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Dipeptide-based models of nickel superoxide dismutase: solvent effects highlight a critical role to Ni-S bonding and active site stabilization.

ABSTRACT: Nickel superoxide dismutase (Ni-SOD) catalyzes the disproportionation of the superoxide radical to O(2) and H(2)O(2) utilizing the Ni(III/II) redox couple. The Ni center in Ni-SOD resides in an unusual coordination environment that is distinct from other SODs. In the reduced state (Ni-SOD(red)), Ni(II) is ligated to a primary amine-N from His1, anionic carboxamido-N/thiolato-S from Cys2, and a second thiolato-S from Cys6 to complete a NiN(2)S(2) square-planar coordination motif. Utilizing the dipeptide N(2)S(2-) ligand, H(2)N-Gly-l-Cys-OMe (GC-OMeH(2)), an accurate model of the structural and electronic contributions provided by His1 and Cys2 in Ni-SOD(red), we constructed the dinuclear sulfur-bridged metallosynthon, [Ni(2)(GC-OMe)(2)] (1). From 1 we prepared the following monomeric Ni(II)-N(2)S(2) complexes: K[Ni(GC-OMe)(SC(6)H(4)-p-Cl)] (2), K[Ni(GC-OMe)(S(t)Bu)] (3), K[Ni(GC-OMe)(SC(6)H(4)-p-OMe)] (4), and K[Ni(GC-OMe)(SNAc)] (5). The design strategy in utilizing GC-OMe(2-) is analogous to one which we reported before (see Inorg. Chem. 2009, 48, 5620 and Inorg. Chem. 2010, 49, 7080) where Ni-SOD(red) active site mimics can be assembled at will with electronically variant RS(-) ligands. Discussed herein is our initial account pertaining to the aqueous behavior of isolable, small-molecule Ni-SOD model complexes (non-maquette based). Spectroscopic (FTIR, UV-vis, ESI-MS, XAS) and electrochemical (CV) measurements suggest that 2-5 successfully simulate many of the electronic features of Ni-SOD(red). Furthermore, the aqueous studies reveal a dynamic behavior with regard to RS(-) lability and bridging interactions, suggesting a stabilizing role brought about by the protein architecture.

SUBMITTER: Gale EM

PROVIDER: S-EPMC3561719 | biostudies-literature | 2011 Oct

REPOSITORIES: biostudies-literature

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Dipeptide-based models of nickel superoxide dismutase: solvent effects highlight a critical role to Ni-S bonding and active site stabilization.

Gale Eric M EM Cowart Darin M DM Scott Robert A RA Harrop Todd C TC

Inorganic chemistry 20110920 20

Nickel superoxide dismutase (Ni-SOD) catalyzes the disproportionation of the superoxide radical to O(2) and H(2)O(2) utilizing the Ni(III/II) redox couple. The Ni center in Ni-SOD resides in an unusual coordination environment that is distinct from other SODs. In the reduced state (Ni-SOD(red)), Ni(II) is ligated to a primary amine-N from His1, anionic carboxamido-N/thiolato-S from Cys2, and a second thiolato-S from Cys6 to complete a NiN(2)S(2) square-planar coordination motif. Utilizing the di ...[more]

