Department of Organic Chemistry Stockholm University, SE-106 91, Stockholm, Sweden.
Inorg Chem. 2020 Aug 17;59(16):11542-11553. doi: 10.1021/acs.inorgchem.0c01351. Epub 2020 Jul 27.
The superfamily of heme copper oxidases reduces molecular oxygen or nitric oxide, and the active sites comprise a high-spin heme group ( or ) and a non-heme metal (Cu or Fe). The C family of cytochrome oxidases, with the high-spin heme and Cu in the active site, is a subfamily of the heme copper oxidases that can reduce both molecular oxygen, which is the main substrate, and nitric oxide. The mechanism for NO reduction in oxidase is studied here using hybrid density functional theory and compared to other cytochrome oxidases (A and B families), with a high-spin heme and Cu in the active site, and to cytochrome dependent NO reductase, with a high-spin heme and a non-heme Fe in the active site. It is found that the reaction mechanism and the detailed reaction energetics of the oxidases are not similar to those of cytochrome dependent NO reductase, which has the same type of high-spin heme group but a different non-heme metal. This is in contrast to earlier expectations. Instead, the NO reduction mechanism in oxidases is very similar to that in the other cytochrome oxidases, with the same non-heme metal, Cu, and is independent of the type of high-spin heme group. The conclusion is that the type of non-heme metal (Cu or Fe) in the active site of the heme copper oxidases is more important for the reaction mechanisms than the type of high-spin heme, at least for the NO reduction reaction. The reason is that the proton-coupled reduction potentials of the active site cofactors determine the energetics for the NO reduction reaction, and they depend to a larger extent on the non-heme metal. Observed differences in NO reduction reactivity among the various cytochrome oxidases may be explained by differences outside the BNC, affecting the rate of proton transfer, rather than in the BNC itself.
血红素铜氧化酶超家族还原分子氧或一氧化氮，活性部位包含一个高自旋血红素基团（ 或 ）和一个非血红素金属（Cu 或 Fe）。细胞色素 c 氧化酶的 C 家族，其活性部位含有高自旋血红素 和 Cu，是血红素铜氧化酶的一个亚家族，可还原主要底物分子氧和一氧化氮。本文使用杂化密度泛函理论研究了 氧化酶中 NO 还原的机制，并与其他细胞色素 c 氧化酶（A 和 B 家族）进行了比较，这些酶的活性部位含有高自旋血红素 和 Cu，以及细胞色素 c 依赖的 NO 还原酶，其活性部位含有高自旋血红素 和非血红素 Fe。结果发现， 氧化酶的反应机制和详细的反应能学与具有相同类型高自旋血红素基团但具有不同非血红素金属的细胞色素 c 依赖的 NO 还原酶的反应机制不相似。这与早期的预期相反。相反， 氧化酶中的 NO 还原机制与其他细胞色素 c 氧化酶非常相似，具有相同的非血红素金属 Cu，并且与高自旋血红素基团的类型无关。结论是，血红素铜氧化酶活性部位中非血红素金属（Cu 或 Fe）的类型对于反应机制比高自旋血红素的类型更为重要，至少对于 NO 还原反应而言是如此。原因是活性部位辅因子的质子偶联还原电位决定了 NO 还原反应的能学，而它们在更大程度上取决于非血红素金属。各种细胞色素 c 氧化酶之间 NO 还原反应活性的差异可以用 BNC 之外的因素来解释，这些因素影响质子转移的速率，而不是 BNC 本身。
