Shi Tiesheng, Spain Stephen M, Rabenstein Dallas L
Department of Chemistry, University of California, Riverside, California 92521, USA.
J Am Chem Soc. 2004 Jan 28;126(3):790-6. doi: 10.1021/ja030311k.
Acyclic dithiol and cyclic disulfide forms of the peptides Ac-Cys-Pro-Xaa-Cys-NH2 (Xaa = Phe, His, Tyr, Gly, and Thr) and Ac-Cys-Gly-Pro-Cys-NH2 and the peptide Ac-Ala-Gly-Pro-Ala-NH2 were synthesized and characterized by mass spectrometry and NMR spectroscopy. Rate constants kct and ktc for cis-to-trans and trans-to-cis isomerization, respectively, across the Cys-Pro or Gly-Pro peptide bonds were determined by magnetization transfer NMR techniques over a range of temperatures, and activation parameters were derived from the temperature dependence of the rate constants. It was found that constraints imposed by the disulfide bond confer an unexpected rate enhancement for cis/trans isomerization, ranging from a factor of 2 to 13. It is proposed that the rate enhancements are a result of an intramolecular catalysis mechanism in which the NH proton of the Pro-Xaa peptide bond hydrogen bonds to the proline nitrogen in the transition state. The peptides Ac-Cys-Pro-Xaa-Cys-NH2 and Ac-Cys-Gly-Pro-Cys-NH2 are model compounds for proline-containing active sites of the thioredoxin superfamily of oxidoreductase enzymes; the results suggest that the backbones of the active sites of the oxidized form of these enzymes may have unusual conformational flexibility.
合成了肽Ac-Cys-Pro-Xaa-Cys-NH2（Xaa = Phe、His、Tyr、Gly和Thr）、Ac-Cys-Gly-Pro-Cys-NH2的无环二硫醇和环状二硫化物形式以及肽Ac-Ala-Gly-Pro-Ala-NH2，并通过质谱和核磁共振光谱对其进行了表征。通过磁化转移核磁共振技术在一系列温度范围内测定了分别跨越Cys-Pro或Gly-Pro肽键的顺式-反式和顺式-反式异构化的速率常数kct和ktc，并从速率常数的温度依赖性推导出活化参数。结果发现，二硫键施加的限制使顺式/反式异构化的速率意外提高，提高幅度在2到13倍之间。有人提出，速率提高是分子内催化机制的结果，其中Pro-Xaa肽键的NH质子在过渡态与脯氨酸氮形成氢键。肽Ac-Cys-Pro-Xaa-Cys-NH2和Ac-Cys-Gly-Pro-Cys-NH2是氧化还原酶硫氧还蛋白超家族含脯氨酸活性位点的模型化合物；结果表明，这些酶氧化形式活性位点的主链可能具有不同寻常的构象灵活性。
