Departments of †Bioengineering, ‡Medicinal Chemistry, and §Human Genetics, University of Utah , Salt Lake City, Utah 84112, United States.
Biomacromolecules. 2013 Oct 14;14(10):3668-81. doi: 10.1021/bm401036z. Epub 2013 Oct 3.
Caddisflies are aquatic relatives of silk-spinning terrestrial moths and butterflies. Casemaker larvae spin adhesive silk fibers for underwater construction of protective composite cases. The central region of Hesperophylax sp. H-fibroin contains a repeating pattern of three conserved subrepeats, all of which contain one or more (SX)n motifs with extensively phosphorylated serines. Native silk fibers were highly extensible and displayed a distinct yield point, force plateau, and load cycle hysteresis. FTIR spectroscopy of native silk showed a conformational mix of random coil, β-sheet, and turns. Exchanging multivalent ions with Na(+) EDTA disrupted fiber mechanics, shifted the secondary structure ratios from antiparallel β-sheet toward random coil and turns, and caused the fibers to shorten, swell in diameter, and disrupted fiber birefringence. The EDTA effects were reversed by restoring Ca(2+). Molecular dynamic simulations provided theoretical support for a hypothetical structure in which the (pSX)n motifs may assemble into two- and three-stranded, Ca(2+)-stabilized β-sheets.
石蛾是水生的，与吐丝的陆地蛾和蝴蝶有亲缘关系。石蛾幼虫吐出粘性的丝纤维，用于水下建造保护性的复合外壳。Hesperophylax sp. H-丝心蛋白的中心区域包含三个保守亚重复的重复模式，所有这些亚重复都包含一个或多个富含磷酸丝氨酸的(SX)n 基序。天然丝纤维具有很高的可拉伸性，并显示出明显的屈服点、力平台和负载循环滞后。天然丝的傅里叶变换红外光谱显示了无规卷曲、β-折叠和转角的构象混合。用 Na(+) EDTA 交换多价离子会破坏纤维力学性能，使二级结构比例从反平行β-折叠向无规卷曲和转角转变，并导致纤维缩短、直径膨胀和纤维双折射破坏。EDTA 的影响可以通过恢复 Ca(2+)来逆转。分子动力学模拟为一个假设结构提供了理论支持，在该结构中，(pSX)n 基序可能组装成两条和三条链、Ca(2+)稳定的β-折叠。
