13C ENDOR Spectroscopy of Lipoxygenase-Substrate Complexes Reveals the Structural Basis for C-H Activation by Tunneling

A Foundational Shift in Models for Enzyme Function

This Perspective addresses the unresolved, and still hotly contested, question of how enzymes transition from stable enzyme-substrate (ES) complexes to successful, femtosecond barrier crossings. By extending Marcus theory to enzyme-catalyzed reactions, we argue that environmental reorganization of the protein scaffold, together with associated water molecules, achieves the intersection of reactant and product potential energy surfaces. After discussing the experimentally demonstrated importance of reduced activation enthalpy in enzyme-catalyzed transformations, we describe new methodologies that measure the temperature dependence of (i) time-averaged hydrogen/deuterium exchange into backbone amides and (ii) time-dependent Stokes shifts to longer emission wavelengths in appended chromophores at the protein/water interface. These methods not only identify specific pathways for the transfer of thermal energy from solvent to the reacting bonds of bound substrates but also suggest that collective thermally activated protein restructuring must occur very rapidly (on the ns-ps time scale) over long distances. Based on these findings, we introduce a comprehensive model for how barrier crossing takes place from the ES complex. This exploits the structural preorganization inherent in protein folding and subsequent conformational sampling, which optimally positions essential catalytic components within ES ground states and correctly places reactive bonds in the substrate(s) relative to embedded energy transfer networks connecting the protein surface to the active site. The existence of these anisotropic energy distribution pathways introduces a new dimension into the ongoing quest for improved de novo enzyme design.

Quantum life science: biological nano quantum sensors, quantum technology-based hyperpolarized MRI/NMR, quantum biology, and quantum biotechnology

The emerging field of quantum life science combines principles from quantum physics and biology to study fundamental life processes at the molecular level. Quantum mechanics, which describes the properties of small particles, can help explain how quantum phenomena such as tunnelling, superposition, and entanglement may play a role in biological systems. However, capturing these effects in living systems is a formidable challenge, as it involves dealing with dissipation and decoherence caused by the surrounding environment. We overview the current status of the quantum life sciences from technologies and topics in quantum biology. Technologies such as biological nano quantum sensors, quantum technology-based hyperpolarized MRI/NMR, high-speed 2D electronic spectrometers, and computer simulations are being developed to address these challenges. These interdisciplinary fields have the potential to revolutionize our understanding of living organisms and lead to advancements in genetics, molecular biology, medicine, and bioengineering.

Host-guest geometry in paramagnetic cavitands elucidated by 19F electron-nuclear double resonance

Elucidating structural information of supramolecular host-guest systems is pivotal for understanding molecular recognition and designing functional materials. This study explores the binding modes of fluorinated benzylamine guests in cyclodextrin-based paramagnetic cavitands, employing Gd(III)-capped cyclodextrins (Gd-α-CD and Gd-β-CD, comprising six and seven glucopyranoside units, respectively) and high-field 19F electron-nuclear double resonance (ENDOR). The 19F ENDOR spectra revealed distinct behaviors based on the fluorine position and cyclodextrin cavity size. For para-fluorinated benzylamine guests, Gd-β-CD displayed a bimodal distribution of Gd-F distances, corresponding to two distinct binding modes, whereas Gd-α-CD exhibited a single binding mode. In contrast, meta-fluorinated benzylamines demonstrated a single binding mode for both Gd-α-CD and Gd-β-CD, underscoring the influence of cavity size and fluorine substitution in the guest on binding specificity. ENDOR measurements performed at the EPR central transition of Gd(III) are generally expected to yield Gd-F distances without orientation-specific details. Surprisingly, in Gd-CDs systems, an unexpected orientation selectivity was observed, enabling the extraction of both Gd-F distances and orientation of the guest molecule relative to the cavitand's Gd(III) zero-field splitting (ZFS) tensor. This two-faceted capability of 19F-ENDOR allows for determining host-guest complexation geometry and provides insights into ZFS orientation within the cavitand structure.

Spin labels for 19F ENDOR distance determination: resolution, sensitivity and distance predictability

19F electron-nuclear double resonance (ENDOR) has emerged as an attractive method for determining distance distributions in biomolecules in the range of 0.7-2 nm, which is not easily accessible by pulsed electron dipolar spectroscopy. The 19F ENDOR approach relies on spin labeling, and in this work, we compare various labels' performance. Four protein variants of GB1 and ubiquitin bearing fluorinated residues were labeled at the same site with nitroxide and trityl radicals and a Gd(III) chelate. Additionally, a double-histidine variant of GB1 was labeled with a Cu(II) nitrilotriacetic acid chelate. ENDOR measurements were carried out at W-band (95 GHz) where 19F signals are well separated from 1H signals. Differences in sensitivity were observed, with Gd(III) chelates providing the highest signal-to-noise ratio. The new trityl label, OXMA, devoid of methyl groups, exhibited a sufficiently long phase memory time to provide an acceptable sensitivity. However, the longer tether of this label effectively reduces the maximum accessible distance between the 19F and the Cα of the spin-labeling site. The nitroxide and Cu(II) labels provide valuable additional geometric insights via orientation selection. Prediction of electron-nuclear distances based on the known structures of the proteins were the closest to the experimental values for Gd(III) labels, and distances obtained for Cu(II) labeled GB1 are in good agreement with previously published NMR results. Overall, our results offer valuable guidance for selecting optimal spin labels for 19F ENDOR distance measurement in proteins.

Solvent-tuned perovskite heterostructures enable visual linoleic acid assay and edible oil species discrimination via wavelength shift

Linoleic acid (LA) detection and edible oils discrimination are essential for food safety. Recently, CsPbBr3@SiO2 heterostructures have been widely applied in edible oil assays, while deep insights into solvent effects on their structure and performance are often overlooked. Based on the suitable polarity and viscosity of cyclohexane, we prepared CsPbBr3@SiO2 Janus nanoparticles (JNPs) with high stability in edible oil and fast halogen-exchange (FHE) efficiency with oleylammonium iodide (OLAI). LA is selectively oxidized by lipoxidase to yield hydroxylated derivative (oxLA) capable of reacting with OLAI, thereby bridging LA content to naked-eye fluorescence color changes through the anti-FHE reaction. The established method for LA in edible oils exhibited consistent results with GC-MS analysis (p > 0.05). Since the LA content difference between edible oils, we further utilized chemometrics to accurately distinguish (100%) the species of edible oils. Overall, such elaborated CsPbBr3@SiO2 JNPs enable a refreshing strategy for edible oil discrimination.

Terminal Hydride Complex of High-Spin Mn

The iron-molybdenum cofactor of nitrogenase (FeMoco) catalyzes fixation of N2 via Fe hydride intermediates. Our understanding of these species has relied heavily on the characterization of well-defined 3d metal hydride complexes, which serve as putative spectroscopic models. Although the Fe ions in FeMoco, a weak-field cluster, are expected to adopt locally high-spin Fe2+/3+ configurations, synthetically accessible hydride complexes featuring d5 or d6 electron counts are almost exclusively low-spin. We report herein the isolation of a terminal hydride complex of four-coordinate, high-spin (d5; S = 5/2) Mn2+. Electron paramagnetic resonance and electron-nuclear double resonance studies reveal an unusually large degree of spin density on the hydrido ligand. In light of the isoelectronic relationship between Mn2+ and Fe3+, our results are expected to inform our understanding of the valence electronic structures of reactive hydride intermediates derived from FeMoco.

Substitution of the mononuclear, non-heme iron cofactor in lipoxygenases for structural studies

This Chapter describes methods for the biosynthetic substitution of the mononuclear, non-heme iron in plant and animal lipoxygenases (LOXs). Substitution of this iron center for a manganese ion results in an inactive, yet faithful structural surrogate of the LOX enzymes. This metal ion substitution permits structural and dynamical studies of enzyme-substrate complexes in solution and immobilized on lipid membrane surfaces. Representative procedures for two LOXs, soybean lipoxygenase (SLO) from plants and human epithelial 15-lipoxygenase-2 (15-LOX-2) from mammals, are described as examples.

