Biosynthetic origin of the saw-toothed profile in d13C and d2Y of n-alkanes and systematic isotopic differences between n-, iso- and anteiso-alkanes in leaf waxes of land plants
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The n-fatty acids containing an even number of carbons (ECN-n-FAs) in higher plants are iosynthesisedby repetitive addition of a two carbon unit from malonyl-ACP. The n-alkanes containing an odd numberof carbon atoms (OCN-n-alkanes) are generally formed by the decarboxylation of ECN-n-FAs, but it isunknown how the less abundant even-carbon-numbered alkanes (ECN-n-alkanes) are biosynthesisedin higher plants. There is a distinctive compositional pattern of incorporation of stable carbon (13C) and hydrogen (2H) isotopes in co-existing ECN- and OCN-n-alkanes in leaves of higher plants, such that the OCN n-alkanes are relatively enriched in 13C but relatively depleted in 2H 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 13C 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, valinefor 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 enrichedin deuterium (2H) than the iso-alkanes which in turn were more enriched than the anteiso-alkanes. Wepropose therefore that the depletion of 2H in the iso-alkanes, relative to the n-alkanes is the consequence of accepting highly 2H-depleted hydrogen atoms from NADPH during their biosynthesis. The anteisoalkanes 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 isoalkanes.
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