Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77005-1892, USA.
J Biol Chem. 2012 Sep 28;287(40):33163-78. doi: 10.1074/jbc.M112.392258. Epub 2012 Aug 1.
Although molecular dynamics simulations suggest multiple interior pathways for O(2) entry into and exit from globins, most experiments indicate well defined single pathways. In 2001, we highlighted the effects of large-to-small amino acid replacements on rates for ligand entry and exit onto the three-dimensional structure of sperm whale myoglobin. The resultant map argued strongly for ligand movement through a short channel from the heme iron to solvent that is gated by the distal histidine (His-64(E7)) near the solvent edge of the porphyrin ring. In this work, we have applied the same mutagenesis mapping strategy to the neuronal mini-hemoglobin from Cerebratulus lacteus (CerHb), which has a large internal tunnel from the heme iron to the C-terminal ends of the E and H helices, a direction that is 180° opposite to the E7 channel. Detailed comparisons of the new CerHb map with expanded results for Mb show unambiguously that the dominant (>90%) ligand pathway in CerHb is through the internal tunnel, and the major (>75%) ligand pathway in Mb is through the E7 gate. These results demonstrate that: 1) mutagenesis mapping can identify internal pathways when they exist; 2) molecular dynamics simulations need to be refined to address discrepancies with experimental observations; and 3) alternative pathways have evolved in globins to meet specific physiological demands.
虽然分子动力学模拟表明氧（O(2)）进入和离开球蛋白有多种内部途径，但大多数实验表明存在明确的单一途径。2001 年，我们强调了大到小氨基酸取代对配体进入和离开三维结构的速度的影响。鲸鱼肌红蛋白。由此产生的图谱强烈表明配体通过从血红素铁到溶剂的短通道移动，该通道由位于卟啉环溶剂边缘的远端组氨酸（His-64（E7））控制。在这项工作中，我们将相同的诱变作图策略应用于来自 Cerebratulus lacteus（CerHb）的神经元小型血红蛋白，该蛋白具有从血红素铁到 E 和 H 螺旋末端的大内部隧道，该方向与 E7 通道相反 180°。新的 CerHb 图谱与 Mb 的扩展结果的详细比较清楚地表明，CerHb 中主要（>90%）配体途径是通过内部隧道，而 Mb 中的主要（>75%）配体途径是通过 E7 门。这些结果表明：1）诱变作图可以在存在内部途径时识别它们；2）需要改进分子动力学模拟以解决与实验观察的差异；3）球蛋白中已经进化出替代途径以满足特定的生理需求。