Impact of N-Glycosylation on Protein Structure and Dynamics Linked to Enzymatic C-H Activation in the M. oryzae Lipoxygenase

Lipoxygenases (LOXs) from pathogenic fungi are potential therapeutic targets for defense against plant and select human diseases. In contrast to the canonical LOXs in plants and animals, fungal LOXs are unique in having appended N-linked glycans. Such important post-translational modifications (PTMs) endow proteins with altered structure, stability, and/or function. In this study, we present the structural and functional outcomes of removing or altering these surface carbohydrates on the LOX from the devastating rice blast fungus, M. oryzae, MoLOX. Alteration of the PTMs did notinfluence the active site enzyme-substrate ground state structures as visualized by electron-nuclear double resonance (ENDOR) spectroscopy. However, removal of the eight N-linked glycans by asparagine-to-glutamine mutagenesis nonetheless led to a change in substrate selectivity and an elevated activation energy for the reaction with substrate linoleic acid, as determined by kinetic measurements. Comparative hydrogen-deuterium exchange mass spectrometry (HDX-MS) analysis of wild-type and Asn-to-Gln MoLOX variants revealed a regionally defined impact on the dynamics of the arched helix that covers the active site. Guided by these HDX results, a single glycan sequon knockout was generated at position 72, and its comparative substrate selectivity from kinetics nearly matched that of the Asn-to-Gln variant. The cumulative data from model glyco-enzyme MoLOX showcase how the presence, alteration, or removal of even a single N-linked glycan can influence the structural integrity and dynamics of the protein that are linked to an enzyme's catalytic proficiency, while indicating that extensive glycosylation protects the enzyme during pathogenesis by protecting it from protease degradation.

Study of the Effects of Remote Heavy Group Vibrations on the Temperature Dependence of Hydride Kinetic Isotope Effects of the NADH/NAD+ Model Reactions

It has recently been observed that the temperature(T)-dependence of KIEs in H-tunneling reactions, characterized by isotopic activation energy difference (ΔEa = EaD - EaH), is correlated to the rigidity of the tunneling ready states (TRSs) in enzymes. A more rigid system with narrowly distributed H-donor-acceptor distances (DADs) at the TRSs gives rise to a weaker T-dependence of KIEs (i.e., a smaller ΔEa). Theoreticians have attempted to develop new H-tunneling models to explain this, but none has been universally accepted. In order to further understand the observations in enzymes and provide useful data to build new theoretical models, we have studied the electronic and solvent effects on ΔEa's for the hydride-tunneling reactions of NADH/NAD+ analogues. We found that a tighter charge-transfer (CT) complex system gives rises to a smaller ΔEa, consistent with the enzyme observations. In this paper, we use the remote heavy group (R) vibrational effects to mediate the system rigidity to study the rigidity-ΔEa relationship. The specific hypothesis is that slower vibrations of a heavier remote group would broaden the DAD distributions and increase the ΔEa value. Four NADH/NAD+ systems were studied in acetonitrile but most of such heavy group vibrations do not appear to significantly increase the ΔEa. The remote heavy group vibrations in these systems may have not affected the CT complexation rigidity to a degree that can significantly increase the DADs, and further, the ΔEa values.

Extending the Range of Distances Accessible by 19F Electron-Nuclear Double Resonance in Proteins Using High-Spin Gd(III) Labels

Fluorine electron-nuclear double resonance (19F ENDOR) has recently emerged as a valuable tool in structural biology for distance determination between F atoms and a paramagnetic center, either intrinsic or conjugated to a biomolecule via spin labeling. Such measurements allow access to distances too short to be measured by double electron-electron resonance (DEER). To further extend the accessible distance range, we exploit the high-spin properties of Gd(III) and focus on transitions other than the central transition (|-1/2⟩ ↔ |+1/2⟩), that become more populated at high magnetic fields and low temperatures. This increases the spectral resolution up to ca. 7 times, thus raising the long-distance limit of 19F ENDOR almost 2-fold. We first demonstrate this on a model fluorine-containing Gd(III) complex with a well-resolved 19F spectrum in conventional central transition measurements and show quantitative agreement between the experimental spectra and theoretical predictions. We then validate our approach on two proteins labeled with 19F and Gd(III), in which the Gd-F distance is too long to produce a well-resolved 19F ENDOR doublet when measured at the central transition. By focusing on the |-5/2⟩ ↔ |-3/2⟩ and |-7/2⟩ ↔ |-5/2⟩ EPR transitions, a resolution enhancement of 4.5- and 7-fold was obtained, respectively. We also present data analysis strategies to handle contributions of different electron spin manifolds to the ENDOR spectrum. Our new extended 19F ENDOR approach may be applicable to Gd-F distances as large as 20 Å, widening the current ENDOR distance window.

Structural Effects on the Temperature Dependence of Hydride Kinetic Isotope Effects of the NADH/NAD+ Model Reactions in Acetonitrile: Charge-Transfer Complex Tightness Is a Key

It has recently frequently been found that the kinetic isotope effect (KIE) is independent of temperature (T) in H-tunneling reactions in enzymes but becomes dependent on T in their mutants. Many enzymologists found that the trend is related to different donor-acceptor distances (DADs) at tunneling-ready states (TRSs), which could be sampled by protein dynamics. That is, a more rigid system of densely populated short DADs gives rise to a weaker T dependence of KIEs. Theoreticians have attempted to develop H-tunneling theories to explain the observations, but none have been universally accepted. It is reasonable to assume that the DAD sampling concept, if it exists, applies to the H-transfer reactions in solution, as well. In this work, we designed NADH/NAD+ model reactions to investigate their structural effects on the T dependence of hydride KIEs in acetonitrile. Hammett correlations together with N-CH3/CD3 secondary KIEs were used to provide the electronic structure of the TRSs and thus the rigidity of their charge-transfer complexation vibrations. In all three pairs of reactions, a weaker T dependence of KIEs always corresponds to a steeper Hammett slope on the substituted hydride acceptors. It was found that a tighter/rigid charge-transfer complexation system corresponds with a weaker T dependence of KIEs, consistent with the observations in enzymes.

ENDOR Spectroscopy Reveals the "Free" 5'-Deoxyadenosyl Radical in a Radical SAM Enzyme Active Site Actually is Chaperoned by Close Interaction with the Methionine-Bound [4Fe-4S]2+ Cluster

1/2H and 13C hyperfine coupling constants to 5'-deoxyadenosyl (5'-dAdo•) radical trapped within the active site of the radical S-adenosyl-l-methionine (SAM) enzyme, pyruvate formate lyase-activating enzyme (PFL-AE), both in the absence of substrate and the presence of a reactive peptide-model of the PFL substrate, are completely characteristic of a classical organic free radical whose unpaired electron is localized in the 2pπ orbital of the sp2 C5'-carbon (J. Am. Chem. Soc. 2019, 141, 12139-12146). However, prior electron-nuclear double resonance (ENDOR) measurements had indicated that this 5'-dAdo• free radical is never truly "free": tight van der Waals contact with its target partners and active-site residues guide it in carrying out the exquisitely precise, regioselective reactions that are hallmarks of RS enzymes. Here, our understanding of how the active site chaperones 5'-dAdo• is extended through the finding that this apparently unexceptional organic free radical has an anomalous g-tensor and exhibits significant 57Fe, 13C, 15N, and 2H hyperfine couplings to the adjacent, isotopically labeled, methionine-bound [4Fe-4S]2+ cluster cogenerated with 5'-dAdo• during homolytic cleavage of cluster-bound SAM. The origin of the 57Fe couplings through nonbonded radical-cluster contact is illuminated by a formal exchange-coupling model and broken symmetry-density functional theory computations. Incorporation of ENDOR-derived distances from C5'(dAdo•) to labeled-methionine as structural constraints yields a model for active-site positioning of 5'-dAdo• with a short, nonbonded C5'-Fe distance (∼3 Å). This distance involves substantial motion of 5'-dAdo• toward the unique Fe of the [4Fe-4S]2+ cluster upon S-C(5') bond-cleavage, plausibly an initial step toward formation of the Fe-C5' bond of the organometallic complex, Ω, the central intermediate in catalysis by radical-SAM enzymes.

