RSB, Australian National University, Canberra, ACT 2601, Australia.
Phytochemistry. 2010 Mar;71(4):388-403. doi: 10.1016/j.phytochem.2009.11.009. Epub 2010 Jan 6.
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.
高等植物中含偶数碳原子的 n-脂肪酸（ECN-n-FAs）是通过丙二酰-ACP 的重复添加两个碳原子单元生物合成的。含奇数碳原子的 n-烷烃（OCN-n-烷烃）通常是通过 ECN-n-FAs 的脱羧形成的，但尚不清楚高等植物中较少的含偶数碳原子数的烷烃（ECN-n-烷烃）是如何生物合成的。在高等植物叶片中，共存的 ECN-和 OCN-n-烷烃中稳定碳（(13)C）和氢（(2)H）同位素的掺入具有独特的组成模式，使得 OCN n-烷烃相对于 ECN-n-烷烃相对富集（(13)C，但相对贫化（(2)H）。这与 OCN-n-脂肪酸具有丙酸盐前体一致，丙酸盐前体是由丙酮酸还原产生的。提出了一条途径，即丙酮酸通过酶促还原产生丙酸盐，然后与叶绿体中的 CoSH（辅酶 A 硫醇）硫酯化形成末端前体分子丙酰 CoA。然后，丙酰 CoA 与丙二酰-ACP 的 2-碳单元重复延伸/延长，形成长链 OCN-n-脂肪酸。先前 Grice 等人发现，烟草叶蜡中的 anteiso-和 iso-烷烃相对于 n-烷烃系统地富集（(13)C。这与它们假定的各自前体（n-烷烃的丙酮酸前体、iso-烷烃的缬氨酸前体和 anteiso-烷烃的异亮氨酸前体）的同位素组成一致。本研究补充了 Grice 等人的研究（2008 年），并研究了氢同位素的分布。发现 n-烷烃比 iso-烷烃更富集氘（(2)H），而 iso-烷烃比 anteiso-烷烃更富集氘。因此，我们提出，iso-烷烃中（2)H 的消耗相对于 n-烷烃是由于其生物合成过程中接受了高度（2)H 贫化的氢原子。anteiso-烷烃再次进一步消耗，因为其前体异亮氨酸中有三个 NADPH 衍生的氢原子，而缬氨酸（iso-烷烃的前体）中只有一个 NADPH 衍生的氢原子。
