Synthesis and reactivity of synthetic analogs for nickel redox enzymes: Superoxide dismutase and acetyl coenzyme A synthase

Synthetic model complexes of nickel-containing enzyme active sites are important tools for understanding the structure-function properties of the active sites as well as for investigating the enzyme mechanisms. The synthesis and reactivity of structural mimics and catalytic-intermediate analogs can provide insight into how the diverse chemistry of nickel-containing enzymes occurs. Reported herein are two synthetic models of redox active nickel-dependant metalloproteins, nickel-containing superoxide dismutase and acetyl coenzyme A synthase.;Nickel-containing superoxide dismutase, Ni SOD, contains a mononuclear nickel active site found at the N-terminus with active site ligands found within the first six residues of the protein sequence.1 The active site sequence is unstructured in the absence of Ni2+ providing a novel target for synthetic modeling. A Ni SOD model system was designed utilizing short peptides as ligands based on the native active site amino acid sequence, NH2-His-Cys-Gly-Gly-Pro-Cys-COOH. The system is used to investigate the minimum construct necessary to establish the native peptide fold and metal coordination.;The [Ni(HCGGPC)]- complex was determined to be monomeric by mass spectrometry and optical spectral titrations with K d = 4.7 +/- 0.4 x 10-12 M. One- and two-dimensional NMR spectral data suggests the nickel coordination sphere is the same as found in the enzyme. Metal selectivity of the peptide was measured comparing Ni 2+, Zn2+, and Co2+. Ni2+ is preferred two orders of magnitude over Zn2+ and three orders of magnitude over Co2+. The reactivity of the model complex with O2·- is similar to that observed for the enzyme.1 Reaction of the complex with substoichiometric KO2 results in a Ni3+ species with a similar EPR spectrum as that observed in the enzyme. The superoxide dismutase activity of the nickel complex was measured to be, IC50=191 +/- 9 muM much reduced compared to the native enzyme. The decreased activity is due to the product-catalyzed decomposition of the complex suggesting substrate channeling and the active site pocket of the enzyme provide essential oxidative protection of the active site thiols.;Acetyl coenzyme A synthase, ACS, catalyzes the production of acetyl coenzyme A from one-carbon units, CO and CH3. A recently proposed ACS catalytic mechanism involves a zero valent nickel as the complex to accept a methyl group from methylcobalt species during catalytic turnover.2 Synthetic analog studies were designed to probe the feasibility of Ni(0) as the methyl acceptor. Reaction of [triphos]Ni(PPh3) (triphos = bis(2-diphenylphosphinoethyl)phenyl phosphine) with CH3Co(dmgBF2)2py proceeds quantitatively yielding [Ni(triphos)(Me)]+ and Co(I). 3 Kinetic data of methyl transfer and the rate of other alkyls (Et, iPr) suggests that the mechanism of alkyl transfer proceeds via an SN2 pathway similar to the ACS enzyme. The nickel-alkyls complexes possessing beta-hydrogen undergo beta-hydrogen elimination forming a nickel-hydride complex characterized by NMR, FT-IR spectroscopies and X-ray crystallography. This system is also used to model another intermediate along the catalytic pathway, the Ni-acyl complex, to help determined the importance of the order of substrate binding to the ACS active site during catalysis.