Cuesta-Seijo Jose A, Nielsen Morten M, Marri Lucia, Tanaka Hidenori, Beeren Sophie R, Palcic Monica M
Carlsberg Laboratory, Gamle Carlsberg Vej 10, 1799 Copenhagen V, Denmark.
Acta Crystallogr D Biol Crystallogr. 2013 Jun;69(Pt 6):1013-25. doi: 10.1107/S090744491300440X. Epub 2013 May 14.
Starch, a polymer of glucose, is the major source of calories in the human diet. It has numerous industrial uses, including as a raw material for the production of first-generation bioethanol. Several classes of enzymes take part in starch biosynthesis, of which starch synthases (SSs) carry out chain elongation of both amylose and amylopectin. Plants have five classes of SS, each with different roles. The products of the reaction of SS are well known, but details of the reaction mechanism remain obscure and even less is known of how different SSs select different substrates for elongation, how they compete with each other and how their activities are regulated. Here, the first crystal structure of a soluble starch synthase is presented: that of starch synthase I (SSI) from barley refined to 2.7 Å resolution. The structure captures an open conformation of the enzyme with a surface-bound maltooligosaccharide and a disulfide bridge that precludes formation of the active site. The maltooligosaccharide-binding site is involved in substrate recognition, while the disulfide bridge is reflective of redox regulation of SSI. Activity measurements on several SSI mutants supporting these roles are also presented.
淀粉是葡萄糖的聚合物，是人类饮食中热量的主要来源。它有许多工业用途，包括作为生产第一代生物乙醇的原料。几类酶参与淀粉生物合成，其中淀粉合酶（SSs）负责直链淀粉和支链淀粉的链延伸。植物有五类SS，每类都有不同的作用。SS反应的产物是众所周知的，但反应机制的细节仍不清楚，对于不同的SS如何选择不同的底物进行延伸、它们如何相互竞争以及它们的活性如何调节，人们了解得更少。在此，展示了可溶性淀粉合酶的首个晶体结构：来自大麦的淀粉合酶I（SSI）的晶体结构，分辨率达到2.7 Å。该结构捕捉到了酶的开放构象，带有一个表面结合的麦芽寡糖和一个阻止活性位点形成的二硫键。麦芽寡糖结合位点参与底物识别，而二硫键反映了SSI的氧化还原调节。还展示了对几个支持这些作用的SSI突变体的活性测量结果。
