Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland.
Department of Chemistry, Ludwigs-Maximilians University, Butenandtstrasse 5-13, 81377 Munich, Germany.
Inorg Chem. 2021 Nov 1;60(21):15948-15967. doi: 10.1021/acs.inorgchem.1c00933. Epub 2021 Sep 3.
According to the current paradigm, the metal-hydroxo bond in a six-coordinate porphyrin complex is believed to be significantly less reactive in ligand substitution than the analogous metal-aqua bond, due to a much higher strength of the former bond. Here, we report kinetic studies for nitric oxide (NO) binding to a heme-protein model, acetylated microperoxidase-11 (AcMP-11), that challenge this paradigm. In the studied pH range 7.4-12.6, ferric AcMP-11 exists in three acid-base forms, assigned in the literature as [(AcMP-11)Fe(HO)(HisH)] (), [(AcMP-11)Fe(OH)(HisH)] (), and [(AcMP-11)Fe(OH)(His)] (). From the pH dependence of the second-order rate constant for NO binding (), we determined individual rate constants characterizing forms -, revealing only a ca. 10-fold decrease in the NO binding rate on going from ( = 3.8 × 10 M s) to ( = 4.0 × 10 M s) and the inertness of . These findings lead to the abandonment of the dissociatively activated mechanism, in which the reaction rate can be directly correlated with the Fe-OH bond energy, as the mechanistic explanation for the process with regard to . The reactivity of is accounted for through the existence of a tautomeric equilibrium between the major [(AcMP-11)Fe(OH)(HisH)] () and minor [(AcMP-11)Fe(HO)(His)] () species, of which the second one is assigned as the NO binding target due to its labile Fe-OH bond. The proposed mechanism is further substantiated by quantum-chemical calculations, which confirmed both the significant labilization of the Fe-OH bond in the [(AcMP-11)Fe(HO)(His)] tautomer and the feasibility of the tautomer formation, especially after introducing empirical corrections to the computed relative acidities of the HO and HisH ligands based on the experimental p values. It is shown that the "effective lability" of the axial ligand (OH/HO) in may be comparable to the lability of the HO ligand in .
根据目前的模式，六配位卟啉配合物中的金属-羟键被认为在配体取代反应中比类似的金属-水键的反应性低得多，这是由于前者键的强度要高得多。在这里，我们报告了对一种血红素蛋白模型（乙酰化微过氧化物酶-11（AcMP-11））的一氧化氮（NO）结合的动力学研究，这对这一模式提出了挑战。在所研究的 pH 值范围 7.4-12.6 下，三价 AcMP-11 存在于三种酸碱形式中，在文献中分别被指定为 [(AcMP-11)Fe(HO)(HisH)] ()、[(AcMP-11)Fe(OH)(HisH)] ()和[(AcMP-11)Fe(OH)(His)] ()。从 NO 结合的二级速率常数随 pH 的变化（），我们确定了表征形式 - 的单独速率常数，表明从（= 3.8 × 10 M s）到（= 4.0 × 10 M s），NO 结合速率仅降低了约 10 倍，而保持了的惰性。这些发现导致放弃了离解激活机制，在该机制中，反应速率可以直接与 Fe-OH 键能相关联，因为该机制对于关于的过程不能提供合理的解释。的反应性可以通过主要的 [(AcMP-11)Fe(OH)(HisH)] ()和次要的 [(AcMP-11)Fe(HO)(His)] ()物种之间的互变异构平衡来解释，其中第二个物种被指定为 NO 结合靶标，因为它的 Fe-OH 键不稳定。该提议的机制通过量子化学计算得到进一步证实，该计算证实了 [(AcMP-11)Fe(HO)(His)] 互变异构体中 Fe-OH 键的明显不稳定，并且特别是在根据实验 p 值对计算的 HO 和 HisH 配体的相对酸度进行经验校正后，互变异构体的形成是可行的。结果表明，轴向配体（OH/HO）在中的“有效不稳定”性可能与 HO 配体在中的不稳定相似。
