Structure of Thermus thermophilus HB8 aspartate aminotransferase and its complex with maleate.
Structure of Thermus thermophilus HB8 aspartate aminotransferase and its complex with maleate.
复制标题
嗜热栖热菌 HB8 天冬氨酸转氨酶及其与马来酸复合物的结构。
作者：
T. Nakai;K. Okada;Shohei Akutsu;I. Miyahara;Shinichi Kawaguchi;R. Kato;S. Kuramitsu;K. Hirotsu
用X射线晶体学方法分别测定了嗜热栖热菌HB 8的5 ′-磷酸吡哆醛型天冬氨酸氨基转移酶（AspAT）和5 ′-磷酸吡哆胺型天冬氨酸氨基转移酶（AspAT）与马来酸盐复合物的三维结构。该酶是同二聚体，亚基的多肽链折叠成一个臂、一个小结构域和一个大结构域。来自许多种属的AspAT被分类为氨基转移酶亚组Ia和Ib。该酶属于亚组Ib，其序列与大肠杆菌、猪胞质和鸡线粒体AspAT的一级序列的同一性小于16%，而后者属于亚组Ia，其序列的同一性大于40%，并且其三维结构非常相似，活性位点残基几乎完全保守。对Ⅰ b亚群AspAT的首次X-射线分析表明，Ⅰ a亚群AspAT与Ⅰ b亚群酶的总体结构和活性中心结构基本上是保守的，但两者之间存在两个明显的差异。(1)在AspAT Ia亚组中，底物（或抑制剂）结合诱导小结构域整体大幅移动以关闭活性位点。然而，在亚组Ib的酶中，只有小结构域的N-末端区域（Lys 13-Val 30）接近活性位点以与马来酸酯相互作用。(2)在AspAT亚组Ia中，Arg 292识别底物的侧链羧酸;然而，亚组Ib中酶的残基292不是Arg，并且代替Arg 292，Lys 109与侧链羧酸形成盐桥。该酶的热稳定性至少部分是通过β-转角中Pro残基的高含量和与嗜温AspAT相比分子表面上盐桥数目的显著增加来实现的。
The three-dimensional structures of pyridoxal 5'-phosphate-type aspartate aminotransferase (AspAT) from Thermus thermophilus HB8 and pyridoxamine 5'-phosphate type one in complex with maleate have been determined by X-ray crystallography at 1.8 and 2.6 A resolution, respectively. The enzyme is a homodimer, and the polypeptide chain of the subunit is folded into one arm, one small domain, and one large domain. AspATs from many species were classified into aminotransferase subgroups Ia and Ib. The enzyme belongs to subgroup Ib, its sequence being less than 16% identical to the primary sequences of Escherichia coli, pig cytosolic, and chicken mitochondrial AspATs, which belong to subgroup Ia whose sequences are more than 40% identical and whose three-dimensional structures are quite similar with the active site residues almost completely conserved. The first X-ray analysis of AspAT subgroup Ib indicated that the overall and the active site structures are essentially conserved between the AspATs of subgroup Ia and the enzyme of subgroup Ib, but there are two distinct differences between them. (1) In AspAT subgroup Ia, substrate (or inhibitor) binding induces a large movement of the small domain as a whole to close the active site. However, in the enzyme of subgroup Ib, only the N-terminal region (Lys13-Val30) of the small domain approaches the active site to interact with the maleate. (2) In AspAT subgroup Ia, Arg292 recognizes the side chain carboxylate of the substrate; however, residue 292 of the enzyme in subgroup Ib is not Arg, and in place of Arg292, Lys109 forms a salt bridge with the side chain carboxylate. The thermostability of the enzyme is attained at least in part by the high content of Pro residues in the beta-turns and the marked increase in the number of salt bridges on the molecular surface compared with the mesophilic AspAT.