Biosynthetic origin of the saw-toothed profile in delta(13)C and delta(2)H of n-alkanes and systematic isotopic differences between n-, iso- and anteiso-alkanes in leaf waxes of land plants.
The n-fatty acids containing an even number of carbons (ECN-n-FAs) in higher plants are biosynthesised by repetitive addition of a two carbon unit from malonyl-ACP. The n-alkanes containing an odd number of carbon atoms (OCN-n-alkanes) are generally formed by the decarboxylation of ECN-n-FAs, but it is unknown how the less abundant even-carbon-numbered alkanes (ECN-n-alkanes) are biosynthesised in higher plants. There is a distinctive compositional pattern of incorporation of stable carbon ((13)C) and hydrogen ((2)H) isotopes in co-existing ECN- and OCN-n-alkanes in leaves of higher plants, such that the OCN n-alkanes are relatively enriched in (13)C but relatively depleted in (2)H against the ECN-n-alkanes. This is consistent with the OCN-n-fatty acids having a propionate precursor which is derived from reduction of pyruvate. A tentative pathway is presented with propionate produced by enzymatic reduction of pyruvate which is then thio-esterified with CoSH (coenzyme A thiol) in the chloroplast to form the terminal precursor molecule propionyl-CoA. This is then repetitively extended/elongated with the 2-carbon unit from malonyl-ACP to form the long chain OCN-n-fatty acids. The anteiso- and iso-alkanes in Nicotiana tabacum leaf waxes have previously been found to be systematically enriched in (13)C compared with the n-alkanes by Grice et al. (2008). This is consistent with the isotopic composition of their putative respective precursors (pyruvate as precursor for n-alkanes, valine for iso-alkanes and isoleucine for anteiso-alkanes). The current study complements that of Grice et al. (2008) and looks at the distribution of hydrogen isotopes. The n-alkanes were found to be more enriched in deuterium ((2)H) than the iso-alkanes which in turn were more enriched than the anteiso-alkanes. We propose therefore that the depletion of (2)H in the iso-alkanes, relative to the n-alkanes is the consequence of accepting highly (2)H-depleted hydrogen atoms from NADPH during their biosynthesis. The anteiso-alkanes are further depleted again because there are three NADPH-derived hydrogen atoms in their precursor isoleucine, as compared with only one NADPH-derived hydrogen in valine, the precursor of the iso-alkanes.
Zhou
Youping
Y
RSB, Australian National University, Canberra, ACT 2601, Australia. youping.zhou@act.gov.au
Grice
Kliti
K
Stuart-Williams
Hilary
H
Farquhar
Graham D
GD
Hocart
Charles H
CH
Lu
Hong
H
Liu
Weiguo
W
D000473:Alkanes / Q000737:chemistry* / Q000378:metabolism*; D053898:Biosynthetic Pathways; D002247:Carbon Isotopes / Q000378:metabolism; D003903:Deuterium / Q000378:metabolism*; D005227:Fatty Acids / Q000096:biosynthesis / Q000737:chemistry; D007536:Isomerism; D010084:Oxidation-Reduction; D063245:Plant Development; D018515:Plant Leaves / Q000254:growth & development / Q000378:metabolism*; D010944:Plants / Q000378:metabolism*; D011422:Propionates / Q000378:metabolism; D015203:Reproducibility of Results; D014885:Waxes / Q000378:metabolism*
D000473:Alkanes; D002247:Carbon Isotopes; D005227:Fatty Acids; D011422:Propionates; D014885:Waxes; D003903:Deuterium