Cuesta-Seijo Jose A, Nielsen Morten M, Ruzanski Christian, Krucewicz Katarzyna, Beeren Sophie R, Rydhal Maja G, Yoshimura Yayoi, Striebeck Alexander, Motawia Mohammed S, Willats William G T, Palcic Monica M
Carlsberg Research Laboratory Copenhagen, Denmark.
Department of Plant and Environmental Sciences, University of Copenhagen Frederiksberg, Copenhagen, Denmark.
Front Plant Sci. 2016 Jan 28;6:1265. doi: 10.3389/fpls.2015.01265. eCollection 2015.
Starch is the main storage polysaccharide in cereals and the major source of calories in the human diet. It is synthesized by a panel of enzymes including five classes of starch synthases (SSs). While the overall starch synthase (SS) reaction is known, the functional differences between the five SS classes are poorly understood. Much of our knowledge comes from analyzing mutant plants with altered SS activities, but the resulting data are often difficult to interpret as a result of pleitropic effects, competition between enzymes, overlaps in enzyme activity and disruption of multi-enzyme complexes. Here we provide a detailed biochemical study of the activity of all five classes of SSs in barley endosperm. Each enzyme was produced recombinantly in E. coli and the properties and modes of action in vitro were studied in isolation from other SSs and other substrate modifying activities. Our results define the mode of action of each SS class in unprecedented detail; we analyze their substrate selection, temperature dependence and stability, substrate affinity and temporal abundance during barley development. Our results are at variance with some generally accepted ideas about starch biosynthesis and might lead to the reinterpretation of results obtained in planta. In particular, they indicate that granule bound SS is capable of processive action even in the absence of a starch matrix, that SSI has no elongation limit, and that SSIV, believed to be critical for the initiation of starch granules, has maltoligosaccharides and not polysaccharides as its preferred substrates.
淀粉是谷物中的主要储存多糖，也是人类饮食中热量的主要来源。它由一组酶合成，包括五类淀粉合酶（SSs）。虽然淀粉合酶（SS）的总体反应是已知的，但对这五类SS之间的功能差异了解甚少。我们的许多知识来自对具有改变的SS活性的突变植物的分析，但由于多效性效应、酶之间的竞争、酶活性重叠以及多酶复合物的破坏，所得数据往往难以解释。在这里，我们对大麦胚乳中所有五类SS的活性进行了详细的生化研究。每种酶都在大肠杆菌中重组产生，并在与其他SS和其他底物修饰活性分离的情况下研究其体外性质和作用模式。我们的结果以前所未有的细节定义了每类SS的作用模式；我们分析了它们在大麦发育过程中的底物选择、温度依赖性和稳定性、底物亲和力以及时间丰度。我们的结果与一些关于淀粉生物合成的普遍接受的观点不一致，可能会导致对在植物中获得的结果进行重新解释。特别是，它们表明颗粒结合型SS即使在没有淀粉基质的情况下也能够进行连续作用，SSI没有延伸极限，并且被认为对淀粉颗粒起始至关重要的SSIV，其优选底物是麦芽寡糖而非多糖。
