HNO/NO Conversion Mechanisms of Cu-Based HNO Probes with Implications for Cu,Zn-SOD

Abstract

HNO has broad biological effects and pharmacological activities. Direct HNO probes for in vivo applications were recently reported, which are CuII-based complexes having fluorescence reporters with reaction to HNO resulting in CuI systems and the release of NO. Their coordination environments are similar to that in Cu,Zn-superoxide dismutase (SOD), which plays a significant role in cellular HNO/NO conversion. However, none of these conversion mechanisms are known. A quantum chemical investigation was performed here to provide structural, energetic, and electronic profiles of HNO/NO conversion pathways via the first CuII-based direct HNO probe. Results not only are consistent with experimental observations but also provide numerous structural and mechanistic details unknown before. Results also suggest the first HNO/NO conversion mechanism for Cu,Zn-SOD, as well as useful guidelines for future design of metal-based HNO probes. These results shall facilitate development of direct HNO probes and studies of HNO/NO conversions via metal complexes and metalloproteins.

Mechanisms of HNO Reactions with Ferric Heme Proteins

“…Active‐site models along both the HNO binding and conversion pathways for metMb (called Mb in model names) and CAT containing proximal and nearby residues that could influence HNO/NO binding were investigated, using Cα‐truncated residues and nonsubstituted porphyrins as based on previous studies . All models were subject to geometry optimization and frequency analysis, in a medium to simulate the bulk protein environment using the PCM approach, with a DFT method that yields excellent predictions of experimental HNO reactivities (see the Supporting Information for computational details and the results of all conformations and electronic states studied, with the results of the most favorable species shown in Figure and Table ).…”

“…After HNO binds to the ferric center, the reductive nitrosylation was found to occur with a proton‐coupled electron transfer (PCET) transition state, Mb‐TS , to form the final ferrous NO heme product, as indicated by 0.142 e spin density for Fe and 0.858 e spin density for NO, and the doubly protonated distal His64 residue in Mb‐I‐6 (see Figure ). Unlike the PCET mechanism of HNO‐to‐NO conversion mediated by the non‐heme metal system, which has a reasonably high barrier, the PCET process via metMb is basically barrierless, with a large thermodynamic tendency (Δ G =−13.69 kcal mol −1 ; see Table ).…”

Recent advances in the chemical biology of nitroxyl (HNO) detection and generation

“…In the fluorescence turn-on process, HNO reduces Cu II to Cu I , restoring the emission of BOT1 and forming NO (Figure 2) and both electrochemical and quantum chemical studies support this mechanism. 47,48 As this mechanism relies on the reducing power of HNO, other biological reductants may reduce Cu II in these complexes yielding fluorescence and generating false positives. Treatment of Cu II [BOT1] with cysteine restores the emission of BOT1 and positive ion electrospray mass spectrometry shows the reduced species as the Cu I complex.…”

Mechanisms of HNO Reactions with Ferric Heme Proteins

“…In particular, fewer heme-based HNO probes have been developed, [1f, 13] in contrast with many non-heme based HNO probes. [16] …”

“…[17] All models were subject to geometry optimization and frequency analysis, in a medium to simulate the bulk protein environment using the PCM approach, [18] with a DFT method that yields excellent predictions of experimental HNO reactivities, [16h, 17a, 19] see Supporting Information (SI) for computational details and results of all conformations and electronic states studied, with results of the most favorable species shown in Figure 1 and Table 1.…”