Thirty years with three-dimensional structures of lipoxygenases

The X-ray crystal structures of soybean lipoxygenase (LOX) and rabbit 15-LOX were reported in the 1990s. Subsequent 3D structures demonstrated a conserved U-like shape of the substrate cavities as reviewed here. The 8-LOX:arachidonic acid (AA) complex showed AA bound to the substrate cavity carboxylate-out with C10 at 3.4 Å from the iron metal center. A recent cryo-electron microscopy (EM) analysis of the 12-LOX:AA complex illustrated AA in the same position as in the 8-LOX:AA complex. The 15- and 12-LOX complexes with isoenzyme-specific inhibitors/substrate mimics confirmed the U-fold. 5-LOX oxidizes AA to leukotriene A4, the first step in biosynthesis of mediators of asthma. The X-ray structure showed that the entrance to the substrate cavity was closed to AA by Phe and Tyr residues of a partly unfolded α2-helix. Recent X-ray analysis revealed that soaking with inhibitors shifted the short α2-helix to a long and continuous, which opened the substrate cavity. The α2-helix also adopted two conformations in 15-LOX. 12-LOX dimers consisted of one closed and one open subunit with an elongated α2-helix. 13C-ENDOR-MD computations of the 9-MnLOX:linoleate complex showed carboxylate-out position with C11 placed 3.4 ± 0.1 Å from the catalytic water. 3D structures have provided a solid ground for future research.

Correlation of temperature dependence of hydride kinetic isotope effects with donor-acceptor distances in two solvents of different polarities

Recently observed nearly temperature (T)-independent kinetic isotope effects (KIEs) in wild-type enzymes and T-dependent KIEs in variants were used to suggest that H-tunneling in enzymes is assisted by the fast protein vibrations that help sample short donor-acceptor distances (DADs). This supports the recently proposed role of protein vibrations in DAD sampling catalysis. However, use of T-dependence of KIEs to suggest DAD sampling associated with protein vibrations is debated. We have formulated a hypothesis regarding the correlation and designed experiments in solution to investigate it. The hypothesis is, a more rigid system with shorter DADTRS's at the tunneling ready states (TRSs) gives rise to a weaker T-dependence of KIEs, i.e., a smaller ΔEa (= EaD - EaH). In a former work, the solvent effects of acetonitrile versus chloroform on the ΔEa of NADH/NAD+ model reactions were determined, and the DADPRC's of the productive reactant complexes (PRCs) were computed to substitute the DADTRS for the DADTRS-ΔEa correlation study. A smaller ΔEa was found in the more polar acetonitrile where the positively charged PRC is better solvated and has a shorter DADPRC, indirectly supporting the hypothesis. In this work, the TRS structures of different DADTRS's for the hydride tunneling reaction from 1,3-dimethyl-2-phenylimidazoline to 10-methylacridinium were computed. The N-CH3/CD3 secondary KIEs on both reactants were calculated and fitted to the observed values to find the DADTRS order in both solutions. It was found that the equilibrium DADTRS is shorter in acetonitrile than in chloroform. Results directly support the DADTRS-ΔEa correlation hypothesis as well as the explanation that links T-dependence of KIEs to DAD sampling catalysis in enzymes.

13C Electron Nuclear Double Resonance Spectroscopy-Guided Molecular Dynamics Computations Reveal the Structure of the Enzyme-Substrate Complex of an Active, N-Linked Glycosylated Lipoxygenase

Lipoxygenase (LOX) enzymes produce important cell-signaling mediators, yet attempts to capture and characterize LOX-substrate complexes by X-ray co-crystallography are commonly unsuccessful, requiring development of alternative structural methods. We previously reported the structure of the complex of soybean lipoxygenase, SLO, with substrate linoleic acid (LA), as visualized through the integration of 13C/1H electron nuclear double resonance (ENDOR) spectroscopy and molecular dynamics (MD) computations. However, this required substitution of the catalytic mononuclear, nonheme iron by the structurally faithful, yet inactive Mn2+ ion as a spin probe. Unlike canonical Fe-LOXs from plants and animals, LOXs from pathogenic fungi contain active mononuclear Mn2+ metallocenters. Here, we report the ground-state active-site structure of the native, fully glycosylated fungal LOX from rice blast pathogen Magnaporthe oryzae, MoLOX complexed with LA, as obtained through the 13C/1H ENDOR-guided MD approach. The catalytically important distance between the hydrogen donor, carbon-11 (C11), and the acceptor, Mn-bound oxygen, (donor-acceptor distance, DAD) for the MoLOX-LA complex derived in this fashion is 3.4 ± 0.1 Å. The difference of the MoLOX-LA DAD from that of the SLO-LA complex, 3.1 ± 0.1 Å, is functionally important, although is only 0.3 Å, despite the MoLOX complex having a Mn-C11 distance of 5.4 Å and a "carboxylate-out" substrate-binding orientation, whereas the SLO complex has a 4.9 Å Mn-C11 distance and a "carboxylate-in" substrate orientation. The results provide structural insights into reactivity differences across the LOX family, give a foundation for guiding development of MoLOX inhibitors, and highlight the robustness of the ENDOR-guided MD approach to describe LOX-substrate structures.

Protein Dynamics and Enzymatic Catalysis

This Perspective presents a review of our work and that of others in the highly controversial topic of the coupling of protein dynamics to reaction in enzymes. We have been involved in studying this topic for many years. Thus, this perspective will naturally present our own views, but it also is designed to present an overview of the variety of viewpoints of this topic, both experimental and theoretical. This is obviously a large and contentious topic.

Diversity of the manganese lipoxygenase gene family - A mini-review

Analyses of fungal genomes of escalate from biological and evolutionary investigations. The biochemical analyses of putative enzymes will inevitably lag behind and only a selection will be characterized. Plant-pathogenic fungi secrete manganese-lipoxygenases (MnLOX), which oxidize unsaturated fatty acids to hydroperoxides to support infection. Six MnLOX have been characterized so far including the 3D structures of these enzymes of the Rice blast and the Take-all fungi. The goal was to use this information to evaluate MnLOX-related gene transcripts to find informative specimens for further studies. Phylogenetic analysis, determinants of catalytic activities, and the C-terminal amino acid sequences divided 54 transcripts into three major subfamilies. The six MnLOX belonged to the same "prototype" subfamily with conserved residues in catalytic determinants and C-terminal sequences. The second subfamily retained the secretion mechanism, presumably necessary for uptake of Mn²⁺, but differed in catalytic determinants and by cysteine replacement of an invariant Leu residue for positioning ("clamping") of fatty acids. The third subfamily contrasted with alanine in the Gly/Ala switch for regiospecific oxidation and a minority contained unprecedented C-terminal sequences or lacked secretion signals. With these exceptions, biochemical analyses of transcripts of the three subfamilies appear to have reasonable prospects to find active enzymes.

Solvent Effects on the Temperature Dependence of Hydride Kinetic Isotope Effects: Correlation to the Donor-Acceptor Distances

Protein structural effects on the temperature (T) dependence of kinetic isotope effects (KIEs) in H-tunneling reactions have recently been used to discuss about the role of enzyme thermal motions in catalysis. Frequently observed nearly T-independent KIEs in the wild-type enzymes and T-dependent KIEs in variants suggest that H-tunneling in the former is assisted by the naturally evolved protein constructive vibrations that help sample short donor-acceptor distances (DADs) needed. This explanation that correlates the T-dependence of KIEs with DAD sampling has been highly debated as simulations following other H-tunneling models sometimes gave alternative explanations. In this paper, solvent effects on the T-dependence of KIEs of two hydride tunneling reactions of NADH/NAD⁺ analogues (represented by ΔE_a = E_aD - E_aH) were determined in attempts to replicate the observations in enzymes and test the protein vibration-assisted DAD sampling concept. Effects of selected aprotic solvents on the DAD_PRC's of the productive reactant complexes (PRCs) and the DAD_TRS's of the activated tunneling ready states (TRSs) were obtained through computations and analyses of the kinetic data, including 2° KIEs, respectively. A weaker T-dependence of KIEs (i.e., smaller ΔE_a) was found in a more polar aprotic solvent in which the system has a shorter average DAD_PRC and DAD_TRS. Further results show that a charge-transfer (CT) complexation made of a stronger donor/acceptor gives rise to a smaller ΔE_a. Overall, the shorter and less broadly distributed DADs resulting from the stronger CT complexation vibrations give rise to a smaller ΔE_a. Our results appear to support the explanation that links the T-dependence of KIEs to the donor-acceptor rigidity in enzymes.

