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1. Mimicking Elementary Reactions of Manganese Lipoxygenase Using Mn-hydroxo and Mn-alkylperoxo Complexes

   https://doi.org/10.3390/molecules26237151  

   Opalade, Adedamola A. ; Grotemeyer, Elizabeth N. ; Jackson, Timothy A. ( December 2021 , Molecules)

   Manganese lipoxygenase (MnLOX) is an enzyme that converts polyunsaturated fatty acids to alkyl hydroperoxides. In proposed mechanisms for this enzyme, the transfer of a hydrogen atom from a substrate C-H bond to an active-site MnIII-hydroxo center initiates substrate oxidation. In some proposed mechanisms, the active-site MnIII-hydroxo complex is regenerated by the reaction of a MnIII-alkylperoxo intermediate with water by a ligand substitution reaction. In a recent study, we described a pair of MnIII-hydroxo and MnIII-alkylperoxo complexes supported by the same amide-containing pentadentate ligand (6Medpaq). In this present work, we describe the reaction of the MnIII-hydroxo unit in C-H and O-H bond oxidation processes, thus mimicking one of the elementary reactions of the MnLOX enzyme. An analysis of kinetic data shows that the MnIII-hydroxo complex [MnIII(OH)(6Medpaq)]+ oxidizes TEMPOH (2,2′-6,6′-tetramethylpiperidine-1-ol) faster than the majority of previously reported MnIII-hydroxo complexes. Using a combination of cyclic voltammetry and electronic structure computations, we demonstrate that the weak MnIII-N(pyridine) bonds lead to a higher MnIII/II reduction potential, increasing the driving force for substrate oxidation reactions and accounting for the faster reaction rate. In addition, we demonstrate that the MnIII-alkylperoxo complex [MnIII(OOtBu)(6Medpaq)]+ reacts with water to obtain the corresponding MnIII-hydroxo species, thus mimicking the ligand substitution stepmore »proposed for MnLOX.« less
2. Molecular mechanism of the chitinolytic peroxygenase reaction

   https://doi.org/10.1073/pnas.1904889117  

   Bissaro, Bastien ; Streit, Bennett ; Isaksen, Ingvild ; Eijsink, Vincent G. H. ; Beckham, Gregg T. ; DuBois, Jennifer L. ; Røhr, Åsmund K. ( January 2020 , Proceedings of the National Academy of Sciences)

   Lytic polysaccharide monooxygenases (LPMOs) are a recently discovered class of monocopper enzymes broadly distributed across the tree of life. Recent reports indicate that LPMOs can use H₂O₂as an oxidant and thus carry out a novel type of peroxygenase reaction involving unprecedented copper chemistry. Here, we present a combined computational and experimental analysis of the H₂O₂-mediated reaction mechanism. In silico studies, based on a model of the enzyme in complex with a crystalline substrate, suggest that a network of hydrogen bonds, involving both the enzyme and the substrate, brings H₂O₂into a strained reactive conformation and guides a derived hydroxyl radical toward formation of a copper–oxyl intermediate. The initial cleavage of H₂O₂and subsequent hydrogen atom abstraction from chitin by the copper–oxyl intermediate are the main energy barriers. Stopped-flow fluorimetry experiments demonstrated that the priming reduction of LPMO–Cu(II) to LPMO–Cu(I) is a fast process compared to the reoxidation reactions. Using conditions resulting in single oxidative events, we found that reoxidation of LPMO–Cu(I) is 2,000-fold faster with H₂O₂than with O₂, the latter being several orders of magnitude slower than rates reported for other monooxygenases. The presence of substrate accelerated reoxidation by H₂O₂, whereas reoxidation by O₂became slower, supporting the peroxygenase paradigm. These insights intomore »the peroxygenase nature of LPMOs will aid in the development and application of enzymatic and synthetic copper catalysts and contribute to a further understanding of the roles of LPMOs in nature, varying from biomass conversion to chitinolytic pathogenesis-defense mechanisms.

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3. Imaging active site chemistry and protonation states: NMR crystallography of the tryptophan synthase α-aminoacrylate intermediate

   https://doi.org/10.1073/pnas.2109235119  

   Holmes, Jacob B. ; Liu, Viktoriia ; Caulkins, Bethany G. ; Hilario, Eduardo ; Ghosh, Rittik K. ; Drago, Victoria N. ; Young, Robert P. ; Romero, Jennifer A. ; Gill, Adam D. ; Bogie, Paul M. ; et al ( January 2022 , Proceedings of the National Academy of Sciences)

