Influence of protein dynamics on the metal-sites of ovotransferrin

Molecular Rotational Correlation Times and Nanoviscosity Determined by 111m Cd Perturbed Angular Correlation (PAC) of γ-rays Spectroscopy

The nanoviscosity experienced by molecules in solution may be determined through measurement of the molecular rotational correlation time, τ_c , for example, by fluorescence and NMR spectroscopy. With this work, we apply PAC spectroscopy to determine the rate of rotational diffusion, λ=1/τ_c , of a de novo designed protein, TRIL12AL16C, in solutions with viscosities, ξ, from 1.7 to 88 mPa⋅s. TRIL12AL16C was selected as molecular probe because it exhibits minimal effects due to intramolecular dynamics and static line broadening, allowing for exclusive elucidation of molecular rotational diffusion. Diffusion rates determined by PAC data agree well with literature data from fluorescence and NMR spectroscopy, and scales linearly with 1/ξ in agreement with the Stokes-Einstein-Debye model. PAC experiments require only trace amounts (∼10¹¹ ) of probe nuclei and can be conducted over a broad range of sample temperatures and pressures. Moreover, most materials are relatively transparent to γ-rays. Thus, PAC spectroscopy could find applications under circumstances where conventional techniques cannot be applied, spanning from the physics of liquids to in-vivo biochemistry.

Nanosecond Dynamics at Protein Metal Sites: An Application of Perturbed Angular Correlation (PAC) of γ-Rays Spectroscopy

Metalloproteins are essential to numerous reactions in nature, and constitute approximately one-third of all known proteins. Molecular dynamics of proteins has been elucidated with great success both by experimental and theoretical methods, revealing atomic level details of function involving the organic constituents on a broad spectrum of time scales. However, the characterization of dynamics at biomolecular metal sites on nanosecond time scales is scarce in the literature. The aqua ions of many biologically relevant metal ions exhibit exchange of water molecules on the nanosecond time scale or faster, often defining their reactivity in aqueous solution, and this is presumably also a relevant time scale for the making and breaking of coordination bonds between metal ions and ligands at protein metal sites. Ligand exchange dynamics is critical for a variety of elementary steps of reactions in metallobiochemistry, for example, association and dissociation of metal bound water, association of substrate and dissociation of product in the catalytic cycle of metalloenzymes, at regulatory metal sites which require binding and dissociation of metal ions, as well as in the transport of metal ions across cell membranes or between proteins involved in metal ion homeostasis. In Perturbed Angular Correlation of γ-rays (PAC) spectroscopy, the correlation in time and space of two γ-rays emitted successively in a nuclear decay is recorded, reflecting the hyperfine interactions of the PAC probe nucleus with the surroundings. This allows for characterization of molecular and electronic structure as well as nanosecond dynamics at the PAC probe binding site. Herein, selected examples describing the application of PAC spectroscopy in probing the dynamics at protein metal sites are presented, including (1) exchange of Cd²⁺ bound water in de novo designed synthetic proteins, and the effect of remote mutations on metal site dynamics; (2) dynamics at the β-lactamase active site, where the metal ion appears to jump between the two adjacent sites; (3) structural relaxation in small blue copper proteins upon ¹¹¹Ag⁺ to ¹¹¹Cd²⁺ transformation in radioactive nuclear decay; (4) metal ion transfer between two HAH1 proteins with change in coordination number; and (5) metal ion sensor proteins with two coexisting metal site structures. With this Account, we hope to make our modest contribution to the field and perhaps spur additional interest in dynamics at protein metal sites, which we consider to be severely underexplored. Relatively little is known about detailed atomic motions at metal sites, for example, how ligand exchange processes affect protein function, and how the amino acid composition of the protein may control this facet of metal site characteristics. We also aim to provide the reader with a qualitative impression of the possibilities offered by PAC spectroscopy in bioinorganic chemistry, especially when elucidating dynamics at protein metal sites, and finally present data that may serve as benchmarks on a relevant time scale for development and tests of theoretical molecular dynamics methods applied to biomolecular metal sites.

Plastocyanin binding to photosystem I as a function of the charge state of the metal ion: effect of metal site conformation

The binding of Ag- and Cd-substituted plastocyanin to reduced photosystem 1 of spinach has been studied through the rotational correlation time of plastocyanin measured by the technique of perturbed angular correlation of gamma-rays (PAC). Ag and Cd are used as models for native Cu(I) and Cu(II), respectively. A dissociation constant of 5 microM was found for Ag-plastocyanin, whereas the dissociation constant was at least 24 times higher for Cd-plastocyanin. PAC was further used to characterize the structure of the metal site of Cd- and Ag-plastocyanin. The Cd spectra are characteristic of a planar configuration of one cysteine and two histidines. However, the spectra show an unusual peak broadening and a high degree of internal motion, interpreted as motion of one of the histidines within the plane. (111)Ag decays to (111)Cd, followed by the emission of two gamma-rays used for the PAC experiment. The (111)Ag PAC spectra indicate that one of the coordinating histidines has a different position in the Ag protein than in the Cd protein but that the decay of Ag to Cd causes a relaxation of the position of this histidine to the position in the Cd protein within 20 ns. Binding of Ag-plastocyanin to photosystem I stabilized the Ag metal site structure so that no relaxation was observed on a time scale of 100 ns. This stabilization of the Ag structure upon binding indicates that the metal site structure is involved in regulating how the dissociation constant for plastocyanin depends on the charge of the metal ion.

Metal-ion binding properties of the transferrins: a vanadium-51 NMR study

Transferrins can bind a wide range of di- and trivalent metal ions. They have a bilobal structure where each domain contains a deep cleft that binds a metal ion along with a synergistic anion. In this work, the binding of vanadate as VO2+ to the transferrins was studied by 51V quadrupolar central transition (QCT) NMR. Information about differences in the symmetry and motion of the bound metal ion was obtained from chemical shift and line width differences for serotransferrin (sTf), lactoferrin (lTf), and ovotransferrrin(oTf). The effects of pH, ionic strength, and temperature on the 51V QCT NMR spectra of the bound VO2+ cations showed that the N-lobe binding site of sTf is unique as compared to the other proteins. Properties of the quadrupolar central transition were also investigated, revealing that temperature, magnetic field strength, and NMR pulse angle all induce predictable changes on the second-order dynamic frequency shift, spectral line width, and optimal pulse angle in the 51V NMR spectra. Analysis of NMR spectra of V(V)2-oTf and V(V)2-sTf at three magnetic fields allowed an estimation of the quadrupolar coupling constants for these binding sites. This indicates that the degree of coordination symmetry in the binding sites is as follows: sTf N < sTf C < oTf N, C. Carbon-13 NMR studies revealed that VO2+ binding, in contrast to di- and trivalent metal ions, has no requirement for a synergistic anion.