PMID: 21932766

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Project description:Nickel superoxide dismutase (NiSOD) is a recently discovered superoxide dismutase that utilizes the Ni(III)/Ni(II) couple to facilitate the disproportionation of O(2)(*-) into H(2)O(2) and O(2). A key structural component of NiSOD is an elongated axial His-imidazole Ni(III) bond (2.3-2.6 A) that is the result of a H-bonding network between His(1), Glu(17), and Arg(47). Herein we utilize metallopeptide based mimics of NiSOD with His(1) epsilon-nitrogen substituted imidazoles to approximate the electronic influence of this H-bonding network ({Ni(III/II)(SOD(M1)-Im-X)} X = Me, H, DNP, and Tos; SOD(M1)-Im-X = H'CDLPCGVYDPA where H' is an N-substituted His). All reduced {Ni(II)(SOD(M1)-Im-X)} are similar to one another as assessed by electronic absorption spectroscopy, circular dichroism (CD) spectroscopy, and Ni K-edge x-ray absorption (XAS). This indicates that the change in His(1) is having little influence on the square-planar Ni(II)N(2)S(2) center. In contrast, changes to the axial His(1) ligand impart differential spectroscopic properties on the oxidized {Ni(III)(SOD(M1)-Im-X)} metallopeptides. Resonance Raman spectroscopy (405 nm excitation) in conjunction with a normal coordinate analysis indicates that as the axial His imidazole is made less Lewis basic there is an increase in Ni(III)-S bond strength in the equatorial plane, with force constants for the Ni-S bond trans to the amine ranging from 1.54 to 1.70 mdyn A(-1). The rhombic electron paramagnetic resonance (EPR) spectra of the four oxidized metallopeptides are all consistent with low-spin Ni(III) contained in a square pyramidal coordination environment, but show changes in the hyperfine coupling to (14)N along g(z). This is attributable to a reorientation of the g(z) vector in the more (along the Ni(III)-N(imidazole) bond) versus less (along the S-Ni(III)-N(amine) bond) Lewis basic imidazole bases. This reorientation of g(z) along the xy plane translates into a decrease in A(zz) by approximately 20 MHz. A decrease in Lewis-basicity of the axial imidazole also translates into a 2 orders of magnitude increase in SOD catalysis across the metallopeptide series, with k(cat) ranging from 6(1) x 10(6) M(-1) s(-1) for the metallopeptide with the most Lewis basic imidazole to 6(2) x 10(8) M(-1) s(-1) for the metallopeptide with the least basic imidazole. This likely results from a fine-tuning of the electron transfer properties of the Ni-center, which optimize it for SOD catalysis.

Project description:Superoxide dismutases (SODs) utilize a ping-pong mechanism in which a redox-active metal cycles between oxidized and reduced forms that differ by one electron to catalyze the disproportionation of superoxide to dioxygen and hydrogen peroxide. Nickel-dependent SOD (NiSOD) is a unique biological solution for controlling superoxide levels. This enzyme relies on the use of cysteinate ligands to bring the Ni(III/II) redox couple into the range required for catalysis (?300 mV vs. NHE). The use of cysteine thiolates, which are not found in any other SOD, is a curious choice because of their well-known oxidation by peroxide and dioxygen. The NiSOD active site cysteinate ligands are resistant to oxidation, and prior studies of synthetic and computational models point to the backbone N-donors in the active site (the N-terminal amine and the amide N atom of Cys2) as being involved in stabilizing the cysteines to oxidation. To test the role of the backbone N-donors, we have constructed a variant of NiSOD wherein an alanine residue was added to the N-terminus (Ala0-NiSOD), effectively altering the amine ligand to an amide. X-ray absorption, electronic absorption, and magnetic circular dichroism (MCD) spectroscopic analyses of as-isolated Ala0-NiSOD coupled with density functional theory (DFT) geometry optimized models that were evaluated on the basis of the spectroscopic data within the framework of DFT and time-dependent DFT computations are consistent with a diamagnetic Ni(II) site with two cysteinate, one His1 amide, and one Cys2 amidate ligands. The variant protein is catalytically inactive, has an altered electronic absorption spectrum associated with the nickel site, and is sensitive to oxidation. Mass spectrometric analysis of the protein exposed to air shows the presence of a mixture of oxidation products, the principal ones being a disulfide, a bis-sulfenate, and a bis-sulfinate derived from the active site cysteine ligands. Details of the electronic structure of the Ni(III) site available from the DFT calculations point to subtle changes in the unpaired spin density on the S-donors as being responsible for the altered sensitivity of Ala0-NiSOD to O2.

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Dipeptide-based models of nickel superoxide dismutase: solvent effects highlight a critical role to Ni-S bonding and active site stabilization.

Publications

Dipeptide-based models of nickel superoxide dismutase: solvent effects highlight a critical role to Ni-S bonding and active site stabilization.

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