Albers E, Larsson C, Lidén G, Niklasson C, Gustafsson L
Department of Chemical Reaction Engineering, Chalmers University of Technology, Göteborg, Sweden.
Appl Environ Microbiol. 1996 Sep;62(9):3187-95. doi: 10.1128/aem.62.9.3187-3195.1996.
To prevent the loss of raw material in ethanol production by anaerobic yeast cultures, glycerol formation has to be reduced. In theory, this may be done by providing the yeast with amino acids, since the de novo cell synthesis of amino acids from glucose and ammonia gives rise to a surplus of NADH, which has to be reoxidized by the formation of glycerol. An industrial strain of Saccharomyces cerevisiae was cultivated in batch cultures with different nitrogen sources, i.e., ammonium salt, glutamic acid, and a mixture of amino acids, with 20 g of glucose per liter as the carbon and energy source. The effects of the nitrogen source on metabolite formation, growth, and cell composition were measured. The glycerol yields obtained with glutamic acid (0.17 mol/mol of glucose) or with the mixture of amino acids (0.10 mol/mol) as a nitrogen source were clearly lower than those for ammonium-grown cultures (0.21 mol/mol). In addition, the ethanol yield increased for growth on both glutamic acid (by 9%) and the mixture of amino acids (by 14%). Glutamic acid has a large influence on the formation of products; the production of, for example, alpha-ketoglutaric acid, succinic acid, and acetic acid, increased compared with their production with the other nitrogen sources. Cultures grown on amino acids have a higher specific growth rate (0.52 h-1) than cultures of both ammonium-grown (0.45 h-1) and glutamic acid-grown (0.33 h-1) cells. Although the product yields differed, similar compositions of the cells were attained. The NADH produced in the amino acid, RNA, and extracellular metabolite syntheses was calculated together with the corresponding glycerol formation. The lower-range values of the theoretically calculated yields of glycerol were in good agreement with the experimental yields, which may indicate that the regulation of metabolism succeeds in the most efficient balancing of the redox potential.
为防止厌氧酵母培养物在乙醇生产过程中原料损失，必须减少甘油的生成。理论上，这可以通过为酵母提供氨基酸来实现，因为由葡萄糖和氨从头合成氨基酸会产生过量的NADH，而NADH必须通过甘油的生成来重新氧化。将一株工业酿酒酵母在分批培养中用不同的氮源进行培养，即铵盐、谷氨酸和氨基酸混合物，以每升20克葡萄糖作为碳源和能源。测定了氮源对代谢物形成、生长和细胞组成的影响。以谷氨酸（0.17摩尔/摩尔葡萄糖）或氨基酸混合物（0.10摩尔/摩尔）作为氮源时获得的甘油产量明显低于以铵盐培养的培养物（0.21摩尔/摩尔）。此外，以谷氨酸（提高9%）和氨基酸混合物（提高14%）为氮源生长时，乙醇产量增加。谷氨酸对产物形成有很大影响；例如，与使用其他氮源时相比，α-酮戊二酸、琥珀酸和乙酸的产量增加。以氨基酸为氮源生长的培养物比以铵盐（0.45 h-1）和谷氨酸（0.33 h-1）为氮源生长的培养物具有更高的比生长速率（0.52 h-1）。尽管产物产量不同，但细胞组成相似。计算了氨基酸、RNA和细胞外代谢物合成过程中产生的NADH以及相应的甘油生成量。理论计算的甘油产量的较低值与实验产量吻合良好，这可能表明代谢调节成功实现了氧化还原电位的最有效平衡。
