Crystal Structure of S-ovalbumin as a Non-loop-inserted Thermostabilized Serpin Form*
Crystal Structure of S-ovalbumin as a Non-loop-inserted Thermostabilized Serpin Form*
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DOI：
10.1074/jbc.m305926200
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发表时间：
2003-09
影响因子：
4.8
通讯作者：
M. Yamasaki;N. Takahashi;M. Hirose
中科院分区：
文献类型：
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作者：
M. Yamasaki;N. Takahashi;M. Hirose
卵清蛋白是丝氨酸蛋白酶抑制剂（serpin）的非抑制性成员，在升高的pH条件下转化为热稳定形式S-卵清蛋白。S-卵清蛋白形成的结构机制一直是食品科学和丝氨酸蛋白酶抑制剂结构生物学中的一个难题。基于通常观察到的丝氨酸蛋白酶抑制剂通过将反应性中心环插入近端β-折叠而实现热稳定，最广泛接受的假设模型包括部分环插入。在这里，我们证明，第一次，在1.9-A的分辨率的S-卵清蛋白的晶体结构。该结构明确排除了部分环插入机制;整体结构，包括反应中心环结构，几乎与天然卵清蛋白相同，除了链1A的前一环远离链2A的显著运动。最引人注目的发现是Ser-164、Ser-236和Ser-320采取d-氨基酸残基构型。这些化学转化可以直接与从天然卵清蛋白到S-卵清蛋白的转化的不可逆和逐步性质有关。随着侧链构象的变化，Phe-99的χ1和Met-241的χ3值发生了显著的变化。前一种构象变化导致丝氨酸蛋白酶抑制剂中高度保守的Phe-180和Phe-378周围疏水核心的溶剂可及性降低。这可能会给S-卵清蛋白的结构稳定性的热力学优势。
Ovalbumin, a non-inhibitory member of serine proteinase inhibitors (serpin), is transformed into a heat-stabilized form, S-ovalbumin, under elevated pH conditions. The structural mechanism for the S-ovalbumin formation has long been a puzzling question in food science and serpin structural biology. On the basis of the commonly observed serpin thermostabilization by insertion of the reactive center loop into the proximal β-sheet, the most widely accepted hypothetical model has included partial loop insertion. Here we demonstrate, for the first time, the crystal structure of S-ovalbumin at 1.9-Å resolution. This structure unequivocally excludes the partial loop insertion mechanism; the overall structure, including the reactive center loop structure, is almost the same as that of native ovalbumin, except for the significant motion of the preceding loop of strand 1A away from strand 2A. The most striking finding is that Ser-164, Ser-236, and Ser-320 take the d-amino acid residue configuration. These chemical inversions can be directly related to the irreversible and stepwise nature of the transformation from native ovalbumin to S-ovalbumin. As conformational changes of the side chains, significant alternations are found in the values of the χ1 of Phe-99 and the χ3 of Met-241. The former conformational change leads to the decreased solvent accessibility of the hydrophobic core around Phe-99, which includes Phe-180 and Phe-378, the highly conserved residues in serpin. This may give a thermodynamic advantage to the structural stability of S-ovalbumin.