Department of Microbiology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa.
Appl Microbiol Biotechnol. 2012 Sep;95(6):1377-88. doi: 10.1007/s00253-012-4248-0. Epub 2012 Jul 14.
Next to cellulose, starch is the most abundant hexose polymer in plants, an import food and feed source and a preferred substrate for the production of many industrial products. Efficient starch hydrolysis requires the activities of both α-1,4 and α-1,6-debranching hydrolases, such as endo-amylases, exo-amylases, debranching enzymes, and transferases. Although amylases are widely distributed in nature, only about 10 % of amylolytic enzymes are able to hydrolyse raw or unmodified starch, with a combination of α-amylases and glucoamylases as minimum requirement for the complete hydrolysis of raw starch. The cost-effective conversion of raw starch for the production of biofuels and other important by-products requires the expression of starch-hydrolysing enzymes in a fermenting yeast strain to achieve liquefaction, hydrolysis, and fermentation (Consolidated Bioprocessing, CBP) by a single organism. The status of engineering amylolytic activities into Saccharomyces cerevisiae as fermentative host is highlighted and progress as well as challenges towards a true CBP organism for raw starch is discussed. Conversion of raw starch by yeast secreting or displaying α-amylases and glucoamylases on their surface has been demonstrated, although not at high starch loading or conversion rates that will be economically viable on industrial scale. Once efficient conversion of raw starch can be demonstrated at commercial level, engineering of yeast to utilize alternative substrates and produce alternative chemicals as part of a sustainable biorefinery can be pursued to ensure the rightful place of starch converting yeasts in the envisaged bio-economy of the future.
除纤维素外，淀粉是植物中含量最丰富的六碳聚合物，是重要的食物和饲料来源，也是许多工业产品的首选底物。高效的淀粉水解需要α-1,4 和 α-1,6 分支水解酶（如内切淀粉酶、外切淀粉酶、分支酶和转移酶）的活性。尽管淀粉酶广泛存在于自然界中，但只有约 10%的淀粉酶能够水解原淀粉或未经修饰的淀粉，α-淀粉酶和葡萄糖淀粉酶的组合是原淀粉完全水解的最低要求。为了以经济有效的方式将原淀粉转化为生物燃料和其他重要的副产品，需要在发酵酵母菌株中表达淀粉水解酶，以实现液化、水解和发酵（整合生物加工，CBP）由单个生物体完成。本文强调了将淀粉水解活性工程化到酿酒酵母中作为发酵宿主的状态，并讨论了实现真正的原淀粉 CBP 生物体的进展和挑战。酵母分泌或在其表面展示α-淀粉酶和葡萄糖淀粉酶来转化原淀粉已经得到了证明，尽管在工业规模上的淀粉负载或转化率方面还没有达到经济可行的水平。一旦能够在商业规模上实现原淀粉的高效转化，就可以对酵母进行工程改造，以利用替代底物并生产替代化学品，作为可持续生物精炼厂的一部分，以确保淀粉转化酵母在未来预期的生物经济中的应有地位。
