Structure and Mechanism Leading to Formation of the Cysteine Sulfinate Product Complex of a Biomimetic Cysteine Dioxygenase Model

Abstract

Cysteine dioxygenase is a unique nonheme iron enzyme that is involved in the metabolism of cysteine in the body. It contains an iron active site with an unusual 3-His ligation to the protein, which contrasts with the structural features of common nonheme iron dioxygenases. Recently, some of us reported a truly biomimetic model for this enzyme, namely a trispyrazolylborato iron(II) cysteinato complex, which not only has a structure very similar to the enzyme-substrate complex but also represents a functional model: Treatment of the model with dioxygen leads to cysteine dioxygenation, as shown by isolating the cysteine part of the product in the course of the work-up. However, little is known on the conversion mechanism and, so far, not even the structure of the actual product complex had been characterised, which is also unknown in case of the enzyme. In a multidisciplinary approach including density functional theory calculations and X-ray absorption spectroscopy, we have now determined the structure of the actual sulfinato complex for the first time. The Cys-SO2 (-) functional group was found to be bound in an η(2) -O,O-coordination mode, which, based on the excellent resemblance between model and enzyme, also provides the first support for a corresponding binding mode within the enzymatic product complex. Indeed, this is again confirmed by theory, which had predicted a η(2) -O,O-binding mode for synthetic as well as the natural enzyme.

Synthesis, X-ray Structures, Electronic Properties, and O2/NO Reactivities of Thiol Dioxygenase Active-Site Models

“…However, we also considered the possibility that the S = 1 Fe/O 2 adduct consists of a low-spin Fe(III) ion ferromagnetically coupled to O 2 •− , as this configuration yielded the lowest-energy Fe/O 2 model in de Visser’s study of complex 3 . 39 The resulting computational models are labeled spin [ 2-O 2 ] (HS,LS) and spin [ 4-O 2 ] (HS,LS) , where HS and LS indicate whether the Fe center is high-spin or low-spin, respectively. Metric parameters of the geometry-optimized structures are provided in the Supporting Information (Table S3).…”

“…Metric parameters of the geometry-optimized structures are provided in the Supporting Information (Table S3). Whereas de Visser found that O 2 binding to 3 triggers dissociation of a pyrazole donor regardless of spin state, 39 nearly allo of our optimized Fe/O 2 models are 6C with Fe-N TIP/Tp bonds lengths ≤ 2.31 Å; the only exceptions being the S =3 [ 2-O 2 ] HS and S =2 [ 2-O 2 ] HS models, in which one of the Fe-N TIP bonds elongates to ~2.4 Å during geometry optimization to yield quasi-6C structures (Table S3). …”

“…De Visser and others have explored the O 2 -activation mechanisms of CDO and select TDO models using computational methods. 39–42 …”

Utilizing the Trispyrazolyl Borate Ligand for the Mimicking of O2-Activating Mononuclear Nonheme Iron Enzymes

“…The theoretical analysis could rationalize that the reaction of 10 with O 2 is comparatively slow: O 2 binding was found to be endergonic and thus represents the rate-determining step. 21 An independent theoretical work (using similar methods) published very recently arrived at similar conclusions, although specific energies and structures varied somewhat. 22 In 2012, Goldberg et al presented a further iron(II) thiolate complex that reacted with O 2 via dioxygenation at the S atom.…”

The Role of Nonheme Transition Metal-Oxo, -Peroxo, and -Superoxo Intermediates in Enzyme Catalysis and Reactions of Bioinspired Complexes

“…An extensive set of computational studies on CDO enzymes 35 -37 and model complexes was performed. 38,39 The calculations showed that the 3-His ligand system in CDO is essential for complete dioxygenase activity and significant amount of cysteine sulfoxide product is predicted in CDO with a 2-His/1-Asp ligand system. Furthermore, the ligand environment is important for spin state ordering and catalysis.…”