Gasymov O K, Abduragimov A R, Yusifov T N, Glasgow B J
Departments of Pathology and Ophthalmology, UCLA School of Medicine, 100 Stein Plaza, Los Angeles, CA 90095, USA.
Biochim Biophys Acta. 1998 Jul 28;1386(1):145-56. doi: 10.1016/s0167-4838(98)00092-2.
Structural and conformational changes in tear lipocalins were detected in association with ligand binding and release. Circular dichroism measurements demonstrated that ligand binding induces beta structure formation, aromatic side chain asymmetry, and a more rigid state in tear lipocalins (TL). The exposure of the tyrosyl component is less in apo-TL than in holo-TL. The sole tryptophan residue, Trp17, is buried in both holo- and apo-TL. The steady state exposure of Trp17 is the same in holo- and apo-TL, but the dynamic exposure is two-fold greater in apo-TL. Maneuvers to unfold the protein with urea or incubation in an acidic environment resulted in increased exposure of aromatic amino acids. Electron paramagnetic resonance studies verified that lipids are liberated from TL in an acidic environment. Acidic pH promotes conformational changes in TL involving aromatic residues, particularly the conserved residue Trp17. These changes are associated with lipid release. The liberation of lipid from the cavity of TL under acidic conditions involves a molten globule state of the protein. We postulate that TL, exposed to the steep surface pH gradient that exists at lipid-aqueous interfaces, would release lipid in association with a molten globule transition. The data suggest a plausible regulatory mechanism for lipid delivery from lipocalins at the tear film surface.
在与配体结合和释放相关的过程中，检测到泪液脂质运载蛋白的结构和构象变化。圆二色性测量表明，配体结合会诱导泪液脂质运载蛋白（TL）中β结构的形成、芳香族侧链不对称性以及更刚性的状态。与全TL相比，脱辅基TL中酪氨酰成分的暴露较少。唯一的色氨酸残基Trp17在全TL和脱辅基TL中均被埋藏。Trp17的稳态暴露在全TL和脱辅基TL中相同，但动态暴露在脱辅基TL中是全TL的两倍。用尿素使蛋白质展开或在酸性环境中孵育会导致芳香族氨基酸的暴露增加。电子顺磁共振研究证实，在酸性环境中脂质从TL中释放出来。酸性pH促进TL中涉及芳香族残基，特别是保守残基Trp17的构象变化。这些变化与脂质释放有关。在酸性条件下，脂质从TL腔中释放涉及蛋白质的熔球态。我们推测，暴露于脂质 - 水界面存在的陡峭表面pH梯度下的TL，会伴随着熔球态转变释放脂质。这些数据表明了泪膜表面脂质运载蛋白脂质递送的一种合理调节机制。