Iron and manganese lipoxygenases of plant pathogenic fungi and their role in biosynthesis of jasmonates

Lipoxygenases (LOX) contain catalytic iron (FeLOX), but fungi also produce LOX with catalytic manganese (MnLOX). In this review, the 3D structures and properties of fungal LOX are compared and contrasted along with their associations with pathogenicity. The 3D structures and properties of two MnLOX (Magnaporthe oryzae, Geaumannomyces graminis) and the catalysis of five additional MnLOX have provided information on the metal center, substrate binding, oxygenation, tentative O₂ channels, and biosynthesis of exclusive hydroperoxides. In addition, the genomes of other plant pathogens also code for putative MnLOX. Crystals of the 13S-FeLOX of Fusarium graminearum revealed an unusual altered geometry of the Fe ligands between mono- and dimeric structures, influenced by a wrapping sequence extension near the C-terminal of the dimers. In plants, the enzymes involved in jasmonate synthesis are well documented whereas the fungal pathway is yet to be fully elucidated. Conversion of deuterium-labeled 18:3n-3, 18:2n-6, and their 13S-hydroperoxides to jasmonates established 13S-FeLOX of F. oxysporum in the biosynthesis, while subsequent enzymes lacked sequence homologues in plants. The Rice-blast (M. oryzae) and the Take-all (G. graminis) fungi secrete MnLOX to support infection, invasive hyphal growth, and cell membrane oxidation, contributing to their devastating impact on world production of rice and wheat.

Structural and Thermodynamic Effects on the Kinetics of C-H Oxidation by Multisite Proton-Coupled Electron Transfer in Fluorenyl Benzoates

Our recent experimental and theoretical investigations have shown that fluorene C-H bonds can be activated through a mechanism in which the proton and electron are transferred from the C-H bond to a separate base and oxidant in a concerted, elementary step. This multisite proton-coupled electron transfer (MS-PCET) mechanism for C-H bond activation was shown to be facilitated by shorter proton donor-acceptor distances. With the goal of intentionally modulating this donor-acceptor distance, we have now studied C-H MS-PCET in the 3-methyl-substituted fluorenyl benzoate (2-Flr-3-Me-BzO^-). This derivative was readily oxidized by ferrocenium oxidants by initial C-H MS-PCET, with rate constants that were 6-21 times larger than those for 2-Flr-BzO^- with the same oxidants. Structural comparisons by X-ray crystallography and by computations showed that addition of the 3-methyl group caused the expected steric compression; however, the relevant C···O^- proton donor-acceptor distance was longer, due to a twist of the carboxylate group. The structural changes induced by the 3-Me group increased the basicity of the carboxylate, weakened the C-H bond, and reduced the reorganization energy for C-H bond cleavage. Thus, the rate enhancement for 2-Flr-3-Me-BzO^- was due to effects on the thermochemistry and kinetic barrier, rather than from compression of the C···O^- proton donor-acceptor distance. These results highlight both the challenges of controlling molecules on the 0.1 Å length scale and the variety of parameters that affect PCET rate constants.

Switching on H-Tunneling through Conformational Control

H-tunneling is a ubiquitous phenomenon, relevant to fields from biochemistry to materials science, but harnessing it for mastering the manipulation of chemical structures still remains nearly illusory. Here, we demonstrate how to switch on H-tunneling by conformational control using external radiation. This is outlined with a triplet 2-hydroxyphenylnitrene generated in an N₂ matrix at 10 K by UV-irradiation of an azide precursor. The anti-orientation of the nitrene's OH moiety was converted to syn by selective vibrational excitation at the 2ν(OH) frequency, thereby moving the H atom closer to the vicinal nitrene center. This triggers spontaneous H-tunneling to a singlet 6-imino-2,4-cyclohexadienone. Computations reveal that such fast H-tunneling occurs through crossing the triplet-to-singlet potential energy surfaces. Our experimental realization provides an exciting novel strategy to attain control over tunneling, opening new avenues for directing chemical transformations.

Direct Correlation between Donor-Acceptor Distance and Temperature Dependence of Kinetic Isotope Effects in Hydride-Tunneling Reactions of NADH/NAD+ Analogues

Recent study of structural effects on primary kinetic isotope effects (1° KIEs) of H-transfer reactions in enzymes and solution revealed that a more rigid reaction system gave rise to a weaker temperature dependence of 1° KIEs, i.e., a smaller isotopic activation energy difference (ΔE_a = E_aD - E_aH). This has been explained within the contemporary vibrationally assisted activated H-tunneling (VA-AHT) model in which rigidity is defined according to the density of donor-acceptor distance (DAD_TRS) populations at the tunneling ready state (TRS) sampled by heavy atom motions. To test the relationship between DAD_TRS and ΔE_a in the model, we developed a computational method to obtain the TRS structures for H-transfer reactions. The method was applied to three hydride transfer reactions of NADH/NAD⁺ analogues for which the ΔE_a's as well as secondary (2°) KIEs have been reported. The 2° KIEs computed from each TRS structure were fitted to the observed values to obtain the optimal TRSs/DAD_TRS's. It was found that a shorter DAD_TRS does correspond with a smaller ΔE_a. This appears to support the VA-AHT model. Moreover, an analysis of hybridizations at the bent TRS structures shows that rehybridizations at the donor-acceptor centers are much more advanced than predicted from the classical mechanism. This implies that more orbital preparations are required for the nonclassical H-tunneling to take place.

Determining electron-nucleus distances and Fermi contact couplings from ENDOR spectra

The hyperfine coupling between an electron spin and a nuclear spin depends on the Fermi contact coupling a_iso and, through dipolar coupling, the distance r between the electron and the nucleus. It is measured with electron-nuclear double resonance (ENDOR) spectroscopy and provides insight into the electronic and spatial structure of paramagnetic centers. The analysis and interpretation of ENDOR spectra is commonly done by ordinary least-squares fitting. As this is an ill-posed, inverse mathematical problem, this is challenging, in particular for spectra that show features from several nuclei or where the hyperfine coupling parameters are distributed. We introduce a novel Tikhonov-type regularization approach that analyzes an experimental ENDOR spectrum in terms of a complete non-parametric distribution over r and a_iso. The approach uses a penalty function similar to the cross entropy between the fitted distribution and a Bayesian prior distribution that is derived from density functional theory calculations. Additionally, we show that smoothness regularization, commonly used for a similar purpose in double electron-electron resonance (DEER) spectroscopy, is not suited for ENDOR. We demonstrate that the novel approach is able to identify and quantitate ligand protons with electron-nucleus distances between 4 and 9 Å in a series of vanadyl porphyrin compounds.

Identification of Thermal Conduits That Link the Protein-Water Interface to the Active Site Loop and Catalytic Base in Enolase

We report here on the salient role of protein mobility in accessing conformational landscapes that enable efficient enzyme catalysis. We are focused on yeast enolase, a highly conserved lyase with a TIM barrel domain and catalytic loop, as part of a larger study of the relationship of site selective protein motions to chemical reactivity within superfamilies. Enthalpically hindered variants were developed by replacement of a conserved hydrophobic side chain (Leu 343) with smaller side chains. Leu343 is proximal to the active site base in enolase, and comparative pH rate profiles for the valine and alanine variants indicate a role for side chain hydrophobicity in tuning the pK_a of the catalytic base. However, the magnitude of a substrate deuterium isotope effect is almost identical for wild-type (WT) and Leu343Ala, supporting an unchanged rate-determining proton abstraction step. The introduced hydrophobic side chains at position 343 lead to a discontinuous break in both activity and activation energy as a function of side chain volume. Hydrogen-deuterium exchange mass spectrometry (HDX-MS) experiments were performed as a function of time and temperature for WT and Leu343Ala, and provide a spatially resolved map of changes in protein flexibility following mutation. Impacts on protein flexibility are localized to specific networks that arise at the protein-solvent interface and terminate in a loop that has been shown by X-ray crystallography to close over the active site. These interrelated effects are discussed in the context of long-range, solvent-accessible and thermally activated networks that play key roles in tuning the precise distances and interactions among reactants.

