Insights into the Mechanism of Aromatic Ring Cleavage of Noncatecholic Compound 2-Aminophenol by Aminophenol Dioxygenase: A Quantum Mechanics/Molecular Mechanics Study

Abstract

2-Aminophenol 1,6-dioxygenase (APD) is an extradiol dioxygenase responsible for the ring cleavage of 2aminophenol (2AP) at the position ortho to the hydroxyl substituent. To elucidate the reaction mechanism, we conducted quantum mechanical/molecular mechanical (QM/ MM) calculations. The mode of binding of the substrate (monodentate or bidentate) to the iron center was found to have a crucial role in dioxygen activation. The Fe−O 2 adducts with 2AP bound bidentately has a quintet ground state having a Fe III −superoxo character, while the Fe−O 2 adducts with a monodentately bound substrate has been characterized as a substrate radical−Fe II −superoxide. Unlike other extradiol dioxygenases that cleave catechol analogues using the superoxo moiety of the Fe−O 2 adducts to attack the substrate, we found here an Fe II −O(H)O intermediate formed through two sequential proton-coupled electron transfer steps from the initial Fe III −superoxo species is responsible for the attack. Importantly, the second-sphere His195 residue acts as an acid−base catalyst to mediate proton transfer (associated with electron transfer). The study presented here expands our understanding of the extradiol dioxygenases, especially those catalyzing the ring cleavage of noncatecholic substrates.

How To Produce Methane Precursor in the Upper Ocean by An Untypical Non‐Heme Fe‐Dependent Methylphosphonate Synthase?

“…It is clearly an adduct of Fe(II) complexed with a triplet‐state dioxygen. This kind of spin state was also reported in the previous study on 2‐aminophenol 1,6‐dioxygenase (APD) . Interestingly, other ES states have lower spin sums of O p and O d (entries 1, 2, 4, and 5 in Table ) compared with the ground state, showing that they are most likely Fe(III)‐superoxide radicals (Fe(III)−O−O .…”

“…This is consistent with the geometrical parameters that they have a little longer O p −O d and shorter Fe−O p bond lengths than the triplet‐state ground state (Figure ). It was further noticed that the three high‐spin ES states (entries 3–5 in Table ) are much lower in energy than the two low‐spin states (entries 1 and 2 in Table ), which is common in non‐heme iron enzymes . If not otherwise indicated, the ES complex mentioned below is the ES ground state, i. e., the triplet‐state ES formed by antiferromagnetic coupling between Fe(II) and dioxygen (entry 3 in Table ).…”

“…This kind of spin state was also reported in the previous study on 2-aminophenol 1,6dioxygenase (APD). [67] Interestingly, other ES states have lower spin sums of O p and O d (entries 1, 2, 4, and 5 in Table 1) compared with the ground state, showing that they are most likely Fe(III)-superoxide radicals (Fe(III)À OÀ O * ). This is consistent with the geometrical parameters that they have a little longer O p À O d and shorter FeÀ O p bond lengths than the triplet-state ground state ( Figure 3).…”

“…It was further noticed that the three high-spin ES states (entries 3-5 in Table 1) are much lower in energy than the two low-spin states (entries 1 and 2 in Table 1), which is common in non-heme iron enzymes. [19,60,[67][68][69] If not otherwise indicated, the ES complex mentioned below is the ES ground state, i. e., the triplet-state ES formed by antiferromag-netic coupling between Fe(II) and dioxygen (entry 3 in Table 1). It is worth mentioning that the subsequent MPnS reaction pathways in the singlet, triplet, quintet, and septet states are all calculated and the one in the quintet state is favorite, which will be discussed in detail later.…”

Transition-metal-mediated reduction and reversible double-cyclization of cyanuric triazide to an asymmetric bitetrazolate involving cleavage of the six-membered aromatic ring

“…The drastic ligand rearrangement resulting in the loss of the six-membered aromatic ring and the appearance of two new five-membered aromatic heterocycles was unexpected, but may be rationalized by a ligand reduction to extrude N 2 , accompanied by a rearrangement of the resulting imido species involving N-C cleavage, and closure of the azides with ortho azine nitrogen atoms (Scheme 2). This isomerization of a sensitive cyanuric polyazide into more stable tetrazole ring systems represents a novel transformation into a new type of energetic moiety: one involving the cleavage of a six member aromatic ring, which is an exceedingly rare phenomenon usually discussed in the context of the 2-aminophenol dioxygenase enzyme 52 and model chemistry thereof. 53,54 However, charge counting considerations lead to a formal oxidation state of 2+ on the manganese ions, suggesting that the metal ion has not been oxidized.…”