Biosynthetic origin of the saw-toothed profile in δ13 C and δ2H 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 alkalies (ECN-n-alkanes) are biosynthesised in higher plants There is a distinctive compositional pattern of incorporation of stable carbon (C-13) and hydrogen (H-2) isotopes in co-existing ECN- and OCN-n-alkanes in leaves of higher plants, such that the OCN n-alkanes are relatively enriched in C-13 but relatively depleted in H-2 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 C-13 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 (H-2) than the iso-alkanes which in turn were more enriched than the anteiso-alkanes We propose therefore that the depletion of H-2 in the iso-alkanes, relative to the n-alkanes is the consequence of accepting highly H-2-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 (C) 2009 Elsevier Ltd All rights reserved