Murkin Andrew S, Tanner Martin E
Contribution from the Department of Chemistry, University of British Columbia, Vancouver, British Columbia, V6T 1Z1, Canada.
J Org Chem. 2002 Nov 29;67(24):8389-94. doi: 10.1021/jo0204653.
Ribosomally produced peptides that contain D-amino acids have been isolated from a number of vertebrate and invertebrate sources. In each case, the D-amino acids are introduced by a posttranslational modification of a parent peptide containing only amino acids of the L-configuration. The only known enzyme to catalyze such a reaction is the peptide epimerase (also known as peptide isomerase) from the venom of the funnel web spider, Agelenopsis aperta. This enzyme interconverts two 48-amino-acid-long peptide toxins that differ only by the stereochemistry at a single serine residue. In this paper we report the synthesis and testing of two pentapeptide analogues that contain modified amino acids at the site normally occupied by the substrate serine residue. When the L-chloroalanine-containing peptide 3 was incubated with the epimerase it was converted into the dehydroalanine-containing peptide 4 via an elimination of HCl. The dehydroalanine peptide 4 was independently synthesized and found to act as a potent inhibitor of the epimerase (IC50 = 0.5 microM). These results support a direct deprotonation/reprotonation mechanism in which a carbanionic intermediate is formed. The observed inhibition by 4 can be attributed to the sp(2)-hybridization of the alpha-carbon in the dehydroalanine unit that mimics the planar geometry of the anionic intermediate.
已从多种脊椎动物和无脊椎动物来源中分离出含有D-氨基酸的核糖体合成肽。在每种情况下，D-氨基酸都是通过对仅含有L-构型氨基酸的亲本肽进行翻译后修饰而引入的。唯一已知催化此类反应的酶是来自漏斗网蜘蛛（Agelenopsis aperta）毒液中的肽表异构酶（也称为肽异构酶）。该酶可使两种仅在单个丝氨酸残基的立体化学上不同的48个氨基酸长的肽毒素相互转化。在本文中，我们报告了两种五肽类似物的合成和测试，它们在通常被底物丝氨酸残基占据的位点含有修饰的氨基酸。当将含L-氯丙氨酸的肽3与表异构酶一起孵育时，它通过消除HCl转化为含脱氢丙氨酸的肽4。含脱氢丙氨酸的肽4是独立合成的，并且发现它是表异构酶的有效抑制剂（IC50 = 0.5 microM）。这些结果支持一种直接去质子化/再质子化机制，其中形成了碳负离子中间体。观察到的4的抑制作用可归因于脱氢丙氨酸单元中α-碳的sp(2)杂化，它模拟了阴离子中间体的平面几何形状。
