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Iron enzymes activate dioxygen for a large number of biomedically, agriculturally, and environmentally important oxidation reactions. Among those that use non-heme mono-iron cofactors, protein ligand sets as minimal as a pair of cis histidines enable the cofactor to coordinate and approximate as many as three different substrates, leading to an astounding array of different reaction types. Many of these reactions install key functional groups in lucrative natural-product drugs. Over the last 15 years, we have established the intermediacy of iron(IV)-oxo (ferryl) complexes in the reactions of several such enzymes with a range of distinct outcomes.¹ In general, the ferryl intermediates generate substrate radicals by abstracting hydrogen (H•) from unactivated aliphatic carbons,^2-6 initiating formation of new C–O,^2-4 ��–C��/����,^5,6 ��–S,⁷ ��–N,⁸ or C–C bonds.⁹ Hydroxylation is the default outcome, as it results from coupling between the carbon radical and the necessarily adjacent Fe(III)-coordinated oxygen that just generated it (termed “oxygen rebound” by Groves). Thus, the first imperative for enzymes that mediate outcomes other than hydroxylation is to avoid the default rebound, which generally has a low activation barrier. In our current work, we are defining the manifold of other pathways by which the substrate radical can decay and the strategies by which a given protein scaffold (or, in rare cases, the substrate itself) specifies a reaction channel. A generally important parameter is the disposition of the substrate relative to the cofactor,¹⁰ which appears to be controlled not only by the substrate pocket but also by the geometry of the ferryl complex (oxo trans to either one of the two conserved histidine ligands).¹¹ It has therefore been important to have tools to visualize intermediates (especially the ferryl complexes) through the reaction sequences.^12,13 I will summarize several of the Penn State team’s recent successes in defining reaction pathways and explaining control of outcome in this versatile enzyme family.  

1.  Krebs, C., Galonic, D.; Walsh, C. T.; Bollinger, J. M., Jr. "Non-Heme Fe(IV)-Oxo Intermediates," Acc. Chem. Res., 2007, 40, 484-492.

2.  Price, J. C.; Barr, E. W.; Tirupati, B.; Bollinger, J. M., Jr.; Krebs, C.; “The First Direct Characterization of a High-Valent Iron Intermediate in the Reaction of an aKetoglutarate-Dependent Dioxygenase: A High-Spin Fe(IV) Complex in Taurine:a-Ketoglutarate Dioxygenase (TauD) from Escherichia coli,” Biochemistry, 2003, 42, 7497-7508.

3.  Price, J. C.; Barr, E. W.; Glass, T. E.; Krebs, C.; Bollinger, J. M., Jr.; “Evidence for Hydrogen Abstraction from C1 of Taurine by the High-Spin Fe(IV) Intermediate Detected during Oxygen Activation by Taurine:a-Ketoglutarate Dioxygenase (TauD),” J. Am. Chem. Soc., 2003, 125, 13008-13009.

4.  Hoffart, L. M.; Barr, E. W.; Guyer, R. B.; Bollinger, J. M., Jr.; Krebs, C. “Direct spectroscopic detection of a C-H-cleaving high-spin Fe(IV) complex in a prolyl-4-hydroxylase,” Proc. Natl. Acad. Sci. USA, 2006, 103, 14738-14743.

5.  Galonic, D. P.; Barr, E. W.; Walsh, C. T.; Bollinger, J. M., Jr.; Krebs, C. “Two Interconverting Fe(IV) Intermediates in Aliphatic Chlorination by the Halogenase CytC3,” Nat. Chem. Biol., 2007, 3, 113-116.

6.  Matthews, M. L.; Krest, C. M.; Barr, E. W.; Vaillancourt, F. H.; Walsh, C. T.; Green, M. T.; Krebs, C.; Bollinger, J. M., Jr. “Substrate-Triggered Formation and Remarkable Stability of the C-H Bond-Cleaving Chloroferryl Intermediate in the Aliphatic Halogenase, SyrB2,” Biochemistry, 2009, 48, 4331-4343.