Fatty Acid Allosteric Regulation of C-H Activation in Plant and Animal Lipoxygenases

Lipoxygenases (LOXs) catalyze the (per) oxidation of fatty acids that serve as important mediators for cell signaling and inflammation. These reactions are initiated by a C-H activation step that is allosterically regulated in plant and animal enzymes. LOXs from higher eukaryotes are equipped with an N-terminal PLAT (Polycystin-1, Lipoxygenase, Alpha-Toxin) domain that has been implicated to bind to small molecule allosteric effectors, which in turn modulate substrate specificity and the rate-limiting steps of catalysis. Herein, the kinetic and structural evidence that describes the allosteric regulation of plant and animal lipoxygenase chemistry by fatty acids and their derivatives are summarized.

15-Lipoxygenase-1 biosynthesis of 7S,14S-diHDHA implicates 15-lipoxygenase-2 in biosynthesis of resolvin D5

The two oxylipins 7S,14S-dihydroxydocosahexaenoic acid (diHDHA) and 7S,17S-diHDHA [resolvin D5 (RvD5)] have been found in macrophages and infectious inflammatory exudates and are believed to function as specialized pro-resolving mediators (SPMs). Their biosynthesis is thought to proceed through sequential oxidations of DHA by lipoxygenase (LOX) enzymes, specifically, by human 5-LOX (h5-LOX) first to 7(S)-hydroxy-4Z,8E,10Z,13Z,16Z,19Z-DHA (7S-HDHA), followed by human platelet 12-LOX (h12-LOX) to form 7(S),14(S)-dihydroxy-4Z,8E,10Z,12E,16Z,19Z-DHA (7S,14S-diHDHA) or human reticulocyte 15-LOX-1 (h15-LOX-1) to form RvD5. In this work, we determined that oxidation of 7(S)-hydroperoxy-4Z,8E,10Z,13Z,16Z,19Z-DHA to 7S,14S-diHDHA is performed with similar kinetics by either h12-LOX or h15-LOX-1. The oxidation at C14 of DHA by h12-LOX was expected, but the noncanonical reaction of h15-LOX-1 to make over 80% 7S,14S-diHDHA was larger than expected. Results of computer modeling suggested that the alcohol on C7 of 7S-HDHA hydrogen bonds with the backbone carbonyl of Ile399, forcing the hydrogen abstraction from C12 to oxygenate on C14 but not C17. This result raised questions regarding the synthesis of RvD5. Strikingly, we found that h15-LOX-2 oxygenates 7S-HDHA almost exclusively at C17, forming RvD5 with faster kinetics than does h15-LOX-1. The presence of h15-LOX-2 in neutrophils and macrophages suggests that it may have a greater role in biosynthesizing SPMs than previously thought. We also determined that the reactions of h5-LOX with 14(S)-hydroperoxy-4Z,7Z,10Z,12E,16Z,19Z-DHA and 17(S)-hydroperoxy-4Z,7Z,10Z,13Z,15E,19Z-DHA are kinetically slow compared with DHA, suggesting that these reactions may be minor biosynthetic routes in vivo. Additionally, we show that 7S,14S-diHDHA and RvD5 have anti-aggregation properties with platelets at low micromolar potencies, which could directly regulate clot resolution.

Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights into Aqueous Chemistry?

Recent experimental studies of kinetic isotope effects (KIE-s) and hydrogen tunnelling comprising three proton-coupled electron transfer (PCET) oxidations of ascorbate monoanion, (a) in aqueous reaction solutions, (b) in the mixed water-organic cosolvent systems, (c) in aqueous solutions of various salts and (d) in fairly diluted aqueous solutions of the various partial hydrophobes are reviewed. A number of new insights into the wealth of the kinetic isotope phenomena in the PCET reactions have been obtained. The modulation of KIE-s and hydrogen tunnelling observed when partially hydrophobic solutes are added into water reaction solution, in the case of fairly diluted solutions is revealed as the strong linear correlation of the isotopic ratios of the Arrhenius prefactors Ah/Ad and the isotopic differences in activation energies ΔE_a (D,H). The observation has been proposed to be a signature of the involvement of the collective intermolecular excitonic vibrational dynamics of water in activation processes and aqueous chemistry.

Moving Through Barriers in Science and Life

This first serious attempt at an autobiographical accounting has forced me to sit still long enough to compile my thoughts about a long personal and scientific journey. I especially hope that my trajectory will be of interest and perhaps beneficial to much younger women who are just getting started in their careers. To paraphrase from Virginia Woolf's writings in A Room of One's Own at the beginning of the 20th century, "for most of history Anonymous was a Woman." However, Ms. Woolf is also quoted as saying "nothing has really happened until it has been described," a harbinger of the enormous historical changes that were about to be enacted and recorded by women in the sciences and other disciplines. The progress in my chosen field of study-the chemical basis of enzyme action-has also been remarkable, from the first description of an enzyme's 3D structure to a growing and deep understanding of the origins of enzyme catalysis.

The Soybean Lipoxygenase-Substrate Complex: Correlation between the Properties of Tunneling-Ready States and ENDOR-Detected Structures of Ground States

Hydrogen tunneling in enzymatic C-H activation requires a dynamical sampling among ground-state enzyme-substrate (E-S) conformations, which transiently generates a tunneling-ready state (TRS). The TRS is characterized by a hydrogen donor-acceptor distance (DAD) of 2.7 Å, ∼0.5 Å shorter than the dominant DAD of optimized ground states. Recently, a high-resolution, 13C electron-nuclear double-resonance (ENDOR) approach was developed to characterize the ground-state structure of the complex of the linoleic acid (LA) substrate with soybean lipoxygenase (SLO). The resulting enzyme-substrate model revealed two ground-state conformers with different distances between the target C11 of LA and the catalytically active cofactor [Fe(III)-OH]: the active conformer "a", with a van der Waals DAD of 3.1 Å between C11 and metal-bound hydroxide, and an inactive conformer "b", with a distance that is almost 1 Å longer. Herein, the structure of the E-S complex is examined for a series of six variants in which subtle structural modifications of SLO have been introduced either at a hydrophobic side chain near the bound substrate or at a remote residue within a protein network whose flexibility influences hydrogen transfer. A remarkable correlation is found between the ENDOR-derived population of the active ground-state conformer a and the kinetically derived differential enthalpic barrier for D versus H transfer, ΔEa, with the latter increasing as the fraction of conformer a decreases. As proposed, ΔEa provides a "ruler" for the DAD within the TRS. ENDOR measurements further corroborate the previous identification of a dynamical network coupling the buried active site of SLO to the surface. This study shows that subtle imperfections within the initial ground-state structures of E-S complexes are accompanied by compromised geometries at the TRS.

Nuclear quantum effects in enzymatic reactions: simulation of the kinetic isotope effect of phenylethylamine oxidation catalyzed by monoamine oxidase A

By using computational techniques for quantizing nuclear motion one can accurately reproduce kinetic isotope effect of enzymatic reactions, as demonstrated for phenylethylamine oxidation catalyzed by the monoamine oxidase A enzyme.

Challenges in constructing accurate methods for hydrogen transfer reactions in large biological assemblies: rare events sampling for mechanistic discovery and tensor networks for quantum nuclear effects

We present two methods that address the computational complexities arising in hydrogen transfer reactions in enzyme active sites.

EPR Spectroscopic Studies of Lipoxygenases

Polyunsaturated fatty acids are sources of diverse natural, and chemically designed products. The enzyme lipoxygenase selectively oxidizes fatty acid acyl chains using controlled free radical chemistry; the products are regio- and stereo-chemically unique hydroperoxides. A conserved structural fold of ≈600 amino acids harbors a long and narrow substrate channel and a well-shielded catalytic iron. Oxygen, a co-substrate, is blocked from the active site until a hydrogen atom is abstracted from substrate bis-allylic carbon, in a non-heme iron redox cycle. EPR spectroscopy of ferric intermediates in lipoxygenase catalysis reveals changes in the metal coordination and leads to a proposal on the nature of the reactive intermediate. Remarkably, free radicals are so well controlled in lipoxygenase chemistry that spin label technology can be applied as well. The current level of understanding of steps in lipoxygenase catalysis, from the EPR perspective, will be reviewed.

