Crystal structure of human uroporphyrinogen III synthase
Crystal structure of human uroporphyrinogen III synthase
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DOI：
10.1093/emboj/20.21.5832
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发表时间：
2001-11-01
期刊：
影响因子：
11.4
通讯作者：
Hill, CP
中科院分区：
文献类型：
--
作者：
Mathews, MAA;Schubert, HL;Hill, CP
尿卟啉原III合酶，U3 S，是卟啉生物合成途径中的第四种酶，催化线性四吡咯羟甲基胆烷环化为大环尿卟啉原III，其用于几种不同的途径以形成血红素、西罗血红素、叶绿素、F-430和维生素B-12。U3 S活性在所有生物体中都是必不可少的，而人类的活性降低会导致常染色体隐性遗传疾病先天性红细胞生成性卟啉症。我们已经在1.85埃分辨率下确定了重组人U3 S的晶体结构。蛋白质折叠成由β-梯连接的两个α/β结构域。活性位点似乎位于结构域之间，并且在晶体学上独立的分子之间观察到的相对结构域位置的变化表明在催化循环中可能重要的灵活性的存在。通过突变蛋白质中具有可滴定侧链的四个不变残基中的每一个来探测可能的催化机制。此外，其他六个高度保守和可滴定的侧链也发生了突变。然而，在任何情况下，这些突变都没有消除酶的活性，这表明该机制不需要酸/碱催化。
Uroporphyrinogen III synthase, U3S, the fourth enzyme in the porphyrin biosynthetic pathway, catalyzes cyclization of the linear tetrapyrrole, hydroxymethylbilane, to the macrocyclic uroporphyrino gen III, which is used in several different pathways to form heme, siroheme, chlorophyll, F-430 and vitamin B-12. U3S activity is essential in all organisms, and decreased activity in humans leads to the autosomal recessive disorder congenital erythropoetic porphyria. We have determined the crystal structure of recombinant human U3S at 1.85 Angstrom resolution. The protein folds into two alpha/beta domains connected by a beta -ladder. The active site appears to be located between the domains, and variations in relative domain positions observed between crystallographically independent molecules indicates the presence of flexibility that may be important in the catalytic cycle. Possible mechanisms of catalysis were probed by mutating each of the four invariant residues in the protein that have titratable side chains. Additionally, six other highly conserved and titratable side chains were also mutated. In no case, however, did one of these mutations abolish enzyme activity, suggesting that the mechanism does not require acid/base catalysis.