Carnitine biosynthesis
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 Nε-trimethyllysine.[10] 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 γ-butyrobetaine hydroxylase.
Nε-Trimethyllysine hydroxylase
[edit]The first enzyme of the L-carnitine biosynthetic pathway is Nε-trimethyllysine hydroxylase, an iron and 2-oxoglutarate (2OG)-dependent oxygenase that also requires ascorbate.[11] Nε-trimethyllysine hydroxylase catalyses the hydroxylation reaction of Nε-trimethyllysine to 3-hydroxy-Nε-trimethyllysine.
The current consensus theory about the origin of Nε-trimethyllysine in mammals is that mammals utilise lysosomal or proteasomal degradation of proteins containing Nε-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 Nε-trimethyllysine biosynthetic pathway involving Nε-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 Nε-trimethyllysine in mammals does not involve biosynthesis at all, but involves direct dietary intake from vegetable foods.[citation needed] High-performance liquid chromatography (HPLC) analysis has confirmed that vegetables contain a significant amount of Nε-trimethyllysine.[17]
3-Hydroxy-Nε-trimethyllysine aldolase
[edit]The second step of L-carnitine biosynthesis requires the 3-hydroxy-Nε-trimethyllysine aldolase enzyme. 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,[18][19] although whether this is the main catalytic activity of these enzymes remains to be established.
4-N-Trimethylaminobutyraldehyde dehydrogenase
[edit]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-Nε-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
[edit]The final step of L-carnitine biosynthesis is γ-butyrobetaine hydroxylase, a zinc binding enzyme.[24][25][26][27][28][29] Like Nε-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.
![](http://up.wiki.x.io/wikipedia/commons/thumb/3/38/CarnitineBiosynthesis.png/600px-CarnitineBiosynthesis.png)
References
[edit]- ^ Activation and Transportation of Fatty Acids for Metabolism via Carnitine Shuttle Pathway (with Animation)
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- ^ Strijbis, K.; Vaz, F, M.; Distel, B. Enzymology of the carnitine biosynthesis pathway. IUBMB Life 2010, 62, 357–362.
- ^ Hulse, J. D.; Ellis, S. R.; Henderson, L. M. Carnitine biosynthesis. β-Hydroxylation of trimethyllysine by an α-ketoglutarate-dependent mitochondrial dioxygenase. J. Biol. Chem. 1978, 253, 1654–1659.
- ^ Vaz, F. M.; Ofman, R.; Westinga, K.; Back, J. W.; Wanders, R. J. A. Molecular and biochemical characterization of rat ε-N-trimethyllysine hydroxylase, the first enzyme of carnitine biosynthesis. J. Biol. Chem. 2001, 276, 33512–33517.
- ^ Bremer, J. Biosynthesis of carnitine in vivo. Biochim. Biophys. Acta 1961, 48, 622–624.
- ^ Wolf, G.; Berger, C. R. A. Studies on the biosynthesis and turnover of carnitine. Arch. Biochem. Biophys. 1961, 92, 360–365.
- ^ Paik, W. K.; Nochumson, S.; Kim, S. Carnitine biosynthesis via protein methylation. Trends Biochem. Sci. 1977, 2, 159–161.
- ^ Khan-Siddiqui, L.; Bamji, M. S. Lysine-carnitine conversion in normal and undernourished adult man – suggestion of a nonpeptidyl pathway. Am. J. Clin. Nutr. 1983, 37, 93–98.
- ^ Rebouche, C. J.; Broquist, H. P. Carnitine biosynthesis in Neurospora crassa: enzymatic conversion of lysine to ε-N-trimethyllysine. J. Bacteriol. 1976, 126, 1207–1214.
- ^ Servillo, L.; Giovane, A.; Cautela, D.; Castaldo, D.; Balestrieri, M. L. Where Does Nε-Trimethyllysine for the Carnitine Biosynthesis in Mammals Come from? PLoS ONE 2014, 9, e84589.
- ^ McNeil, J. B.; Flynn, J.; Tsao, N.; Monschau, N.; Stahmann, K. P.; Haynes, R. H.; McIntosh, E. M.; Pearlman, R. E. Glycine metabolism in Candida albicans: characterization of the serine hydroxymethyltransferase (SHM1, SHM2) and threonine aldolase (GLY1) genes. Yeast 2000, 16, 167–175.
- ^ Schirch, L.; Peterson, D. Purification and properties of mitochondrial serine hydroxymethyltransferas. J. Biol. Chem. 1980, 255, 7801–7806.
- ^ Hulse, J. D.; Henderson, L. M. Carnitine biosynthesis. Purification of 4-N’-trimethylaminobutyraldehyde dehydrogenase from beef liver. J. Biol. Chem. 1980, 255, 1146–1151.
- ^ Vaz, F. M.; Fouchier, S. W.; Ofman, R.; Sommer, M.; Wanders, R. J. A. Molecular and biochemical characterization of rat γ-trimethylaminobutyraldehyde dehydrogenase and evidence for the involvement of human aldehyde dehydrogenase 9 in carnitine biosynthesis. J. Biol. Chem. 2000, 275, 7390–7394.
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- ^ Lin, S. W.; Chen, J. C.; Hsu, L. C.; Hsieh, C. L.; Yoshida, A. Human γ-aminobutyraldehyde dehydrogenase (ALDH9): cDNA sequence, genomic organization, polymorphism, chromosomal localization, and tissue expression. Genomics 1996, 34, 376–380
- ^ Vaz, F. M.; van Gool, S.; Ofman, R.; Ijlst, L.; Wanders, R. J. Carnitine Biosynthesis: Identification of the cDNA Encoding Human γ-Butyrobetaine Hydroxylase. Biochem. Biophys. Res. Commun. 1998, 250, 506–510.
- ^ Rigault, C.; Le Borgne, F.; Demarquoy, J. Genomic structure, alternative maturation and tissue expression of the human BBOX1 gene. Biochim. Biophys. Acta 2006, 1761, 1469–1481.
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