Comparative kinetic isotope effects on first- and second-order rate constants of soybean lipoxygenase variants uncover a substrate-binding network

Lipoxygenases are widespread enzymes found in virtually all eukaryotes, including fungi, and, more recently, in prokaryotes. These enzymes act on long-chain polyunsaturated fatty acid substrates (C18 to C20), raising questions regarding how the substrate threads its way from solvent to the active site. Herein, we report a comparison of the temperature dependence of isotope effects on first- and second-order rate constants among single-site variants of the prototypic plant enzyme soybean lipoxygenase-1 substituted at amino acid residues inferred to impact substrate binding. We created 10 protein variants including four amino acid positions, Val-750, Ile-552, Ile-839, and Trp-500, located within a previously proposed substrate portal. The conversion of these bulky hydrophobic side chains to smaller side chains is concluded to increase the mobility of flanking helices, giving rise to increased off rates for substrate dissociation from the enzyme. In this manner, we identified a specific "binding network" that can regulate movement of the substrate from the solvent to the active site. Taken together with our previous findings on C-H and O2 activation of soybean lipoxygenase-1, these results support the emergence of multiple complementary networks within a single protein scaffold that modulate different steps along the enzymatic reaction coordinate.

Detecting and Characterizing the Kinetic Activation of Thermal Networks in Proteins: Thermal Transfer from a Distal, Solvent-Exposed Loop to the Active Site in Soybean Lipoxygenase

The rate-limiting chemical reaction catalyzed by soybean lipoxygenase (SLO) involves quantum mechanical tunneling of a hydrogen atom from substrate to its active site ferric-hydroxide cofactor. SLO has emerged as a prototypical system for linking the thermal activation of a protein scaffold to the efficiency of active site chemistry. Significantly, hydrogen-deuterium exchange-mass spectrometry (HDX-MS) experiments on wild type and mutant forms of SLO have uncovered trends in the enthalpic barriers for HDX within a solvent-exposed loop (positions 317-334) that correlate well with trends in the corresponding enthalpic barriers for kcat. A model for this behavior posits that collisions between water and loop 317-334 initiate thermal activation at the protein surface that is then propagated 15-34 Å inward toward the reactive carbon of substrate in proximity to the iron catalyst. In this study, we have prepared protein samples containing cysteine residues either at the tip of the loop 317-334 (Q322C) or on a control loop, 586-603 (S596C). Chemical modification of cysteines with the fluorophore 6-bromoacetyl-2-dimethylaminonaphthalene (Badan, BD) provides site-specific probes for the measurement of fluorescence relaxation lifetimes and Stokes shift decays as a function of temperature. Computational studies indicate that surface water structure is likely to be largely preserved in each sample. While both loops exhibit temperature-independent fluorescence relaxation lifetimes as do the Stokes shifts for S596C-BD, the activation enthalpy for the nanosecond solvent reorganization at Q322C-BD (Ea(ksolv) = 2.8(0.9) kcal/mol)) approximates the enthalpy of activation for catalytic C-H activation (Ea(kcat) = 2.3(0.4) kcal/mol). This study establishes and validates the methodology for measuring rates of rapid local motions at the protein/solvent interface of SLO. These new findings, when combined with previously published correlations between protein motions and the rate-limiting hydride transfer in a thermophilic alcohol dehydrogenase, provide experimental evidence for thermally induced "protein quakes" as the origin of enthalpic barriers in catalysis.

Oxygenation reactions catalyzed by the F557V mutant of soybean lipoxygenase-1: Evidence for two orientations of substrate binding

Plant lipoxygenases oxygenate linoleic acid to produce 13(S)-hydroperoxy-9Z,11E-octadecadienoic acid (13(S)-HPOD) or 9-hydroperoxy-10E,12Z-octadecadienoic acid (9(S)-HPOD). The manner in which these enzymes bind substrates and the mechanisms by which they control regiospecificity are uncertain. Hornung et al. (Proc. Natl. Acad. Sci. USA96 (1999) 4192-4197) have identified an important residue, corresponding to phe-557 in soybean lipoxygenase-1 (SBLO-1). These authors proposed that large residues in this position favored binding of linoleate with the carboxylate group near the surface of the enzyme (tail-first binding), resulting in formation of 13(S)-HPOD. They also proposed that smaller residues in this position facilitate binding of linoleate in a head-first manner with its carboxylate group interacting with a conserved arginine residue (arg-707 in SBLO-1), which leads to 9(S)-HPOD. In the present work, we have tested these proposals on SBLO-1. The F557V mutant produced 33% 9-HPOD (S:R = 87:13) from linoleic acid at pH 7.5, compared with 8% for the wild-type enzyme and 12% with the F557V,R707L double mutant. Experiments with 11(S)-deuteriolinoleic acid indicated that the 9(S)-HPOD produced by the F557V mutant involves removal of hydrogen from the pro-R position on C-11 of linoleic acid, as expected if 9(S)-HPOD results from binding in an orientation that is inverted relative to that leading to 13(S)-HPOD. The product distributions obtained by oxygenation of 10Z,13Z-nonadecadienoic acid and arachidonic acid by the F557V mutant support the hypothesis that ω6 oxygenation results from tail-first binding and ω10 oxygenation from head-first binding. The results demonstrate that the regiospecificity of SBLO-1 can be altered by a mutation that facilitates an alternative mode of substrate binding and adds to the body of evidence that 13(S)-HPOD arises from tail-first binding.

Replication of the Enzymatic Temperature Dependency of the Primary Hydride Kinetic Isotope Effects in Solution: Caused by the Protein-Controlled Rigidity of the Donor-Acceptor Centers?

The change from the temperature independence of the primary (1°) H/D kinetic isotope effects (KIEs) in wild-type enzyme-catalyzed H-transfer reactions (ΔE_a = E_aD - E_aH ∼ 0) to a strong temperature dependence with the mutated enzymes (ΔE_a ≫ 0) has recently been frequently observed. This has prompted some enzymologists to develop new H-tunneling models to correlate ΔE_a with the donor-acceptor distance (DAD) at the tunneling-ready state (TRS) as well as the protein thermal motions/dynamics that sample the short DAD_TRS's for H-tunneling to occur. While extensive evidence supporting or disproving the thermally activated DAD sampling concept has emerged, a comparable study of the simpler bimolecular H-tunneling reactions in solution has not been carried out. In particular, small ΔE_a's (∼0) have not been found. In this paper, we report a study of the hydride-transfer reactions from four NADH models to the same hydride acceptor in acetonitrile. The ΔE_a's were determined: 0.37 (small), 0.60, 0.99, and 1.53 kcal/mol (large). The α-secondary (2°) KIEs on the acceptor that serve as a ruler for the rigidity of reaction centers were previously reported or determined. All possible productive reactant complex (PRC) configurations were computed to provide insight into the structures of the TRS's. Relationships among structures, 2° KIEs, DAD_PRC's, and ΔE_a's were discussed. The more rigid system with more suppressed 2° C-H vibrations at the TRS and more narrowly distributed DAD_PRC's in PRCs gave a smaller ΔE_a. The results replicated the trend observed in enzymes versus mutated enzymes and appeared to support the concepts of different thermally activated DAD_TRS sampling processes in response to the rigid versus flexible donor-acceptor centers.

