Biomacromolecules. 2019 Mar 11;20(3):1178-1189. doi: 10.1021/acs.biomac.8b01503. Epub 2019 Feb 4.
Elastin-like polypeptides (ELPs) are thermoresponsive biopolymers that undergo an LCST-like phase transition in aqueous solutions. The temperature of this LCST-like transition, T , can be tuned by varying the number of repeat units in the ELP, sequence and composition of the repeat units, the solution conditions, and via conjugation to other biomacromolecules. In this study, we show how and why the choice of guest (X) residue in the VPGXG pentad repeat tunes the T of short ELPs, (VPGXG), in the free state and when conjugated to collagen-like peptides (CLPs). In experiments, the (VPGWG) chain (in short, WWWW) has a T < 278 K, while (VPGFG) or FFFF has a T > 353 K in both free ELP and ELP-CLP systems. The T for the FWWF ELP sequence decreases from being >353 K for free ELP to <278 K for the corresponding ELP-CLP system. The decrease in T upon conjugation to CLP has been shown to be due to the crowding of ELP chains that decreases the entropic loss upon ELP aggregation. Even though the net hydrophobicity of ELP has been reasoned to drive the T , the origins of lower T of WWWW compared to FFFF are unclear, as there is disagreement in hydrophobicity scales in how phenylalanine (F) compares to tryptophan (W). Motivated by these experimental observations, we use a combination of atomistic and coarse-grained (CG) molecular dynamics simulations. Atomistic simulations of free and tethered ELPs show that WWWW are more prone to acquire β-turn structures than FFFF at lower temperatures. Also, the atomistically informed CG simulations show that the increased local stiffness in W than F due to the bulkier side chain in W compared to F, alone does not cause the shift in the transition of WWWW versus FFFF. The experimentally observed lower T of WWWW than FFFF is achieved in CG simulations only when the CG model incorporates both the atomistically informed local stiffness and stronger effective attractions localized at the W position versus the F position. The effective interactions localized at the guest residue in the CG model is guided by our atomistically observed increased propensity for β-turn structure in WWWW versus FFFF and by past experimental work of Urry et al. quantifying hydrophobic differences through enthalpy of association for W versus F.
弹性蛋白样多肽（ELPs）是热响应性生物聚合物，在水溶液中经历类似 LCST 的相转变。这种类似 LCST 的转变温度 T 可以通过改变 ELP 中的重复单元数量、重复单元的序列和组成、溶液条件以及与其他生物大分子的缀合来调节。在这项研究中，我们展示了 VPGXG 五肽重复中的客体（X）残基的选择如何以及为什么会调节游离状态和与胶原样肽（CLP）缀合时短 ELPs（VPGXG）的 T。在实验中，（VPGWG）链（简称 WWWW）在游离 ELP 和 ELP-CLP 系统中 T < 278 K，而（VPGFG）或 FFFF 的 T > 353 K。对于 FWWF ELP 序列，从游离 ELP 的 T > 353 K 降低到相应的 ELP-CLP 系统的 T < 278 K。与 CLP 缀合后 T 的降低已被证明是由于 ELP 链的拥挤导致 ELP 聚集时的熵损失减少。尽管已经推理出 ELP 的净疏水性驱动 T ，但与 FFFF 相比，WWWF 较低的 T 的起源尚不清楚，因为在疏水性尺度上，苯丙氨酸（F）与色氨酸（W）的比较存在分歧。受这些实验观察的启发，我们使用了原子和粗粒（CG）分子动力学模拟的组合。自由和键合 ELP 的原子模拟表明，在较低温度下，WWWF 比 FFFF 更容易获得 β-转角结构。此外，原子信息丰富的 CG 模拟表明，由于 W 相对于 F 的较大侧链，与 F 相比，W 中增加的局部刚度不会导致 WWWW 与 FFFF 的转变发生偏移。仅当 CG 模型包含原子观测到的局部刚度增加以及与 F 位置相比在 W 位置处局部化的更强有效吸引力时，在 CG 模拟中才能实现实验观察到的 WWWW 比 FFFF 更低的 T。CG 模型中位于客体残基处的有效相互作用由我们原子观测到的 WWWW 与 FFFF 相比 β-转角结构增加的倾向以及 Urry 等人通过定量测量 W 与 F 的结合焓来量化疏水性差异的过去实验工作指导。
