A Single Outer-Sphere Mutation Stabilizes apo-Mn Superoxide Dismutase by 35 °C and Disfavors Mn Binding

The catalytic active site of Mn-specific superoxide dismutase (MnSOD) is organized around a redox-active Mn ion. The most highly conserved difference between MnSODs and the homologous FeSODs is the origin of a Gln in the second coordination sphere. In MnSODs it derives from the C-terminal domain whereas in FeSODs it derives from the N-terminal domain, yet its side chain occupies almost superimposable positions in the active sites of these two types of SODs. Mutation of this Gln69 to Glu in Escherichia coli FeSOD increased the Fe^3+/2+ reduction midpoint potential by >0.6 V without disrupting the structure or Fe binding [Yikilmaz, E., Rodgers, D. W., and Miller, A.-F. (2006) Biochemistry 45 (4), 1151−1161]. We now describe the analogous Q146E mutant of MnSOD, explaining its low Mn content in terms increased stability of the apo-Mn protein. In 0.8 M guanidinium HCl, Q146E-apoMnSOD displays an apparent melting midpoint temperature (<i>T</i>_m) 35 °C <i>higher</i> that of wild-type (WT) apoMnSOD, whereas the <i>T</i>_m of WT-holoMnSOD is only 20 °C higher than that of WT-apoMnSOD. In contrast, the <i>T</i>_m attributed to Q146E-holoMnSOD is 40 °C <i>lower</i> than that of Q146E-apoMnSOD. Thus, our data refute the notion that the WT residues optimize the structural stability of the protein and instead are consistent with conservation on the basis of enzyme function and therefore ability to bind metal ion. We propose that the WT-MnSOD protein conserves a destabilizing amino acid at position 146 as part of a strategy to favor metal ion binding

Two Major Pre-Nucleation Species that are Conformationally Distinct and in Equilibrium of Self-Association

To understand how solution chemistry governs polymorphic formation of organic crystals, solution NMR measurements of tolfenamic acid were conducted in ethanol. It was unveiled by chemical shift and diffusivity results that the solute molecules self-associated as dimers in solution. Further nOe (nuclear Overhauser effect) analyses indicate that a more twisted conformation became dominant over a planar conformation under the solution conditions that favored the dimer formation. This discovery is rationalized in terms of the energy balance between the conformation and intermolecular hydrogen bonding of the solute molecule, suggesting a significant role of the cooperability between a molecule’s conformation and its intermolecular interaction in determining the nucleation outcome of distinct crystal structures

¹H Dynamic Nuclear Polarization Based on an Endogenous Radical

We demonstrate a 15-fold enhancement of solid-state NMR signals via dynamic nuclear polarization (DNP) based on a stable, naturally occurring radical in a protein: the flavin mononucleotide (FMN) semiquinone of flavodoxin. The line width of flavodoxin’s EPR signal suggests that the dominant DNP mechanism is the solid effect, consistent with the field-dependent DNP enhancement profile. The magnitude of the enhancement as well as the bulk-polarization build-up time constant (τ_B) with which it develops are dependent on the isotopic composition of the protein. Deuteration of the protein to 85% increased the nuclear longitudinal relaxation time <i>T</i>_1n and τ_B by factors of five and seven, respectively. Slowed dissipation of polarization can explain the 2-fold higher maximal enhancement than that obtained in proteated protein, based on the endogenous semiquinone. In contrast, the long τ_B of TOTAPOL-based DNP in nonglassy samples was not accompanied by a similarly important long <i>T</i>_1n, and in this case the enhancement was greatly reduced. The low concentrations of radicals occurring naturally in biological systems limit the magnitude of DNP enhancement that is attainable by this means. However, our enhancement factors of up to 15 can nonetheless make an important difference to the feasibility of applying solid-state NMR to biochemical systems. We speculate that DNP based on endogenous radicals may facilitate MAS NMR characterization of biochemical complexes and even organelles, and could also serve as a source of additional structural and physiological information

Layer-by-Layer-Assembled Laccase Enzyme on Stimuli-Responsive Membranes for Chloro-Organics Degradation

Functionalized membranes provide versatile platforms for the incorporation of biocatalysts and nanostructured materials for efficient and benign environmental remediation. The existing techniques for remediating chloro-organics in water consist of both physical and chemical means mostly using metal oxide-based catalysts, despite associated environmental concerns. To offer bioinspired remediation as an alternative, we herein demonstrate a layer-by-layer approach to immobilize laccase enzyme onto pH-responsive functionalized membranes for the degradation of chloro-organics in water. The efficacy of these bioinspired membranes toward dechlorination of 2,4,6-trichlorophenol (TCP) is demonstrated under a pressure-driven continuous flow mode (convective flow) for the first time to the best of our knowledge. Over 80% of the initial TCP was degraded at an optimum flow rate under an applied air pressure of about 0.7 bar or lower. This corresponds to degradation of a substantial amount of the initial substrate in only 36 s residence time, whereas it takes hours for degradation in a batch reaction. This, in fact, demonstrates an energy efficient flow-through system with potentially large-scale applications. Comparison of the stability of the enzyme in the solution phase versus immobilized on the membrane phase showed a loss of some 65% of enzyme activity in the solution phase after 22 d, whereas the membrane-bound enzyme lost only a negligible percentage of the activity in a comparable time span. Finally, the membrane was exposed to rigorous cycles of TCP degradation trials to study its reusability. The primary results reveal a loss of only 14% of the initial activity after 4 cycles of use in a period of 25 d, demonstrating its potential to be reused. Regeneration of the functionalized membrane was also validated by dislodging the immobilized enzyme, followed by immobilization of fresh enzyme onto the membrane

