Borchert T V, Abagyan R, Kishan K V, Zeelen J P, Wierenga R K
European Molecular Biology Laboratory, Meyerhofstrasse 1, Postfach 102209, D69012 Heidelberg, Germany.
Structure. 1993 Nov 15;1(3):205-13. doi: 10.1016/0969-2126(93)90021-8.
The triosephosphate isomerase (TIM) fold is found in several different classes of enzymes, most of which are oligomers; TIM itself always functions as a very tight dimer. It has recently been shown that a monomeric form of TIM ('monoTIM') can be constructed by replacing a 15-residue interface loop, loop-3, with an eight-residue fragment; modelling suggests that this should result in a short strain-free turn, resulting in the subsequent helix, helix-A3, having an additional turn at its amino terminus.
The crystal structure of monoTIM shows that it retains the characteristic TIM-barrel (betaalpha)8-fold and that the new loop has a structure very close to that predicted. Two other interface loops, loop-1 and loop-4, which contain the active site residues Lys13 and His95, respectively, show significant changes in structure in monoTIM compared with dimeric wild-type TIM.
The observed structural differences between monoTIM and wild-type TIM indicate that the dimeric appearance of TIM determines the location and conformation of two of the four catalytic residues.
磷酸丙糖异构酶（TIM）折叠存在于几种不同类型的酶中，其中大多数是寡聚体；TIM本身总是以非常紧密的二聚体形式发挥作用。最近研究表明，通过用一个八残基片段取代一个15残基的界面环（环3），可以构建出TIM的单体形式（“单TIM”）；模型显示，这应该会形成一个无应变的短转角，使得随后的螺旋（螺旋A3）在其氨基末端有一个额外的转角。
单TIM的晶体结构表明，它保留了特征性的TIM桶状（βα）8折叠结构，并且新环的结构与预测的非常接近。另外两个界面环，环1和环4，分别包含活性位点残基赖氨酸13和组氨酸95，与二聚体野生型TIM相比，单TIM中的这两个环在结构上有显著变化。
单TIM和野生型TIM之间观察到的结构差异表明，TIM的二聚体形式决定了四个催化残基中两个残基的位置和构象。
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