Jeremy M. Berg

Kinetics of metal binding by a zinc finger peptide

Abstract The kinetics of cobalt(II) and zinc(II) binding by a prototypical zinc finger peptide has been examined by stopped-flow spectrometry. Cobalt(II) binding was demonstrated to be second order, first order in each peptide and cobalt(II) concentration. At pH 6.55 in 100 mM HEPES, 100 mM NaCl buffer the second order rate constant was 7.5(±2.1)×10 4 M −1 s −1 . The displacement of cobalt(II) by zinc(II) was examined at pH 7.0 as a function of both cobalt(II) and zinc(II) concentration. The data are consistent with a dissociative mechanism with the rate of zinc(II) binding being substantially faster than cobalt(II) binding. The rate constant for cobalt(II) dissociation was 5.3(± 0.5)×10 −2 s −1 . The ratio of the rate constants for zinc(II) and cobalt(II) binding was found to be 66±3, indicating that the rate constant for zinc(II) is 2.8(±0.4)×10 7 M −1 s −1 at pH 7.0. This indicates that the rate of zinc(II) dissociation is of 1.6(±0.6)×10 −4 s −1 . The magnitude of the kinetic preference for zinc(II) over cobalt(II) revealed in metal binding rate constants is observed for many other zinc metalloproteins despite the fact that these rate constants vary over eight orders of magnitude.

The crystal and molecular structures of dioxo mo(VI) complexes of tripodal, tetradentate N,S-donor ligands

Abstract The crystal and molecular structures of the complexes MoO2((SCH2CH2)2NCH2CH2SCH3), I and MoO2((SCH2CH2)2NCH2CH2N(CH3)2), II, have been determined from X-ray intensity data collected by counter methods. Compound I crystallizes in two forms, Ia and Ib. In form Ia the space group is P21/n with cell parameters a = 7.235(2), b = 7.717(2), c = 24.527(6) A, β = 119.86(2)°, V = 1188(1) A3, Z = 4. In form Ib the space group is P21/c with cell parameters a = 14.945(5), b = 11.925(5), c = 14.878(4) A, β = 114.51(2)°, V = 2413(3) A3, Z = 8. The molecules of I in Ia and Ib are very similar having an octahedral structure with cis oxo groups, trans thiolates (cis to both oxo groups) and N and thioether sulfur atoms trans to oxo groups. Average ditances are Moue5f8O = 1.70, Moue5f8S (thiolate) = 2.40, Moue5f8N = 2.40 and Moue5f8S (thioether) = 2.79 A. Molecule II crystallizes in space group P212121 with a = 7.188(1), b = 22.708(8), c = 7.746(2) A, V = 1246(1) A3 and Z = 4. The coordination about Mo is octahedral with cis oxo groups, trans thiolates and N atoms trans to oxo. Distances in the first coordination sphere are Moue5f8O = 1.705(2), 1.699(2), Moue5f8S = 2.420(1), 2.409(1) and Moue5f8N = 2.372(2), 2.510(2) A. The conformational features of the complexes are discussed. Complex I displays Moue5f8O and Moue5f8S distances which are very similar to those found by EXAFS in sulfite oxidase. This similarity is discussed.

Structural comparison of octahedral MoO22+ complexes of bidentate and linear tetradentate N,S-donor ligands

The structures of MoO2[NH2C(CH3)2CH2S]2 and MoO2[SC(CH3)2CH2NHCH2CH2NHCH2C(CH3)2S] have been determined using X-ray diffraction intensity data collected by counter techniques. MoO2[NH2C(CH3)2CH2S]2 crystallizes in space group Pbca with a = 11.234(3), b = 11.822(3) and c = 20.179(5) A, V = 2680(2) A3 and Z = 8. Its structure is derived from octahedral coordination with cis oxo groups [Moue5f8O = 1.705(3) and 1.705(3)], trans thiolate donors cis to the oxo groups [Moue5f8S = 2.416(1) and 2.402(1) and N donors trans to oxo [Moue5f8N = 2.325(3) and 2.385(4) A]. MoO2[SC(CH3)2CH2NHCH2CH2NHCH2C(CH3)2S] crystallizes in the space group P21/c with a = 10.798(5), b = 6.911(2), c = 20.333(9) A, β = 95.20°, V = 1511(2) A3 and Z = 4. Its structure is very similar to that of MoO2[NH2C(CH3)2CH2S]2 with Moue5f8O = 1.714(2) and 1.710(2), Moue5f8S = 2.415(1) and 2.404(1) and Moue5f8N = 2.316(3) and 2.362(3). The small differences in the geometries of the two compounds are attributed to the constraints of the extra chelate ring in the complex with the tetradentate ligand. The structures in this paper stand in contrast to those reported for complexes of similar ligands wherein steric hindrance produces complexes with a skew trapezoidal bipyramidal structure.

Mo(VI) Complexes of N,S-Donor Ligands: Relevance to Molybdenum Enzymes

The molybdenum enzymes other than nitrogenase have a common molybdenum cofactor1,2 and spectroscopic studies of their molybdenum sites reveal these to be similar although not identical.3 EPR* studies on xanthine oxidase in the Mo(V) oxidation state,4 together with results from model compounds,5 led to the suggestion of sulfur as a donor atom to molybdenum. However, in these original studies and in subsequent investigations, 3,6 the inorganic compounds used in the comparison were not structurally and, at times, not even stoichiometrically defined. In order to provide a comprehensive set of structurally defined oxo-molybdenum complexes containing sulfur-donor ligands for comparison with the enzymes by EPR*, EXAFS* and other spectroscopic probes, we have embarked on an exten sive synthetic and isolation program to obtain relevant compounds in the Mo(IV), Mo(V) and MoiVI) oxidation states. Recently, EXAFS studies on xanthine oxidase and sulfite oxidase8 have confirmed the presence of sulfur and terminal oxo ligands in the coordination sphere of molybdenum and have revealed distinct similarities to some of the model compounds discussed here.9

Structural Chemistry of Molybdenum in Metalloenzymes as Elucidated by EXAFS

One of the most important perspectives in the study of metalloproteins is the relationship between chemical structure and function. A vast body of chemical research has shown structure and function to be irrevocably intertwined; it is upon this foundation that a good deal of the predictive power of chemistry rests. This paper describes a new structural tool, x-ray absorption spectroscopy (XAS), 1’2’3’4 and its application to the study of the coordination environment of molybdenum in biological systems.