Lu S, Reyes V E, Lew R A, Anderson J, Mole J, Humphreys R E, Ciardelli T
Department of Pharmacology, University of Massachusetts Medical School, Worcester 01655.
J Immunol. 1990 Aug 1;145(3):899-904.
We tested the hypothesis that the recurrence of hydrophobic amino acids in a polypeptide at positions falling in an axial, hydrophobic strip if the sequence were coiled as an alpha helix, can lead to helical nucleation on a hydrophobic surface. The hydrophobic surface could anchor such residues, whereas the peptide sequence grows in a helical configuration that is stabilized by hydrogen bonds among carbonyl and amido NH groups along the peptidyl backbone of the helix, and by other intercycle interactions among amino acid side chains. Such bound, helical structures might protect peptides from proteases and/or facilitate transport to a MHC-containing compartment and thus be reflected in the selection of T cell-presented segments. Helical structure in a series of HPLC-purified peptides was estimated from circular dichroism measurements in: 1) 0.01 M phosphate buffer, pH 7.0, 2) that buffer with 45% trifluoroethanol (TFE), and 3) that buffer with di-O-hexadecyl phosphatidylcholine vesicles. By decreasing the dielectric constant of the buffer, TFE enhances intrapeptide interactions generally, whereas the lipid vesicles only provide a surface for hydrophobic interactions. The peptides varied in their strip-of-helix hydrophobicity indices (SOHHI; the mean Kyte-Doolittle hydrophobicities of residues in an axial strip of an alpha helix) and in proline content. Structural order for peptides with helical circular dichroism spectra was estimated as percentage helicity from circular dichroism theta 222 nm values and peptide concentration. A prototypic alpha helical peptide with three cycles plus two amino acids and an axial hydrophobic strip of four leucyl residues (SOHHI = 3.8) was disordered in phosphate buffer, 58% helical in that buffer with 48% TFE, and 36% helical in that buffer with vesicles. Percentage helicity in the presence of vesicles of the subset of peptides without proline followed their SOHHI values. Peptides with multiple prolyl residues had circular dichroism spectra with strong signals, but since they did not have altered spectra in the presence of vesicles relative to phosphate buffer alone, the hydrophobic surface of the vesicle did not appear to stabilize those structures.
如果多肽序列盘绕成α螺旋，那么落在轴向疏水条带上的位置处出现的疏水性氨基酸的重复出现，可能会导致在疏水表面上形成螺旋成核。疏水表面可以锚定这些残基，而肽序列则以螺旋构型生长，该构型通过沿着螺旋肽基主链的羰基和酰胺基NH基团之间的氢键以及氨基酸侧链之间的其他环间相互作用得以稳定。这种结合的螺旋结构可能会保护肽免受蛋白酶的作用和/或促进其转运至含有MHC的区室，从而在T细胞呈递片段的选择中得以体现。通过圆二色性测量来估计一系列经高效液相色谱纯化的肽的螺旋结构：1）在pH 7.0的0.01 M磷酸盐缓冲液中；2）在含有45%三氟乙醇（TFE）的该缓冲液中；3）在含有二 - O - 十六烷基磷脂酰胆碱囊泡的该缓冲液中。通过降低缓冲液的介电常数，TFE通常会增强肽内相互作用，而脂质囊泡仅为疏水相互作用提供一个表面。这些肽在其螺旋条带疏水指数（SOHHI；α螺旋轴向条带中残基的平均Kyte - Doolittle疏水性）和脯氨酸含量方面有所不同。具有螺旋圆二色光谱的肽的结构有序度根据圆二色性θ222 nm值和肽浓度估计为螺旋度百分比。一个具有三个环加两个氨基酸以及四个亮氨酰残基的轴向疏水条带（SOHHI = 3.8）的原型α螺旋肽在磷酸盐缓冲液中无序，在含有48% TFE的该缓冲液中为58%螺旋，在含有囊泡的该缓冲液中为36%螺旋。不含脯氨酸的肽子集在存在囊泡时的螺旋度百分比遵循其SOHHI值。具有多个脯氨酰残基的肽具有带有强信号的圆二色光谱，但由于它们在存在囊泡时相对于单独的磷酸盐缓冲液没有改变光谱，囊泡的疏水表面似乎并未稳定那些结构。
