Wax Ester Fermentation and Its Application for Biofuel Production
Wax Ester Fermentation and Its Application for Biofuel Production
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DOI：
10.1007/978-3-319-54910-1_13
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发表时间：
2017-01-01
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影响因子：
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通讯作者：
Tamoi, Masahiro
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作者：
Inui, Hiroshi;Ishikawa, Takahiro;Tamoi, Masahiro
在厌氧条件下的绿藻细胞中，储存多糖的paramylon被迅速降解并转化为蜡酯。所合成的蜡酯由链长为10 ~ 18的饱和脂肪酸和醇组成，主要成分为肉豆蔻酸和肉豆蔻醇。由于厌氧细胞通过将酰胺转化为蜡酯而获得ATP，因此这种现象被称为“蜡酯发酵”。蜡酯发酵的独特之处在于，与乳酸和乙醇等常见发酵终产物相比，最终产物即蜡酯具有相对较高的分子量，不溶于水，并在细胞中积累。蜡酯发酵过程中一个独特的代谢途径是线粒体脂肪酸合成系统。在这个体系中，脂肪酸是通过β -氧化的逆转来合成的，除了反式-2-烯基辅酶a还原酶而不是酰基辅酶a脱氢酶。因此，在这种脂肪酸合成中，乙酰辅酶a直接作为C2供体，不需要将乙酰辅酶a转化为丙二酰辅酶a，需要ATP。因此，线粒体脂肪酸合成系统使ATP的净增益成为可能，通过从paramylon合成蜡酯。此外，丙酮酸在厌氧细胞中通过一种独特的酶提供乙酰辅酶a，这种酶是氧敏感的丙酮酸：NADP（+）氧化还原酶，而不是常见的丙酮酸脱氢酶多酶复合物。厌氧藻产生的蜡酯是很有前途的生物燃料，因为与其他藻类产生的脂肪酸，如棕榈酸（C-16:0）和硬脂酸（C-18:0）相比，肉豆肉酸（C-14:0）具有较低的凝固点，适合作为喷气燃料。为了提高蜡酯的产量，通过鉴定蜡酯发酵代谢途径相关基因和综合基因/蛋白表达分析，逐渐阐明了蜡酯发酵在好氧和厌氧条件下调控的分子机制。此外，蓝藻卡尔文循环果糖-1,6-二磷酸酶/ sedoheptuose -1,7-二磷酸酶在绿藻中的表达提供了光合作用，增加了paramylon积累，促进了蜡酯的产生。本章将讨论蜡酯发酵的生物化学过程、蜡酯发酵调控的最新进展以及提高生物燃料蜡酯产量的基因工程方法。
In Euglena cells under anaerobic conditions, paramylon, the storage polysaccharide, is promptly degraded and converted to wax esters. The wax esters synthesized are composed of saturated fatty acids and alcohols with chain lengths of 10-18, and the major constituents are myristic acid and myristyl alcohol. Since the anaerobic cells gain ATP through the conversion of paramylon to wax esters, the phenomenon is named "wax ester fermentation". The wax ester fermentation is quite unique in that the end products, i.e. wax esters, have relatively high molecular weights, are insoluble in water, and accumulate in the cells, in contrast to the common fermentation end products such as lactic acid and ethanol.A unique metabolic pathway involved in the wax ester fermentation is the mitochondrial fatty acid synthetic system. In this system, fatty acid are synthesized by the reversal of beta-oxidation with an exception that trans-2-enoyl-CoA reductase functions instead of acyl-CoA dehydrogenase. Therefore, acetyl-CoA is directly used as a C2 donor in this fatty acid synthesis, and the conversion of acetyl-CoA to malonyl-CoA, which requires ATP, is not necessary. Consequently, the mitochondrial fatty acid synthetic system makes possible the net gain of ATP through the synthesis of wax esters from paramylon. In addition, acetyl-CoA is provided in the anaerobic cells from pyruvate by the action of a unique enzyme, oxygen sensitive pyruvate: NADP(+) oxidoreductase, instead of the common pyruvate dehydrogenase multienzyme complex.Wax esters produced by anaerobic Euglena are promising biofuels because myristic acid (C-14:0) in contrast to other algal produced fatty acids, such as palmitic acid (C-16:0) and stearic acid (C-18:0), has a low freezing point making it suitable as a drop-in jet fuel. To improve wax ester production, the molecular mechanisms by which wax ester fermentation is regulated in response to aerobic and anaerobic conditions have been gradually elucidated by identifying individual genes related to the wax ester fermentation metabolic pathway and by comprehensive gene/protein expression analysis. In addition, expression of the cyanobacterial Calvin cycle fructose-1,6-bisphosphatase/sedohepturose-1,7-bisphosphatase, in Euglena provided photosynthesis resulting in increased paramylon accumulation enhancing wax ester production. This chapter will discuss the biochemistry of the wax ester fermentation, recent advances in our understanding of the regulation of the wax ester fermentation and genetic engineering approaches to increase production of wax esters for biofuels.