Characterization and Reevaluation of Initial Reactions of Fatty Acid Biosynthesis in Plants

由于发现菠菜中存在一种以前未报道的3-酮酰基- acp合成酶（sc-KAS），因此必须重新评估植物中脂肪酸生物合成最初涉及的序列反应。该酶对短链脂肪酸具有特异性，可与其他3-酮酰基- acp合成酶区分，因为它对抗生素蓝蛋白和硫霉素敏感。根据对大肠杆菌和菠菜进行的研究，有强有力的证据表明，这种酶在这个序列中取代了乙酰转酰基酶，乙酰转酰基酶是脂肪酸合成中的一种限速反应。本文的目的是对菠菜中sc-KAS进行广泛的生化分析，并分离出sc-KAS的结构基因。虽然对植物中脂肪酸生物合成的调控知之甚少，但有体外证据表明，该序列的初始反应可以影响产品的脂肪酸组成。sc-KAS可能在这一过程中发挥关键作用，并成为我们理解监管的关键。在过去的25年里，脂肪酸的生物合成被认为是由脂肪酸合成酶催化的一系列反应。根据该途径，乙酰转酰基酶是该反应的关键限速酶。然而，3-酮酰基- acp合成酶的发现对这一系列反应提出了挑战。人们认为后一种酶负责目前被认为是由乙酰转酰基酶催化的步骤，这与公认的脂肪酸生物合成教条相反。这是一个重要的发现，将通过纯化、表征和检查这种酶的调节进行研究。了解这种酶在脂肪酸合成中的作用是很重要的，特别是在油籽作物的生产中。如果这种酶的表达可以通过基因工程加以控制，就有可能产生过量的脂肪酸。即使含油量只增加一点点，也会产生重大的经济影响。

The sequence reactions initially involved in fatty acid biosynthesis in plants must be reevaluated in light of the discovery that there is in spinach a previously unreported 3- ketoacyl-ACP synthase (sc-KAS). This enzyme is specific for short chain fatty acids and can be distinguished from other 3- ketoacyl-ACP synthases by its sensitivity to the antibiotics cerulenin and thiolactomycin. Based on work carried out with E. coli, as well as this work with spinach, there is strong evidence that this enzyme replaces in this sequence the enzyme acetyl transacylase, a proposed rate-limiting reaction in fatty acid synthesis. The objectives of this proposal are an extensive biochemical analysis of sc-KAS from spinach and isolation of the structural gene for sc-KAS. While the regulation of fatty acid biosynthesis in plants is poorly understood, there is in vitro evidence that the initial reactions of the sequence can influence the fatty acid composition of the products. The sc-KAS may play a critical role in this process and be the key to our understanding of regulation. For the past 25 years, fatty acid biosynthesis has been illustrated as a series of reactions catalyzed by the enzyme complex, fatty acid synthase. According to the pathway, the enzyme, acetyl transacylase is a key rate-limiting enzyme in this reaction. However, the discovery of the enzyme, 3-ketoacyl-ACP synthase, is challenging this series of reactions. It is believed that the latter enzyme is responsible for the step currently thought to be catalyzed by acetyl transacylase, contrary to the accepted dogma of fatty acid biosynthesis. This is a significant discovery that will be investigated through purification,characterization, and examination of the regulation of this enzyme. Understanding the role of this enzyme in fatty acid synthesis is of importance, particularly in the production of oilseed crops. If expression of this enzyme can be controlled through genetic engineering, there is the potential for over- production of fatty acids. Even if only a small increase in oil content could be achieved, it would have a major economic impact.