Department of Chemistry, Université de Montréal, P.O. Box 6128, Downtown Station, Montréal QC H3C 3J7, Canada.
J Am Chem Soc. 2011 Aug 17;133(32):12493-506. doi: 10.1021/ja203007u. Epub 2011 Jul 21.
The cluster of differentiation 36 (CD36) class B scavenger receptor binds a variety of biologically endogenous ligands in addition to synthetic peptides (i.e., growth hormone-releasing peptides, GHRPs), which modulate biological function related to anti-angiogenic and anti-atherosclerotic activities. Affinity labeling had previously shown that GHRP-6 analogues such as hexarelin, [2-Me-W(2)]GHRP-6 (1), bind to the lysine-rich domain of the CD36 receptor. Moreover, the azapeptide analogue [aza-F(4)]GHRP-6, 2, exhibited a characteristic β-turn conformation as described by CD and NMR spectroscopy and a slightly higher CD36 binding affinity relative to hexarelin (1.34 and 2.37 μM, respectively), suggesting receptor binding was mediated by the conformation and the aromatic residues of these peptide sequences. Ligand-receptor binding interactions were thus explored using azapeptides to examine influences of side-chain diversity and backbone conformation. In particular, considering that aromatic cation interactions may contribute to binding affinity, we have explored the potential of introducing salt bridges to furnish GHRP-6 azapeptide ligands of the CD36 receptor. Fifteen aza-glutamic acid analogues related to 2 were prepared by submonomer solid-phase synthesis. The azapeptide side chains were installed by novel approaches featuring alkylation of resin-bound semicarbazone with Michael acceptors and activated allylic acetates in the presence of phosphazene base (BTPP). Moreover, certain Michael adducts underwent intramolecular cyclization during semicarbazone deprotection, leading to novel pyrrazoline and aza-pyroglutamate N-terminal residues. Structural studies indicated that contingent on sequence the [aza-Glu]GHRP-6 analogues exhibited CD spectra characteristic of random coil, polyproline type II and β-turn secondary structures in aqueous media. In covalent competition binding studies with the GHRP-6 prototype hexarelin bearing a radiotracer, certain [aza-Glu]GHRP-6 azapeptides retained relatively high (2-27 μM) affinity for the CD36 scavenger receptor.
簇分化 36（CD36）B 类清道夫受体除了合成肽（即生长激素释放肽，GHRPs）之外，还结合多种生物内源性配体，这些配体调节与抗血管生成和抗动脉粥样硬化活性相关的生物学功能。亲和标记先前表明，GHRP-6 类似物，如 hexarelin，[2-Me-W(2)]GHRP-6（1），结合 CD36 受体的赖氨酸丰富域。此外，aza 肽类似物[aza-F(4)]GHRP-6，2，表现出由 CD 和 NMR 光谱描述的特征β-转角构象，并且相对于 hexarelin（分别为 1.34 和 2.37 μM）具有略高的 CD36 结合亲和力，这表明受体结合是由这些肽序列的构象和芳香残基介导的。因此，使用 aza 肽探索配体-受体结合相互作用，以检查侧链多样性和骨架构象的影响。特别是，考虑到芳香阳离子相互作用可能有助于结合亲和力，我们已经探索了引入盐桥以提供 CD36 受体的 GHRP-6 aza 肽配体的潜力。通过亚单体固相合成制备了与 2 相关的 15 种 aza-谷氨酸类似物。通过新颖的方法在膦氮杂环戊二烯（BTPP）存在下用迈克尔受体烷基化树脂结合的半卡巴腙和活化的烯丙基乙酸酯来安装 aza 肽侧链。此外，某些迈克尔加合物在半卡巴腙脱保护期间经历分子内环化，导致新的吡唑啉和 aza-吡咯烷酮 N-末端残基。结构研究表明，取决于序列，[aza-Glu]GHRP-6 类似物在水性介质中表现出随机卷曲、多脯氨酸 II 型和β-转角二级结构的 CD 光谱特征。在与带有放射性示踪剂的 GHRP-6 原型 hexarelin 的共价竞争结合研究中，某些[aza-Glu]GHRP-6 aza 肽保留了相对较高的（2-27 μM）CD36 清道夫受体亲和力。
