Lambert R, Brownlie P D, Woodcock S C, Louie G V, Cooper J C, Warren M J, Jordan P M, Blundell T L, Wood S P
Department of Crystallography, Birkbeck College, London, UK.
Ciba Found Symp. 1994;180:97-104; discussion 105-10. doi: 10.1002/9780470514535.ch6.
The X-ray crystallographic analysis of porphobilinogen deaminase (hydroxymethylbilane synthase, EC 4.3.1.8) shows the polypeptide chain folded into three domains, (1) N-terminal, (2) central and (3) C-terminal, of approximately equal size. Domains 1 and 2 have a similar overall topology, a modified doubly wound parallel beta-sheet. Domain 3 is an open-faced three-stranded antiparallel beta-sheet, with one face covered by three alpha-helices. The active site is located between domains 1 and 2. The dipyrromethane cofactor linked to cysteine 242 protrudes from domain 3 into the mouth of the cleft. Flexible segments between domains 1 and 2 are thought to have a role in a hinge mechanism, facilitating conformational changes. The cleft is lined with positively charged, highly conserved, arginine residues which form ion pairs with the acidic side chains of the cofactor. Aspartic acid 84 has been identified as a critical catalytic residue both by its proximity to the cofactor pyrrole ring nitrogen and by structural and kinetic studies of the Asp-84-->Glu mutant protein. The active site arginine residues have been altered by site-directed mutagenesis to histidine residues. The mutant proteins have been studied crystallographically in order to reconcile the functional changes in the polymerization reaction with structural changes in the enzyme.
对胆色素原脱氨酶（羟甲基胆素合酶，EC 4.3.1.8）的X射线晶体学分析表明，多肽链折叠成三个大小大致相等的结构域：（1）N端结构域，（2）中央结构域和（3）C端结构域。结构域1和2具有相似的整体拓扑结构，即一种经过修饰的双股平行β折叠片层。结构域3是一个开放面的三股反平行β折叠片层，其中一面被三个α螺旋覆盖。活性位点位于结构域1和2之间。与半胱氨酸242相连的二吡咯甲烷辅因子从结构域3突出到裂隙口。结构域1和2之间的柔性片段被认为在一种铰链机制中起作用，促进构象变化。裂隙内衬有带正电荷、高度保守的精氨酸残基，它们与辅因子的酸性侧链形成离子对。天冬氨酸84已被确定为关键催化残基，这是基于其与辅因子吡咯环氮的接近程度以及对Asp-84→Glu突变蛋白的结构和动力学研究。通过定点诱变将活性位点的精氨酸残基改变为组氨酸残基。已对突变蛋白进行晶体学研究，以便将聚合反应中的功能变化与酶的结构变化相协调。