   NMR-assisted crystallography—the integrated application of solid-state NMR, X-ray crystallography, and first-principles computational chemistry—holds significant promise for mechanistic enzymology: by providing atomic-resolution characterization of stable intermediates in enzyme active sites, including hydrogen atom locations and tautomeric equilibria, NMR crystallography offers insight into both structure and chemical dynamics. Here, this integrated approach is used to characterize the tryptophan synthase α-aminoacrylate intermediate, a defining species for pyridoxal-5′-phosphate–dependent enzymes that catalyze β-elimination and replacement reactions. For this intermediate, NMR-assisted crystallography is able to identify the protonation states of the ionizable sites on the cofactor, substrate, and catalytic side chains as well as the location and orientation of crystallographic waters within the active site. Most notable is the water molecule immediately adjacent to the substrate β-carbon, which serves as a hydrogen bond donor to the ε-amino group of the acid–base catalytic residue βLys87. From this analysis, a detailed three-dimensional picture of structure and reactivity emerges, highlighting the fate of the L-serine hydroxyl leaving group and the reaction pathway back to the preceding transition state. Reaction of the α-aminoacrylate intermediate with benzimidazole, an isostere of the natural substrate indole, shows benzimidazole bound in the active site and poised for, but unable to initiate, the subsequent bondmore »formation step. When modeled into the benzimidazole position, indole is positioned with C3 in contact with the α-aminoacrylate C β and aligned for nucleophilic attack. Here, the chemically detailed, three-dimensional structure from NMR-assisted crystallography is key to understanding why benzimidazole does not react, while indole does.« less
4. Coupled Transmembrane Substrate Docking and Helical Unwinding in Intramembrane Proteolysis of Amyloid Precursor Protein

   https://doi.org/10.1038/s41598-018-30015-6  

   Clemente, Nicolina ; Abdine, Alaa ; Ubarretxena-Belandia, Iban ; Wang, Chunyu ( August 2018 , Scientific Reports)

   Intramembrane-cleaving proteases (I-CLiPs) play crucial roles in physiological and pathological processes, such as Alzheimer’s disease and cancer. However, the mechanisms of substrate recognition by I-CLiPs remain poorly understood. The aspartic I-CLiP presenilin is the catalytic subunit of the γ-secretase complex, which releases the amyloid-β peptides (Aβs) through intramembrane proteolysis of the transmembrane domain of the amyloid precursor protein (APPTM). Here we used solution NMR to probe substrate docking of APPTM to the presenilin homologs (PSHs) MCMJR1 and MAMRE50, which cleaved APPTM in the NMR tube. Chemical shift perturbation (CSP) showed juxtamembrane regions of APPTM mediate its docking to MCMJR1. Binding of the substrate to I-CLiP decreased the magnitude of amide proton chemical shifts δ_Hat the C-terminal half of the substrate APPTM, indicating that the docking to the enzyme weakens helical hydrogen bonds and unwinds the substrate transmembrane helix around the initial ε-cleavage site. The APPTM V44M substitution linked to familial AD caused more CSP and helical unwinding around the ε-cleavage site. MAMRE50, which cleaved APPTM at a higher rate, also caused more CSP and helical unwinding in APPTM than MCMJR1. Our data suggest that docking of the substrate transmembrane helix and helical unwinding is coupled in intramembrane proteolysis andmore »FAD mutation modifies enzyme/substrate interaction, providing novel insights into the mechanisms of I-CLiPs and AD drug discovery.

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5. A Hammett Study of Clostridium acetobutylicum Alcohol Dehydrogenase (CaADH): An Enzyme with Remarkable Substrate Promiscuity and Utility for Organic Synthesis

   https://doi.org/10.1055/s-0039-1691576  

   Kudalkar, Gaurav P. ; Tiwari, Virendra K. ; Lee, Joshua D. ; Berkowitz, David B. ( January 2020 , Synlett)

   Described is a physical organic study of the reduction of three sets of carbonyl compounds by the NADPH-dependent enzyme Clostridium acetobutylicum alcohol dehydrogenase (CaADH). Previous studies in our group have shown this enzyme to display broad substrate promiscuity, yet remarkable stereochemical fidelity, in the reduction of carbonyl compounds, including α-, β- and γ-keto esters (d-stereochemistry), as well as α,α-difluorinated-β-keto phosphonate esters (l-stereochemistry). To better mechanistically characterize this promising dehydrogenase enzyme, we report here the results of a Hammett linear free-energy relationship (LFER) study across three distinct classes of carbonyl substrates; namely aryl aldehydes, aryl β-keto esters and aryl trifluoromethyl ketones. Rates are measured by monitoring the decrease in NADPH fluorescence at 460 nm with time across a range of substrate concentrations for each member of each carbonyl compound class. The resulting v 0 versus [S] data are subjected to least-squares hyperbolic fitting to the Michaelis–Menton equation. Hammett plots of log(V max) versus σX yield the following Hammett parameters: (i) for p-substituted aldehydes, ρ = 0.99 ± 0.10, ρ = 0.40 ± 0.09; two domains observed, (ii) for p-substituted β-keto esters ρ = 1.02 ± 0.31, and (iii) for p-substituted aryl trifluoromethyl ketones ρ = –0.97 ± 0.12. The positive sign of ρ indicated for the first two compoundmore »classes suggests that the hydride transfer from the nicotinamide cofactor is at least partially rate-limiting, whereas the negative sign of ρ for the aryl trifluoromethyl ketone class suggests that dehydration of the ketone hydrate may be rate-limiting for this compound class. Consistent with this notion, examination of the 13C NMR spectra for the set of p-substituted aryl trifluo­romethyl ketones in 2% aqueous DMSO reveals significant formation of the hydrate (gem-diol) for this compound family, with compounds bearing the more electron-withdrawing groups showing greater degrees of hydration. This work also presents the first examples of the CaADH-mediated reduction of aryl trifluoromethyl ketones, and chiral HPLC analysis indicates that the parent compound α,α,α-trifluoroacetophenone is enzymatically reduced in 99% ee and 95% yield, providing the (S)-stereoisomer, suggesting yet another compound class for which this enzyme displays high enantioselectivity.« less