7.  Tamanaha, E. Y.; Zhang, B.; Guo, Y.; Chang, W.-c.; Barr, E. W.; Xing, G.; St. Clair, J.; Ye, S.; Neese, F.; Bollinger, J. M., Jr.; Krebs, C. “Spectroscopic evidence for the two C-H-cleaving intermediates of Aspergillus nidulans isopenicillin N synthase," J. Am. Chem. Soc. 2016, 138, 8862-8874.

8.  Matthews, M.L.; Chang, W.-c.; Layne, A.P.; Miles, L.A.; Krebs, C.; Bollinger, J. M., Jr. “Direct Nitration and Azidation of Aliphatic Carbons by an Iron-dependent Halogenase,” Nat. Chem. Biol. 2014, 10, 209-215.

9.  Dunham, N. P.; Chang, W.-c.; Mitchell, A. J.; Martinie, R. J.; Zhang, B.; Bergman, J. A.; Rajakovich, L. J.; Wang, B.; Silakov, A.; Krebs, C.; Boal, A. K.; Bollinger, J. M., Jr. (2018) "Two Distinct Mechanisms for C–C Desaturation by Iron(II)- and 2-(Oxo)glutarate-Dependent Oxygenases: Importance of α-Heteroatom Assistance," J. Am. Chem. Soc., in review.

10. Matthews, M. L.; Neumann, C. S.; Miles, L. A.; Grove, T. L.; Booker, S. J.; Krebs, C; Walsh, C. T.; Bollinger, J. M., Jr. "Substrate positioning controls the partition between halogenation and hydroxylation in the aliphatic halogenase, SyrB2," Proc. Natl. Acad. Sci. USA, 2009, 106, 17723-17728.

11. Martinie, R. J.; Livada, J.; Chang, W-c.; Green, M. T.; Krebs, C.; Bollinger, J. M., Jr.; Silakov, A. "Experimental Correlation of Substrate Position with Reaction Outcome in the Aliphatic Halogenase, SyrB2," J. Am. Chem. Soc. 2015, 137, 6912-6919.

12. Martinie, R. J.; Pollock, C. J.; Matthews, M. L.; Bollinger, J. M., Jr.; Krebs, C; Silakov, A. “Vanadyl as a Stable Structural Mimic of Reactive Ferryl Intermediates in Mononuclear Nonheme-Iron Enzymes,” Inorg. Chem. 2017, 56, 13382-13389.

13. Mitchell, A. J.; Dunham, N. P.; Martinie, R. J.; Bergman, J. A.; Pollock, C. J.; Hu, K.; Allen, B. D.; Chang, W.-c.; Silakov, A.; Bollinger, J. M., Jr.; Krebs, C.; Boal, A. K. “Visualizing the Reaction Cycle in an Iron(II)- and 2-(Oxo)-glutarate-Dependent Hydroxylase,” J. Am. Chem. Soc. 2017, 139, 13830-13836.

Abstract:

Exposure to particulate matter pollution (PM, tiny particles suspended in air) ranks among the top ten global health risks. PM-induced oxidative stress has been suggested as a possible mechanism leading to their health effects. We conducted cellular and acellular measurements of PM-induced oxidative stress through systematic laboratory experiments and ambient field studies. Murine alveolar macrophages were used to measure intracellular reactive oxygen and nitrogen species (ROS/RNS) production, while dithiothreitol (DTT) was used to measure the chemical oxidative potential for each sample. Ambient samples (n = 104) were collected during multiple seasons from rural and urban sites around the greater Atlanta area as part of the Southeastern Center for Air Pollution and Epidemiology (SCAPE). For laboratory studies, SOA were generated from photooxidation of six commonly emitted volatile organic precursors, including isoprene, α-pinene, β-caryophyllene, pentadecane, m-xylene, and naphthalene. Laboratory experiments were conducted in the Georgia Tech Environmental (GTEC) facility under different conditions. For both ambient and laboratory samples, we found that cellular ROS/RNS production was highly dose-dependent, non-linear, and could not be represented by a single concentration measurement. For chemical oxidative potential, precursor identity influenced toxicity significantly, with isoprene and naphthalene SOA having the lowest and highest response, respectively. Both precursor identity and formation conditions influenced inflammatory responses. Several response patterns were identified for SOA precursors whose photooxidation products share similar carbon chain length and functionalities. A significant correlation between ROS/RNS levels and aerosol carbon oxidation state was also observed, which may have significant implications as ambient aerosols have an atmospheric lifetime of a week, over which oxidation state increases due to photochemical aging, potentially resulting in more toxic aerosols.

