酶法替代存在一定安全性问题的化学法对淀粉改性是目前国内外研究热点之一。然而不同淀粉内部结构引起酶改性淀粉生成的产物差异性大，导致其应用稳定性差。因此，如何利用酶法目的性修饰不同淀粉链结构以生产预期的、品质稳定的改性淀粉是尚需解决的核心问题。本项目拟对已鉴定的E. coli BL21(DE3)/pET20b-BE工程菌的分支酶（BE）催化活性中心处四种氨基酸残基进行定点突变，筛选出酶活性、热稳定性和产物特异性高的优质BE突变酶；以淀粉内部结构关键环节—支链淀粉最小构建单元building blocks为切入点，确定与淀粉良好物化性能和缓慢消化特性呈正相关性building blocks结构参数，并以此参数为理论依据，挖掘和锁定酶促反应关键调控因素，构建优质BE突变酶改性淀粉定向转化生成具有良好building blocks目标产物特异性调控机制，为制备预期绿色改性淀粉提供理论依据和实践基础。

Currently, it is one of the hotest research points that enzymic method is being utilized to modify starches instead of chemical method existing certain safety problems at home and abroad. However, different internal structure of starches can produce different modified starches using enzymic method, leading to lower application stability. So, how to purposefully modify different starch chain sturctures using enzymic method to prepare green modified starches with desired and stable qualities has become an urgent issue to be resolved. In this study, in order to establish and select the excellently mutated branching enzyme (BE) with high enzymatic activity, heat stability and product specificity, the site-directed mutagenesis will firstly be carried out to mutate four amino acid residues located at the active center in BE from E. coli BL21(DE3)/pET20b-BE identified in our previous research. Secondly, the key point of starch internal structure- the smallest building blocks of amylopectin will be as a breakthrough point, the optimal structure parameters of building blocks that are positively correlated with the excellent physicochemical and slowly digestible properties of starches will be determined, and they will be as an important theoretical basis for starches modified by using the excellently mutated BE. Finally, on the basis of the optimal structure parameters of building blocks, the key regulation factors in enzymatic reaction will be exploited and located, and then the regulatory mechanism that starches can be directionally converted to target product specificity possessing optimal building blocks by using mutated BE will be established. It will supply a theoretical and technological foundation for preparing green modified starches with the desired quality.