Biophysical Characterization of a Disabled Double Mutant of Soybean Lipoxygenase: The "Undoing" of Precise Substrate Positioning Relative to Metal Cofactor and an Identified Dynamical Network

Soybean lipoxygenase (SLO) has served as a prototype for understanding the molecular origin of enzymatic rate accelerations. The double mutant (DM) L546A/L754A is considered a dramatic outlier, due to the unprecedented size and near temperature-independence of its primary kinetic isotope effect, low catalytic efficiency, and elevated enthalpy of activation. To uncover the physical basis of these features, we herein apply three structural probes: hydrogen-deuterium exchange mass spectrometry, room-temperature X-ray crystallography and EPR spectroscopy on four SLO variants (wild-type (WT) enzyme, DM, and the two parental single mutants, L546A and L754A). DM is found to incorporate features of each parent, with the perturbation at position 546 predominantly influencing thermally activated motions that connect the active site to a protein-solvent interface, while mutation at position 754 disrupts the ligand field and solvation near the cofactor iron. However, the expanded active site in DM leads to more active site water molecules and their associated hydrogen bond network, and the individual features from L546A and L754A alone cannot explain the aggregate kinetic properties for DM. Using recently published QM/MM-derived ground-state SLO-substrate complexes for WT and DM, together with the thorough structural analyses presented herein, we propose that the impairment of DM is the combined result of a repositioning of the reactive carbon of linoleic acid substrate with regard to both the iron cofactor and a catalytically linked dynamic region of protein.

Impact of Mutations on the Binding Pocket of Soybean Lipoxygenase: Implications for Proton-Coupled Electron Transfer

Soybean lipoxygenase catalyzes a proton-coupled electron transfer (PCET) reaction and serves as a prototype for hydrogen tunneling in enzymes due to the unusually high kinetic isotope effect and significant modulation of the rate constant and kinetic isotope effect by mutation. Herein these experimental observations are interpreted in the context of changes to the substrate binding pocket in microsecond molecular dynamics simulations of wild-type and mutant soybean lipoxygenase. The binding pocket exhibits an hourglass shape with residues L546 and L754 bracketing the bottleneck, positioning the linoleic acid substrate for PCET. Mutation of I553 to less bulky residues slightly increases the width of the bottleneck and the volume of the binding pocket. Mutating L546 or L754 to a smaller residue also enlarges this width and volume, and mutating both has an even more dramatic effect. This analysis illustrates how mutation of the substrate binding pocket can be used as a strategy to tune the kinetics.

Understanding Biological Hydrogen Transfer Through the Lens of Temperature Dependent Kinetic Isotope Effects

Hydrogen atom transfer (HAT) is a salient feature of many enzymatic C-H cleavage mechanisms. In systems where kinetic isolation of HAT is achieved, selective labeling of substrate with hydrogen isotopes, such as deuterium, enables the determination of intrinsic kinetic isotope effects (KIEs). While the magnitude of the KIE is itself informative, ultimately the size of the temperature dependence of the KIE, Δ E_a = E_a(D) - E_a(H), serves as a critical, and often misinterpreted (or even ignored) descriptor of the reaction coordinate. As will be highlighted in this Account, Δ E_a is one of the most robust parameters to emerge from studies of enzyme catalyzed hydrogen transfer. Kinetic parameters for C-H reactions via HAT can appear consistent with either classical "over-the-barrier" or "Bell-like tunneling correction" models. However, neither of these models is able to explain the observation of near-zero Δ E_a values with many native enzymes that increase upon extrinsic or intrinsic perturbations to function. Instead, a full tunneling model has been developed that can account for the aggregate trends in the temperature dependence of the KIE. This model is reminiscent of Marcus-like theory for electron tunneling, with the additional incorporation of an H atom donor-acceptor distance (DAD) sampling term for effective wave function overlap; the role of the latter term is manifested in the experimentally determined Δ E_a. Three enzyme systems from this laboratory that illustrate different aspects of HAT are presented: taurine dioxygenase, the dual copper β-monooxygenases, and soybean lipoxygenase (SLO). The latter provides a particularly compelling system for understanding the properties of hydrogen tunneling, showing systematic increases in Δ E_a upon reduction in the size of hydrophobic residues both proximal and distal from the active site iron cofactor. Of note, recent ENDOR-based studies of enzyme-substrate complexes with SLO indicate an increase in DAD for mutants with increased Δ E_a, observations that are inconsistent with "Bell-like correction" models. Overall, the surmounting kinetic and biophysical evidence corroborates a multidimensional approach for understanding HAT, offering a robust mechanistic explanation for the magnitude and trends of the KIE and Δ E_a. Recent DFT and QM/MM computations on SLO are compared to the developed nonadiabatic analytical constructs, providing considerable insight into ground state structures and reactivity. However, QM/MM is unable to readily reproduce the small Δ E_a values characteristic of native enzymes. Future theoretical developments to capture these experimental observations may necessitate a parsing of protein motions for local, substrate deuteration-sensitive modes from isotope-insensitive modes within the larger conformational landscape, in the process providing deeper understanding of how native enzymes have evolved to transiently optimize their active site configurations.

N-linoleoylamino acids as chiral probes of substrate binding by soybean lipoxygenase-1

Lipoxygenases catalyze the oxygenation of polyunsaturated fatty acids and their derivatives to produce conjugated diene hydroperoxides. Soybean lipoxygenase-1 (SBLO-1) has been the subject of intensive structural and mechanistic study, but the manner in which this enzyme binds substrates is uncertain. Previous studies suggest that the fatty acyl group of the substrate binds in an internal cavity near the catalytic iron with the polar end at the surface of the protein or perhaps external to the protein. To test this model, we have investigated two pairs of enantiomeric N-linoleoylamino acids as substrates for SBLO-1. If the amino acid moiety binds external to the protein, the kinetics and product distribution should show little or no sensitivity to the stereochemical configuration of the amino acid moiety. Consistent with this expectation, N-linoleoyl-l-valine (LLV) and N-linoleoyl-d-valine (LDV) are both good substrates with kcat/Km values that are equal within error and about 40% higher than kcat/Km for linoleic acid. Experiments with N-linoleoyl-l-tryptophan (LLT) and N-linoleoyl-d-tryptophan (LDT) were complicated by the low critical micelle concentrations (CMC = 6-8 μM) of these substances. Below the CMC, LDT is a better substrate by a factor of 2.7. The rates of oxygenation of LDT and LLT continue to rise above the CMC, with modest stereoselectivity in favor of the d enantiomer. With all of the substrates tested, the major product is the 13(S)-hydroperoxide, and the distribution of minor products is not appreciably affected by the configuration of the amino acid moiety. The absence of stereoselectivity with LLV and LDV, the modest magnitude of the stereoselectivity with LLT and LDT, and the ability micellar forms of LLT and LDT to increase the concentration of available substrate are all consistent with the hypothesis that the amino acid moiety binds largely external to SBLO-1 and interacts with it only weakly.

Structural basis of noncanonical polyphenol oxidase activity in DLL-II: A lectin from Dolichos lablab

Lectins known to possess an additional enzymatic function are called leczymes. Previous studies reported a unique polyphenol oxidase (PPO) activity in DLL-II-a leczyme from Dolichos lablab. DLL-II shares a high sequence and structural homology with DBL-another leczyme from Dolichos biflorus. Incidentally, DBL possesses lipoxygenase activity, but not the PPO activity. Legume lectins usually possess two metal-binding sites A and B. Although these sites are conserved in both DBL and DLL-II, site A in DLL-II is occupied by Mn²⁺ and site B by Ca²⁺ . In contrast, DLL-II binds Cu²⁺ and Ca²⁺ at sites A and B, respectively. Here, investigating the structural basis of PPO activity in DLL-II, we find that the PPO activity is only dependent on Cu²⁺ , but not Ca²⁺ ; and the lectin activity requires only Ca²⁺ . Further, our analysis suggests that an alternative mechanism of PPO reaction may be operative in DLL-II, which involves a mononuclear Cu²⁺ metal center; this is in contrast to the bi-nuclear Cu²⁺ metal center commonly observed in all PPOs. Importantly, structural and computational approaches employed here, we hypothesize possible PPO binding sites and the corresponding migration channels for accessing the active site.