The human brain with about 10¹¹ neurons and about 10¹⁵ connections in addition to neuroglia which consists of even more numerous cells of various types is one of the most complex and fascinating systems in the universe. The connections between neurons are established through axons, which are long and often thin structures that carry electrical signals also called action potentials. Physiologic function relies on correct timing of the arrival of the electric signals and hence speed at which they travel along the axons, which, in turn, depends strongly on the diameter of the axon, which therefore must be precisely matched to its physiologic function.

The principal determinant of axon diameter in vertebrates are space-filling cytoskeletal polymers called neurofilaments (NFs). Morphometric studies have indeed established a direct correlation between NFs and axonal diameter. In addition to their space-filling role, NFs are also cargo of slow axonal transport and are in relentless but slow movement toward the nerve terminals. The focus of our collaborative research with the Brown-lab at Ohio State University is a new paradigm for the understanding of axon morphology that is rooted in the dual motile and architectural function of NFs as cargo of slow transport and space-filling structures. According to this view, axon caliber is emergent and dynamically determined by changes in the flow of NFs.  We combine fluorescent life imaging methods to characterize the dynamics of the cytoskeleton of the axon, with mathematical and computational modeling to understand how axon caliber is regulated, how morphological structures, such as constrictions at nodes of Ranvier or neurodegenerative disease related swellings, are formed.

Tailoring mesoporous carbons and related materials for energy applications

Sheng Dai ^1,2

¹Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6201, USA

²Department of ��ɫ��Ƶ, University of Tennessee, Knoxville, TN 37996-1600, USA

Carbon materials are ubiquitous in catalysis, separation, and energy storage/conversion.  The creation of well-defined carbon architectures is essential for a number of the aforementioned applications.  Recently, we have developed several methods for the synthesis of carbon materials with controlled mesostructures and compositions. The mesostructures of these carbon materials are highly stable and can be further tailored via graphitization and surface functionalization for catalysis and energy-storage applications.  This presentation will be focused on our recent development in (a) self-assembly approaches to the preparation of carbon composite materials for controlling mesostructures and morphologies and (b) surface modification techniques to control the interfacial chemistry of carbon materials for separation, catalysis, and energy-storage applications.

Abstract

This seminar is directed towards undergraduate and graduate students who want to learn about the many career areas available to them after graduation.  Vince will share his career history, P&G research project examples, and the variety of science-related roles that are required to make them happen.  He will also provide information regarding P&G internship opportunities, and the general steps to prepare and apply for them.

About the Speaker 

Vince graduated in 1991 from the ��ɫ��Ƶ ��ɫ��Ƶ Department with a Bachelor of Science degree in ��ɫ��Ƶ.  He worked in the environmental science field for one year before joining The Procter & Gamble Co. in 1992.  In his 25 years at Procter & Gamble, he has continually expanded his knowledge and expertise via additional coursework in mass spectrometry, formulation chemistry, chromatography, headspace analysis, modeling and simulation, material science, and lab management.  During this time, Vince has been an integral part of many R&D projects across several businesses and corporate R&D functions, and is a go-to expert, leader, and mentor in his area.  He is currently a Senior Scientist in the Baby Care business responsible for new material innovation and development.