Institute of Bioorganic Chemistry, Polish Academy of Sciences, Noskowskiego 12-14, 61-704 Poznań, Poland.
Department of Pharmaceutical Technology and Biochemistry, Gdańsk University of Technology, Narutowicza 11/12, 80-233 Gdańsk, Poland.
J Struct Biol. 2019 Mar 1;205(3):26-33. doi: 10.1016/j.jsb.2019.02.001. Epub 2019 Feb 8.
Aminotransferases catalyze reversibly the transamination reaction by a ping-pong bi-bi mechanism with pyridoxal 5'-phosphate (PLP) as a cofactor. Various aminotransferases acting on a range of substrates have been reported. Aromatic transaminases are able to catalyze the transamination reaction with both aromatic and acidic substrates. Two aminotransferases from C. albicans, Aro8p and Aro9p, have been identified recently, exhibiting different catalytic properties. To elucidate the multiple substrate recognition of the two enzymes we determined the crystal structures of an unliganded CaAro8p, a complex of CaAro8p with the PLP cofactor bound to a substrate, forming an external aldimine, CaAro9p with PLP in the form of internal aldimine, and CaAro9p with a mixture of ligands that have been interpreted as results of the enzymatic reaction. The crystal structures of both enzymes contains in the asymmetric unit a biologically relevant dimer of 55 kDa for CaAro8 and 59 kDa for CaAro9p protein subunits. The ability of the enzymes to process multiple substrates could be related to a feature of their architecture in which the active site resides on one subunit while the substrate-binding site is formed by a long loop extending from the other subunit of the dimeric molecule. The separation of the two functions to different chemical entities could facilitate the evolution of the substrate-binding part and allow it to be flexible without destabilizing the conservative catalytic mechanism.
氨基转移酶通过乒乓双机制可逆地催化转氨基反应，其辅因子为吡哆醛 5'-磷酸（PLP）。已经报道了作用于一系列底物的各种氨基转移酶。芳香族氨基转移酶能够催化芳香族和酸性底物的转氨基反应。最近已经鉴定出两种来自 C. albicans 的氨基转移酶，Aro8p 和 Aro9p，它们具有不同的催化特性。为了阐明两种酶的多种底物识别，我们测定了未配位的 CaAro8p、与结合到底物的 PLP 辅因子形成外部醛亚胺的 CaAro8p 复合物、以内部醛亚胺形式存在 PLP 的 CaAro9p 以及被解释为酶反应结果的配体混合物的 CaAro9p 的晶体结构。两种酶的晶体结构在不对称单位中都包含生物相关的二聚体，CaAro8 的 55 kDa 和 CaAro9p 的 59 kDa 是蛋白质亚基。酶处理多种底物的能力可能与其结构特征有关，其中活性位点位于一个亚基上，而底物结合位点由从二聚体分子的另一个亚基延伸的长环形成。两种功能分离到不同的化学实体可以促进底物结合部分的进化，并允许其具有灵活性而不会使保守的催化机制不稳定。
