methyltransferase has been fully characterised in fungi including Neurospora crassa, such biosynthetic pathway has never been properly characterised in mammals or humans.^[16] A third theory about the origin of N^ε-trimethyllysine in mammals does not involve biosynthesis at all, but involves direct dietary intake from vegetable foods. High-performance liquid chromatography (HPLC) analysis has confirmed that vegetables contain a significant amount of N^ε-trimethyllysine.^[17]

3-Hydroxy-N^ε-trimethyllysine aldolase

The second step of L-carnitine biosynthesis requires the 3-hydroxy-N^ε-trimethyllysine aldolase enzyme. 3-hydroxy-N^ε-trimethyllysine aldolase is a pyridoxal phosphate dependent aldolase, and it catalyses the cleavage of 3-hydroxy-N^ε-trimethyllysine into 4-N-trimethylaminobutyraldehyde and glycine.

The true identity of 3-hydroxy-N^ε-trimethyllysine aldolase is elusive and the mammalian gene encoding 3-hydroxy-N^ε-trimethyllysine aldolase has not been identified. 3-hydroxy-N^ε-trimethyllysine aldolase activity has been demonstrated in both L-threonine aldolase and serine hydroxymethyltransferase,^{[18] [19]} although whether this is the main catalytic activity of these enzymes remains to be established.

4-N-Trimethylaminobutyraldehyde dehydrogenase

See main article: 4-trimethylammoniobutyraldehyde dehydrogenase.

The third enzyme of L-carnitine biosynthesis is 4-N-trimethylaminobutyraldehyde dehydrogenase.^[20] 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-N^ε-trimethyllysine aldolase, 4-N-trimethylaminobutyraldehyde dehydrogenase has been identified and purified from many sources including rat^[21] and Pseudomonas.^[22] 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,^[23] but the true identity of 4-N-trimethylaminobutyraldehyde dehydrogenase remains to be established.

γ-Butyrobetaine hydroxylase

See main article: gamma-butyrobetaine dioxygenase. The final step of L-carnitine biosynthesis is γ-butyrobetaine hydroxylase, a zinc binding enzyme.^{[24] [25] [26] [27] [28] [29]} Like N^ε-trimethyllysine hydroxylase, γ-butyrobetaine hydroxylase is a 2-oxoglutarate and iron(II)-dependent oxygenase. γ-Butyrobetaine hydroxylase catalyses 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,^[30] rat,^{[31] [32] [33]} cow,^[34] guinea pig^[35] and human.^[36] Recombinant human γ-butyrobetaine hydroxylase has also been produced by Escherichia coli^[27] and baculoviruses^[26] systems.

Notes and References

1. http://pharmaxchange.info/press/2013/10/activation-and-transportation-of-fatty-acids-to-the-mitochondria-via-the-carnitine-shuttle-with-animation/ Activation and Transportation of Fatty Acids for Metabolism via Carnitine Shuttle Pathway (with Animation)
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36. Lindstedt, G.; Lindstedt, S.; Nordin, I. γ-Butyrobetaine hydroxylase in human kidney. Scand. J. Clin. Lab. Invest. 1982, 42, 477–485.