Neutron crystallography of copper amine oxidase reveals keto/enolate interconversion of the quinone cofactor and unusual proton sharing.

Recent advances in neutron crystallographic studies have provided structural bases for quantum behaviors of protons observed in enzymatic reactions. Thus, we resolved the neutron crystal structure of a bacterial copper (Cu) amine oxidase (CAO), which contains a prosthetic Cu ion and a protein-derived redox cofactor, topa quinone (TPQ). We solved hitherto unknown structures of the active site, including a keto/enolate equilibrium of the cofactor with a nonplanar quinone ring, unusual proton sharing between the cofactor and the catalytic base, and metal-induced deprotonation of a histidine residue that coordinates to the Cu. Our findings show a refined active-site structure that gives detailed information on the protonation state of dissociable groups, such as the quinone cofactor, which are critical for catalytic reactions.

Crystal structure of a nucleotide-binding domain of fatty acid kinase FakA from Thermus thermophilus HB8.

Fatty acid kinase is necessary for the incorporation of exogenous fatty acids into membrane phospholipids. Fatty acid kinase consists of two components: a kinase component, FakA, that phosphorylates a fatty acid bound to a fatty acid-binding component, FakB. However, the molecular details underlying the phosphotransfer reaction remain to be resolved. We determined the crystal structure of the N-terminal domain of FakA bound to ADP from Thermus thermophilus HB8. The overall structure of this domain showed that the helical barrel fold is similar to the nucleotide-binding component of dihydroxyacetone kinase. The structure of the nucleotide-binding site revealed the roles of the conserved residues in recognition of ADP and Mg2+, but the N-terminal domain of FakA lacked the ADP-capping loop found in the dihydroxyacetone kinase component. Based on the structural similarity to the two subunits of dihydroxyacetone kinase complex, we constructed a model of the complex of T. thermophilus FakB and the N-terminal domain of FakA. In this model, the invariant Arg residue of FakB occupied a position that was spatially similar to that of the catalytically important Arg residue of dihydroxyacetone kinase, which predicted a composite active site in the Fatty acid kinase complex.

Identification of the corticotropin-releasing factor receptor 1 antagonists as inhibitors of Chikungunya virus replication using a Gaussia luciferase-expressing subgenomic replicon.

The Chikungunya virus (CHIKV), an enveloped RNA virus that has been identified in over 40 countries and is considered a growing threat to public health worldwide. However, there is no preventive vaccine or specific therapeutic drug for CHIKV infection. To identify a new inhibitor against CHIKV infection, this study constructed a subgenomic RNA replicon expressing the secretory Gaussia luciferase (Gluc) based on the CHIKV SL11131 strain. Transfection of in vitro-transcribed replicon RNA to BHK-21 cells revealed that Gluc activity in culture supernatants was correlated with the intracellular replication of the replicon genome. Through a chemical compound library screen using the Gluc reporter CHIKV replicon, we identified several compounds that suppressed CHIKV infection in Vero cells. Among the hits identified, CP-154,526, a non-peptide antagonist of the corticotropin-releasing factor receptor type-1 (CRF-R1), showed the strongest anti-CHIKV activity and inhibited CHIKV infection in Huh-7 cells. Interestingly, other CRF-R1 antagonists, R121919 and NGD 98-2, also exhibited inhibitory effects on CHIKV infection. Time-of-drug addition and virus entry assays indicated that CP-154,526 suppressed a post-entry step of infection, suggesting that CRF-R1 antagonists acted on a target in the intracellular replication process of CHIKV. Therefore, the Gluc reporter replicon system established in this study would greatly facilitate the development of antiviral drugs against CHIKV infection.

In crystallo thermodynamic analysis of conformational change of the topaquinone cofactor in bacterial copper amine oxidase.

In the catalytic reaction of copper amine oxidase, the protein-derived redox cofactor topaquinone (TPQ) is reduced by an amine substrate to an aminoresorcinol form (TPQamr), which is in equilibrium with a semiquinone radical (TPQsq). The transition from TPQamr to TPQsq is an endothermic process, accompanied by a significant conformational change of the cofactor. We employed the humid air and glue-coating (HAG) method to capture the equilibrium mixture of TPQamr and TPQsq in noncryocooled crystals of the enzyme from Arthrobacter globiformis and found that the equilibrium shifts more toward TPQsq in crystals than in solution. Thermodynamic analyses of the temperature-dependent equilibrium also revealed that the transition to TPQsq is entropy-driven both in crystals and in solution, giving the thermodynamic parameters that led to experimental determination of the crystal packing effect. Furthermore, we demonstrate that the binding of product aldehyde to the hydrophobic pocket in the active site produces various equilibrium states among two forms of the product Schiff-base, TPQamr, and TPQsq, in a pH-dependent manner. The temperature-controlled HAG method provides a technique for thermodynamic analysis of conformational changes occurring in protein crystals that are hardly scrutinized by conventional cryogenic X-ray crystallography.

