Kroumova A B, Xie Z, Wagner G J
Agronomy Department, University of Kentucky, Lexington 40546-0091.
Proc Natl Acad Sci U S A. 1994 Nov 22;91(24):11437-41. doi: 10.1073/pnas.91.24.11437.
Pathways and enzymes of fatty acid synthase-mediated, long-even-chain (generally C16-C20) fatty acid synthesis are well studied, and general metabolism involved in short-chain (C4-C7) fatty acid biosynthesis is also understood. In contrast, mechanisms of medium-chain (C8-C14) fatty acid synthesis are unclear. Recent work suggests involvement of chain-elongation-terminating thioesterases in medium-chain fatty acid formation in oilseeds and animals. We have shown that iso- and anteiso-branched and straight, odd- and even-length, short-chain fatty acids esterified in plant-trichome-gland-produced sucrose esters are synthesized by using carbon skeletons provided by modified branched-chain amino acid metabolism/catabolism. The principal enzymes involved are those catalyzing leucine biosynthesis in all organisms and those leading to short-chain alcohols in mutant yeasts and alkyl acids in Clostridium species (products often serving as mammalian pheromones). Here we provide evidence that C10-C12 straight medium-chain and C10-C12 branched medium-chain acyl acids of tomato, C6-C8 straight-chain acyl acids of Petunia, and C6 and C8 branched acyl acids of Nicotiana glutinosa are formed by alpha-ketoacid elongation without participation of fatty acid synthase-mediated reactions or -independent thioesterases. This different metabolism suggests greater integration of amino acid and fatty acid metabolism than previously considered and provides other avenues to study and manipulate not only straight even-length but also odd- and even-length straight and branched medium-chain fatty acid biosynthesis.
脂肪酸合酶介导的长偶数链（通常为C16 - C20）脂肪酸合成途径及相关酶已得到充分研究，短链（C4 - C7）脂肪酸生物合成所涉及的一般代谢过程也已为人所知。相比之下，中链（C8 - C14）脂肪酸的合成机制尚不清楚。近期研究表明，链延长终止硫酯酶参与了油料种子和动物中链脂肪酸的形成。我们已经证明，在植物毛状体腺产生的蔗糖酯中酯化的异分支和反异分支、直链、奇数和偶数长度的短链脂肪酸是利用经修饰的支链氨基酸代谢/分解代谢提供的碳骨架合成的。主要的酶包括在所有生物体中催化亮氨酸生物合成的酶，以及在突变酵母中导致短链醇生成和在梭菌属中导致烷基酸生成的酶（这些产物常作为哺乳动物的信息素）。在此，我们提供证据表明，番茄的C10 - C12直链中链和C10 - C12支链中链酰基酸、矮牵牛的C6 - C8直链酰基酸以及烟草的C6和C8支链酰基酸是通过α-酮酸延长形成的，而不涉及脂肪酸合酶介导的反应或非依赖硫酯酶的参与。这种不同的代谢表明氨基酸和脂肪酸代谢的整合程度比之前认为的更高，并且为研究和操纵不仅直链偶数长度的，而且奇数和偶数长度的直链和支链中链脂肪酸生物合成提供了其他途径。
