Tada T, Watanabe Y H, Matsuoka A, Ikeda-Saito M, Imai K, Ni-hei Y, Shikama K
Biological Institute, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan.
Biochim Biophys Acta. 1998 Sep 8;1387(1-2):165-76. doi: 10.1016/s0167-4838(98)00118-6.
Elephant myoglobins both from Asian and African species have a glutamine in place of the usual distal (E7) histidine at position 64. We have isolated native oxymyoglobin directly from the skeletal muscle of African elephant (Loxodonta africana), and examined the autoxidation rate of oxymyoglobin (MbO2) to metmyoglobin (metMb) as a function of pH in 0.1 M buffer at 25 degreesC. As a result, African elephant MbO2 was found to be equally resistant to autoxidation as sperm whale myoglobin. However, the elephant myoglobin exhibited a distinct rate saturation below pH 6. Kinetic analysis of the pH profiles for the autoxidation rate has disclosed that African elephant MbO2 does not show any proton-catalyzed process, such as the one that can play a dominant role in the autoxidation reaction of sperm whale myoglobin by involving the distal histidine as its catalytic residue. Such a greater stability of African elephant MbO2 at low pH could be explained almost completely by the single H64Q mutation of sperm whale myoglobin. In African elephant aqua-metmyoglobin the Soret band was considerably broadened so as to produce another peak in the pentacoordinate 395 nm region. This unique spectral feature was therefore analyzed to show that the myoglobin is in equilibrium between two species, depending upon the presence or absence of a water molecule at the sixth coordinate position.
亚洲和非洲象的肌红蛋白在64位上都有一个谷氨酰胺取代了通常的远端（E7）组氨酸。我们直接从非洲象（Loxodonta africana）的骨骼肌中分离出天然氧合肌红蛋白，并在25℃下于0.1 M缓冲液中研究了氧合肌红蛋白（MbO₂）自动氧化为高铁肌红蛋白（metMb）的速率与pH的关系。结果发现，非洲象的MbO₂与抹香鲸肌红蛋白对自动氧化的抗性相同。然而，象肌红蛋白在pH值低于6时表现出明显的速率饱和现象。对自动氧化速率的pH曲线进行动力学分析表明，非洲象的MbO₂没有显示出任何质子催化过程，比如在抹香鲸肌红蛋白的自动氧化反应中以远端组氨酸作为催化残基而发挥主导作用的那种过程。非洲象MbO₂在低pH下具有更高的稳定性，这几乎可以完全由抹香鲸肌红蛋白的单个H64Q突变来解释。在非洲象的水合高铁肌红蛋白中，Soret带显著变宽，从而在五配位的395 nm区域产生另一个峰。因此，对这一独特的光谱特征进行分析表明，肌红蛋白在两种状态之间处于平衡，这取决于第六配位位置上是否存在水分子。
