FULL PAPER Nickel-substituted iron-dependent cysteine dioxygenase: Implications for the dioxygenation activity of nickel model compounds Amr A. A. Attia | Radu Silaghi-Dumitrescu Departamentul de Chimie, Facultatea de Chimie şi Inginerie Chimica, Universitatea Babeş-Bolyai, Cluj-Napoca, Romania Correspondence Amr A. A. Attia, Departamentul de Chimie, Facultatea de Chimie şi Inginerie Chimica, Universitatea Babeş-Bolyai, Cluj-Napoca, Romania. Email: amrattia@chem.ubbcluj.ro Funding information Babeş-Bolyai University, Grant/Award Number: GTC-31775-2016, SMIS 48801/ 1862; European Regional Development Fund Reported here is a density functional theory study on the ability of Ni-substituted iron-dependent cysteine dioxygenase (CDO) to catalyze the oxidation of cysteine to cysteine sulfinic acid. The first steps of the commonly accepted mechanism for CDO, the O 2 activation mechanism, suggests the binding of O 2 to the metal ion (where redox isomerism takes place converting O 2 to O 2 2 ) followed by the attack of the distal oxygen atom on the cysteine sulfur—in line with most previous evidence. An alternative mechanism entailing the attack of the cysteine sulfur on the proximal oxygen atom of the dioxygen moiety to form a persulfenate intermediate without any redox exchange between the metal ion and the O 2 ligand, is supported by an X-ray crystal structure showing a CDO with a bound cysteine persulfenate, and also supported by data on the oxidation of thiols catalyzed by Ni (II) compounds. Our results show that the O 2 activation mechanism with a Ni-substituted active site follows the same pattern as native CDOs albeit with much higher energy barriers for the for- mation of the intermediates suggesting that the reaction might not be biologically feasible. Conversely, the immediate cleavage of the persulfenate SAO bond in the alternative mechanism suggests that cysteine persulfenate might not be a true intermediate in catalytic cycle of CDOs. KEYWORDS cysteine dioxygenase, density functional theory, nickel, reaction mechanism 1 | INTRODUCTION Cysteine dioxygenases (CDOs) are non-heme iron enzymes that catalyze the conversion of cysteine to cysteine sulfinic acid—a crucial first step in the bioprocess of cysteine metabolism [1–3] (cf. Figure 1). Deficiency of this enzyme has been linked to serious diseases and neurological disorders. [4,5] CDOs are among a small group of non-heme dioxygenases that deviate from the typical 2-histidine-1-carboxylate ligand arrangement to feature three histidines as first shell ligands. [6,7] The substrate-bound crystal structure of CDO shows a penta-coordinated ferrous active site where, in addi- tion to the three histidines ligated to the ferrous ion, the cysteine substrate is bound to the iron as a bidentate ligand via the sulfur and nitrogen atoms. [7,8] One intriguing structural feature that has been reported for CDOs is the presence of a covalent crosslink between Y157 and C93, which con- strains the orientation of the aromatic ring of Y157 in the active site. The details of the purpose of this covalent crosslink is still unknown; to date, several studies have demonstrated a fivefold-tenfold increase in the catalytic efficiency of the enzyme on the formation of this covalent crosslink. [9–11] The labile nature of the reactive intermediates in CDOs has offered ground for several proposed reaction mechanisms. [2,12–19] The generally accepted mechanism is depicted in Figure 2 and is suggested to proceed via the binding of O 2 to the ferrous active site, thus, forming a ferric- superoxo species, followed by the transfer of the distal oxygen atom to the sulfur atom of cysteine, heterolytic cleavage of the OAO bond forming an iron(IV)-oxo adduct and a cysteine sulfenate intermediate, and lastly followed by the transfer of the iron-bound oxygen atom to the cysteine sul- fur to form cysteine sulfinic acid as the final product. [2,12–19] Over the past years, several experimental and computational studies have been Int J Quantum Chem. 2017;e25564. https://doi.org/10.1002/qua.25564 http://q-chem.org VC 2017 Wiley Periodicals, Inc. | 1 of 8 Received: 7 August 2017 | Revised: 18 November 2017 | Accepted: 21 November 2017 DOI: 10.1002/qua.25564