Fundamental Insights into Proton-Coupled Electron Transfer in Soybean Lipoxygenase from Quantum Mechanical/Molecular Mechanical Free Energy Simulations

The proton-coupled electron transfer (PCET) reaction catalyzed by soybean lipoxygenase has served as a prototype for understanding hydrogen tunneling in enzymes. Herein this PCET reaction is studied with mixed quantum mechanical/molecular mechanical (QM/MM) free energy simulations. The free energy surfaces are computed as functions of the proton donor-acceptor (C-O) distance and the proton coordinate, and the potential of mean force is computed as a function of the C-O distance, inherently including anharmonicity. The simulation results are used to calculate the kinetic isotope effects for the wild-type enzyme (WT) and the L546A/L754A double mutant (DM), which have been measured experimentally to be ∼80 and ∼700, respectively. The PCET reaction is found to be exoergic for WT and slightly endoergic for the DM, and the equilibrium C-O distance for the reactant is found to be ∼0.2 Å greater for the DM than for WT. The larger equilibrium distance for the DM, which is due mainly to less optimal substrate binding in the expanded binding cavity, is primarily responsible for its higher kinetic isotope effect. The calculated potentials of mean force are anharmonic and relatively soft at shorter C-O distances, allowing efficient thermal sampling of the shorter distances required for effective hydrogen tunneling. The primarily local electrostatic field at the transferring hydrogen is ∼100 MV/cm in the direction to facilitate proton transfer and increases dramatically as the C-O distance decreases. These simulations suggest that the overall protein environment is important for conformational sampling of active substrate configurations aligned for proton transfer, but the PCET reaction is influenced primarily by local electrostatic effects that facilitate conformational sampling of shorter proton donor-acceptor distances required for effective hydrogen tunneling.

Origins of Enzyme Catalysis: Experimental Findings for C-H Activation, New Models, and Their Relevance to Prevailing Theoretical Constructs

The physical basis for enzymatic rate accelerations is a subject of great fundamental interest and of direct relevance to areas that include the de novo design of green catalysts and the pursuit of new drug regimens. Extensive investigations of C-H activating systems have provided considerable insight into the relationship between an enzyme's overall structure and the catalytic chemistry at its active site. This Perspective highlights recent experimental data for two members of distinct, yet iconic C-H activation enzyme classes, lipoxygenases and prokaryotic alcohol dehydrogenases. The data necessitate a reformulation of the dominant textbook definition of biological catalysis. A multidimensional model emerges that incorporates a range of protein motions that can be parsed into a combination of global stochastic conformational thermal fluctuations and local donor-acceptor distance sampling. These motions are needed to achieve a high degree of precision with regard to internuclear distances, geometries, and charges within the active site. The available model also suggests a physical framework for understanding the empirical enthalpic barrier in enzyme-catalyzed processes. We conclude by addressing the often conflicting interface between computational and experimental chemists, emphasizing the need for computation to predict experimental results in advance of their measurement.

Across the tree of life, radiation resistance is governed by antioxidant Mn2+, gauged by paramagnetic resonance

Despite concerted functional genomic efforts to understand the complex phenotype of ionizing radiation (IR) resistance, a genome sequence cannot predict whether a cell is IR-resistant or not. Instead, we report that absorption-display electron paramagnetic resonance (EPR) spectroscopy of nonirradiated cells is highly diagnostic of IR survival and repair efficiency of DNA double-strand breaks (DSBs) caused by exposure to gamma radiation across archaea, bacteria, and eukaryotes, including fungi and human cells. IR-resistant cells, which are efficient at DSB repair, contain a high cellular content of manganous ions (Mn²⁺) in high-symmetry (H) antioxidant complexes with small metabolites (e.g., orthophosphate, peptides), which exhibit narrow EPR signals (small zero-field splitting). In contrast, Mn²⁺ ions in IR-sensitive cells, which are inefficient at DSB repair, exist largely as low-symmetry (L) complexes with substantially broadened spectra seen with enzymes and strongly chelating ligands. The fraction of cellular Mn²⁺ present as H-complexes (H-Mn²⁺), as measured by EPR of live, nonirradiated Mn-replete cells, is now the strongest known gauge of biological IR resistance between and within organisms representing all three domains of life: Antioxidant H-Mn²⁺ complexes, not antioxidant enzymes (e.g., Mn superoxide dismutase), govern IR survival. As the pool of intracellular metabolites needed to form H-Mn²⁺ complexes depends on the nutritional status of the cell, we conclude that IR resistance is predominantly a metabolic phenomenon. In a cross-kingdom analysis, the vast differences in taxonomic classification, genome size, and radioresistance between cell types studied here support that IR resistance is not controlled by the repertoire of DNA repair and antioxidant enzymes.

Monovalent Cation Activation of the Radical SAM Enzyme Pyruvate Formate-Lyase Activating Enzyme

Pyruvate formate-lyase activating enzyme (PFL-AE) is a radical S-adenosyl-l-methionine (SAM) enzyme that installs a catalytically essential glycyl radical on pyruvate formate-lyase. We show that PFL-AE binds a catalytically essential monovalent cation at its active site, yet another parallel with B₁₂ enzymes, and we characterize this cation site by a combination of structural, biochemical, and spectroscopic approaches. Refinement of the PFL-AE crystal structure reveals Na⁺ as the most likely ion present in the solved structures, and pulsed electron nuclear double resonance (ENDOR) demonstrates that the same cation site is occupied by ²³Na in the solution state of the as-isolated enzyme. A SAM carboxylate-oxygen is an M⁺ ligand, and EPR and circular dichroism spectroscopies reveal that both the site occupancy and the identity of the cation perturb the electronic properties of the SAM-chelated iron–sulfur cluster. ENDOR studies of the PFL-AE/[¹³C-methyl]-SAM complex show that the target sulfonium positioning varies with the cation, while the observation of an isotropic hyperfine coupling to the cation by ENDOR measurements establishes its intimate, SAM-mediated interaction with the cluster. This monovalent cation site controls enzyme activity: (i) PFL-AE in the absence of any simple monovalent cations has little–no activity; and (ii) among monocations, going down Group 1 of the periodic table from Li⁺ to Cs⁺, PFL-AE activity sharply maximizes at K⁺, with NH₄⁺ closely matching the efficacy of K⁺. PFL-AE is thus a type I M⁺-activated enzyme whose M⁺ controls reactivity by interactions with the cosubstrate, SAM, which is bound to the catalytic iron–sulfur cluster.

Enhanced Rigidification within a Double Mutant of Soybean Lipoxygenase Provides Experimental Support for Vibronically Nonadiabatic Proton-Coupled Electron Transfer Models

Soybean lipoxygenase (SLO) is a prototype for nonadiabatic hydrogen tunneling reactions and, as such, has served as the subject of numerous theoretical studies. In this work, we report a nearly temperature-independent kinetic isotope effect (KIE) with an average KIE value of 661 ± 27 for a double mutant (DM) of SLO at six temperatures. The data are well-reproduced within a vibronically nonadiabatic proton-coupled electron transfer model in which the active site has become rigidified compared to wild-type enzyme and single-site mutants. A combined temperature-pressure perturbation further shows that temperature-dependent global motions within DM-SLO are more resistant to perturbation by elevated pressure. These findings provide strong experimental support for the model of hydrogen tunneling in SLO, where optimization of both local protein and ligand motions and distal conformational rearrangements is a prerequisite for effective proton vibrational wave function overlap between the substrate and the active-site iron cofactor.

Visualization of Electron Paramagnetic Resonance Hyperfine Structure Coupling Pathways

The close relation between the EPR hyperfine coupling constant and NMR indirect spin-spin coupling constant is well-known. For example, the Karplus-type dependence of hyperfine constants on the dihedral angle, originally proposed for NMR spin-spin coupling, is widely used in pNMR studies. In the present work we propose a new tool for visualization of hyperfine coupling pathways based on our experience with visualization of NMR indirect spin-spin couplings. The plotted 3D-function is the difference between the total electron densities when the magnetic moment of the nucleus of interest changes its sign and as such is an observable from the physical point of view. It has been shown that it is proportional to the linear response of the spin density to the nuclear spin (i.e., magnetic moment). In contrast to the widely used visualization of spin density, our new approach depicts only the part of the electron cloud of a molecule that is affected by the interaction of the unpaired electron(s) with the desired nuclear magnetic moment. Because visualization of NMR spin-spin couplings and hyperfine interaction is based on the same ideas and done using similar techniques, it allows a direct comparison of the corresponding pathways for the two phenomena so as to analyze their resemblance and/or dissimilarity.