Geometric and Electronic Structures of Manganese-Substituted Iron Superoxide Dismutase

The active-site structures of the oxidized and reduced forms of manganese-substituted iron superoxide dismutase (Mn­(Fe)­SOD) are examined, for the first time, using a combination of spectroscopic and computational methods. On the basis of electronic absorption, circular dichroism (CD), magnetic CD (MCD), and variable-temperature variable-field MCD data obtained for oxidized Mn­(Fe)­SOD, we propose that the active site of this species is virtually identical to that of wild-type manganese SOD (MnSOD), with both containing a metal ion that resides in a trigonal bipyramidal ligand environment. This proposal is corroborated by quantum mechanical/molecular mechanical (QM/MM) computations performed on complete protein models of Mn­(Fe)­SOD in both its oxidized and reduced states and, for comparison, wild-type (WT) MnSOD. The major differences between the QM/MM optimized active sites of WT MnSOD and Mn­(Fe)­SOD are a smaller (His)­N–Mn–N­(His) equatorial angle and a longer (Gln146(69))­NH···O­(sol) H-bond distance in the metal-substituted protein. Importantly, these modest geometric differences are consistent with our spectroscopic data obtained for the oxidized proteins and high-field electron paramagnetic resonance spectra reported previously for reduced Mn­(Fe)­SOD and MnSOD. As Mn­(Fe)­SOD exhibits a reduction midpoint potential (<i>E</i>_m) almost 700 mV higher than that of MnSOD, which has been shown to be sufficient for explaining the lack of SOD activity displayed by the metal-subtituted species (Vance, C. K.; Miller, A. F. <i>Biochemistry</i> <b>2001</b>, <i>40</i>, 13079–13087), <i>E</i>_m’s were computed for our experimentally validated QM/MM optimized models of Mn­(Fe)­SOD and MnSOD. These computations properly reproduce the experimental trend and reveal that the drastically elevated <i>E</i>_m of the metal substituted protein stems from a larger separation between the second-sphere Gln residue and the coordinated solvent in Mn­(Fe)­SOD relative to MnSOD, which causes a weakening of the corresponding H-bond interaction in the oxidized state and alleviates steric crowding in the reduced state

Multitechnique Investigation of the pH Dependence of Phosphate Induced Transformations of ZnO Nanoparticles

In order to properly evaluate the ecological and human health risks of ZnO manufactured nanomaterials (MNMs) released to the environment, it is critical to understand the likely transformation products in various environments, such as soils, surface and ground waters, and wastewater treatment processes. To address this knowledge gap, we examined the transformation of 30 nm ZnO MNMs in the presence of different concentrations of phosphate as a function of time and pH using a variety of orthogonal analytical techniques. The data reveal that ZnO MNMs react with phosphate at various concentrations and transform into two distinct morphological/structural phases: a micrometer scale crystalline zinc phosphate phase (hopeite-like) and a nanoscale phase that likely consists of a ZnO core with an amorphous Zn₃(PO₄)₂ shell. The P species composition was also pH dependent, with 82% occurring as hopeite-like P at pH 6 while only 15% occurred as hopeite-like P at pH 8. These results highlight how reactions of ZnO MNMs with phosphate are influenced by environmental variables, including pH, and may ultimately result in structurally and morphologically heterogeneous end products

The Electron Bifurcating FixABCX Protein Complex from <i>Azotobacter vinelandii</i>: Generation of Low-Potential Reducing Equivalents for Nitrogenase Catalysis

The biological reduction of dinitrogen (N₂) to ammonia (NH₃) by nitrogenase is an energetically demanding reaction that requires low-potential electrons and ATP; however, pathways used to deliver the electrons from central metabolism to the reductants of nitrogenase, ferredoxin or flavodoxin, remain unknown for many diazotrophic microbes. The FixABCX protein complex has been proposed to reduce flavodoxin or ferredoxin using NADH as the electron donor in a process known as electron bifurcation. Herein, the FixABCX complex from <i>Azotobacter vinelandii</i> was purified and demonstrated to catalyze an electron bifurcation reaction: oxidation of NADH (<i>E</i>_m = −320 mV) coupled to reduction of flavodoxin semiquinone (<i>E</i>_m = −460 mV) and reduction of coenzyme Q (<i>E</i>_m = 10 mV). Knocking out <i>fix</i> genes rendered Δ<i>rnf A. vinelandii</i> cells unable to fix dinitrogen, confirming that the FixABCX system provides another route for delivery of electrons to nitrogenase. Characterization of the purified FixABCX complex revealed the presence of flavin and iron–sulfur cofactors confirmed by native mass spectrometry, electron paramagnetic resonance spectroscopy, and transient absorption spectroscopy. Transient absorption spectroscopy further established the presence of a short-lived flavin semiquinone radical, suggesting that a thermodynamically unstable flavin semiquinone may participate as an intermediate in the transfer of an electron to flavodoxin. A structural model of FixABCX, generated using chemical cross-linking in conjunction with homology modeling, revealed plausible electron transfer pathways to both high- and low-potential acceptors. Overall, this study informs a mechanism for electron bifurcation, offering insight into a unique method for delivery of low-potential electrons required for energy-intensive biochemical conversions