The 3-D Structure and Reaction Mechanism in The Highly Organized supra-Molecule of Pyruvate Dehydrogenase Complex.

丙酮酸脱氢酶复合物是由丙酮酸脱氢酶（E1）、二氢脂酰乙酰转移酶（E2）和脂酰胺脱氢酶（E3）三组分酶组成的高度组织的多酶系统之一，特异而高效地催化不可逆的系列反应。根据不同的生物体，这些配合物可分为两种不同对称的E2s组成的中心核心结构。一个在革兰氏阴性菌中具有432对称，另一个在革兰氏阳性菌和真核生物中具有532对称。对从酵母中分离得到的E3的胞囊结构进行了测定。研究发现，晶体有两种不同的结晶习性。用分子置换法求解晶体结构。通过电子密度图的平均化和平面化来改善初始相。用XPLOR在非结晶2重对称约束下，对FRODO构建的分子模型的原子坐标进行了细化。虽然分子包装不同，但分子结构是相似的。与谷胱甘肽还原酶的比较表明，虽然在c端结构域存在较大的原子偏差，但两个结构域之间的活性位点具有几乎相同的三维结构。在具有432对称的不同生物体的E3中也观察到这种相似性。从目前的研究中可以看出，即使两种类型的结构不同，E3s的三级结构在本质上是相同的。值得注意的是，这种结构的强保守性可能是酶的功能抑制。

The pyruvate dehydrogenase complex is one of the highly organized multienzyme system, consisting of multiple copies of three component enzymes, pyruvate dehydrogenase (E1), dihydrolipoyl acetyltransferase (E2), and lipoamide dehydrogenase (E3), which catalyses the irreversible serial reactios specifically and efficiently. The complexes are classified into two types of the central core structures composed of E2s with different symmetries depending on organisms. One has the 432 symmetry in Gram negative bacteria and the other the 532 symmetry in Gram positive bacteria and in eukaryotes. The cyustal structure of the isolated component of E3 from yeast was determined for the latter type. It has been found that there are two types of crystals which are different with each other in crystal habit. The crystal structures were solved by the molecular replacement method. The initial phases were improved by averaging and flattening of electron density map. The atomic coordinates of the molecular model constracted by FRODO were refined by XPLOR with a constrain of non-crystallographic 2-fold symmetry. Although the molecular packings are different, but the molecular structures are similar. Closer comparison with glutathion reductase indicated that although the large atomic deviations are observed in the C-terminal domain, the active site between the two domains has almost the same 3-D structure. This similarity is also observed in E3 from the different organisms with 432 symmetry. From the present investigation, it has been thus revealed that the tertiary structures of E3s are essentially the same even in the different architecture between the two types. It is noticed that such a strong conservation of the structure may be the functional restrain by the enzyme.