Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4, Takano-nishihirakicho, Sakyo-ku, Kyoto, 606-8103, Japan.
Phys Chem Chem Phys. 2013 Apr 28;15(16):6139-48. doi: 10.1039/c3cp43253j. Epub 2013 Mar 20.
Rr-CooA is a CO-sensor heme protein, where binding of CO with the heme group stimulates a transcriptional activator activity of CooA. In this process, the heme undergoes a series of ligand exchanges. In the ferric form, the heme has Cys75 and Pro2 as the axial ligands. In the reduced ferrous form, the heme has His77 instead of Cys75 as an axial ligand with Pro2. Only in the reduced form, CooA can bind CO that replaces Pro2. Model calculations are carried out to elucidate the ligand exchange reactions of CooA. The coordinated proline is found to be the neutral, protonated form. The ligand exchange of cysteine for histidine is reproduced by a relatively small model. This exchange would be mainly due to difference in stability of the non-bonding sulfur p-orbital in Cys75 between the ferric and ferrous states. The selectivity of gas molecules among CO, NO, and O2 in the proteins is explained by the relative stability of products for Rr-CooA. This is also the case for Ch-CooA, where the amino group of the N-terminus and a histidine are coordinated to the iron ion both in the ferric and ferrous states. The ability to bind the gas molecules is a little stronger in Rr-CooA than in Ch-CooA. In the ferric form of Rr-CooA, heme is deformed to a ruffled form whereas heme is planar in the ferrous form, which leads to a red-shifted Q-band in the former.
Rr-CooA 是一种 CO 传感器血红素蛋白，其中 CO 与血红素基团的结合刺激 CooA 的转录激活活性。在这个过程中，血红素经历了一系列的配体交换。在三价铁形式中，血红素有 Cys75 和 Pro2 作为轴向配体。在还原的二价亚铁形式中，血红素有 His77 而不是 Cys75 作为轴向配体，同时保留 Pro2。只有在还原形式中，CooA 才能结合取代 Pro2 的 CO。进行模型计算以阐明 CooA 的配体交换反应。发现配位的脯氨酸呈中性、质子化形式。通过相对较小的模型重现了半胱氨酸取代组氨酸的配体交换。这种交换主要归因于铁的三价和二价状态之间非键合硫 p 轨道在 Cys75 中的稳定性差异。Rr-CooA 中 CO、NO 和 O2 等气体分子在蛋白质中的选择性通过 Rr-CooA 的产物相对稳定性来解释。对于 Ch-CooA 也是如此，其中铁离子在三价和二价状态下均与 N 端的氨基和组氨酸配位。Rr-CooA 结合气体分子的能力比 Ch-CooA 稍强。在 Rr-CooA 的三价铁形式中，血红素变形为皱缩形式，而在二价亚铁形式中血红素呈平面形式，这导致前者的 Q 带发生红移。
