Department of Applied Life Sciences, Graduate School of Agriculture, Kyoto University, Uji, Kyoto 611-0011, Japan.
Chem Biodivers. 2010 Jun;7(6):1634-43. doi: 10.1002/cbdv.200900305.
During storage of shell eggs, an egg-white protein, ovalbumin is converted into a molecular species with higher thermostability (Delta T(m)=8 degrees), which is named as S-ovalbumin. Since the pH of egg white is raised to 9.5 in that process, particular chemical modifications could occur. Although any evidence for relationship between chemical modifications and the thermostabilization had not been established, our X-ray crystallographic model for S-ovalbumin demonstrated that three serine residues (S164, S236, and S320) might be in D-configuration. Then, whether these serine residues contribute to the thermostabilization in S-ovalbumin is investigated by protein-engineering approach. By alkaline treatment, S236G mutant was converted to a more thermostable form, as well as ovalbumin has been converted to S-ovalbumin. Hence, the Ser236 is regarded to have no contribution to S-ovalbumin formation. On the contrary, S164G and S320G did show accelerated conversion to thermostable forms in comparison with ovalbumin or wild-type recombinant protein. When these residues were replaced with valine, which has a slower racemization rate, Delta T(m) after alkaline treatment had become half of that for the wild-type recombinant ovalbumin. Therefore, the process of S-ovalbumin formation is deduced to consist of two steps, which is closely related to the two serine residues.
在贮藏鸡蛋过程中，蛋清蛋白卵清白蛋白转化为具有更高热稳定性的分子物种（Delta T(m)=8 度），称为 S-卵清白蛋白。在此过程中，蛋清的 pH 值升高到 9.5，可能会发生特定的化学修饰。尽管尚未建立化学修饰与热稳定性之间的关系的任何证据，但我们 S-卵清白蛋白的 X 射线晶体结构模型表明，三个丝氨酸残基（S164、S236 和 S320）可能处于 D-构型。然后，通过蛋白质工程方法研究这些丝氨酸残基是否对 S-卵清白蛋白的热稳定性有贡献。通过碱性处理，S236G 突变体转化为更热稳定的形式，而卵清白蛋白也转化为 S-卵清白蛋白。因此，丝氨酸 236 被认为对 S-卵清白蛋白的形成没有贡献。相反，与卵清白蛋白或野生型重组蛋白相比，S164G 和 S320G 确实显示出向热稳定形式的转化加速。当这些残基被具有较慢外消旋化速率的缬氨酸取代时，碱性处理后的 Delta T(m) 变为野生型重组卵清白蛋白的一半。因此，S-卵清白蛋白的形成过程被推断为包括两个步骤，这与两个丝氨酸残基密切相关。