Restriction of Flaviviruses by an Interferon-Stimulated Gene SHFL/C19orf66.

Flaviviruses (the genus Flavivirus of the Flaviviridae family) include many arthropod-borne viruses, often causing life-threatening diseases in humans, such as hemorrhaging and encephalitis. Although the flaviviruses have a significant clinical impact, it has become apparent that flavivirus replication is restricted by cellular factors induced by the interferon (IFN) response, which are called IFN-stimulated genes (ISGs). SHFL (shiftless antiviral inhibitor of ribosomal frameshifting) is a novel ISG that inhibits dengue virus (DENV), West Nile virus (WNV), Zika virus (ZIKV), and Japanese encephalitis virus (JEV) infections. Interestingly, SHFL functions as a broad-spectrum antiviral factor exhibiting suppressive activity against various types of RNA and DNA viruses. In this review, we summarize the current understanding of the molecular mechanisms by which SHFL inhibits flavivirus infection and discuss the molecular basis of the inhibitory mechanism using a predicted tertiary structure of SHFL generated by the program AlphaFold2.

A Lynch syndrome-associated mutation at a Bergerat ATP-binding fold destabilizes the structure of the DNA mismatch repair endonuclease MutL.

In humans, mutations in genes encoding homologs of the DNA mismatch repair endonuclease MutL cause a hereditary cancer that is known as Lynch syndrome. Here, we determined the crystal structures of the N-terminal domain (NTD) of MutL from the thermophilic eubacterium Aquifex aeolicus (aqMutL) complexed with ATP analogs at 1.69-1.73 Å. The structures revealed significant structural similarities to those of a human MutL homolog, postmeiotic segregation increased 2 (PMS2). We introduced five Lynch syndrome-associated mutations clinically found in human PMS2 into the aqMutL NTD and investigated the protein stability, ATPase activity, and DNA-binding ability of these protein variants. Among the mutations studied, the most unexpected results were obtained for the residue Ser34. Ser34 (Ser46 in PMS2) is located at a previously identified Bergerat ATP-binding fold. We found that the S34I aqMutL NTD retains ATPase and DNA-binding activities. Interestingly, CD spectrometry and trypsin-limited proteolysis indicated the disruption of a secondary structure element of the S34I NTD, destabilizing the overall structure of the aqMutL NTD. In agreement with this, the recombinant human PMS2 S46I NTD was easily digested in the host Escherichia coli cells. Moreover, other mutations resulted in reduced DNA-binding or ATPase activity. In summary, using the thermostable aqMutL protein as a model molecule, we have experimentally determined the effects of the mutations on MutL endonuclease; we discuss the pathological effects of the corresponding mutations in human PMS2.

Probing the Catalytic Mechanism of Copper Amine Oxidase from Arthrobacter globiformis with Halide Ions.

The catalytic reaction of copper amine oxidase proceeds through a ping-pong mechanism comprising two half-reactions. In the initial half-reaction, the substrate amine reduces the Tyr-derived cofactor, topa quinone (TPQ), to an aminoresorcinol form (TPQamr) that is in equilibrium with a semiquinone radical (TPQsq) via an intramolecular electron transfer to the active-site copper. We have analyzed this reductive half-reaction in crystals of the copper amine oxidase from Arthrobacter globiformis. Anerobic soaking of the crystals with an amine substrate shifted the equilibrium toward TPQsq in an "on-copper" conformation, in which the 4-OH group ligated axially to the copper center, which was probably reduced to Cu(I). When the crystals were soaked with substrate in the presence of halide ions, which act as uncompetitive and noncompetitive inhibitors with respect to the amine substrate and dioxygen, respectively, the equilibrium in the crystals shifted toward the "off-copper" conformation of TPQamr. The halide ion was bound to the axial position of the copper center, thereby preventing TPQamr from adopting the on-copper conformation. Furthermore, transient kinetic analyses in the presence of viscogen (glycerol) revealed that only the rate constant in the step of TPQamr/TPQsq interconversion is markedly affected by the viscogen, which probably perturbs the conformational change. These findings unequivocally demonstrate that TPQ undergoes large conformational changes during the reductive half-reaction.

A QM/MM study of the L-threonine formation reaction of threonine synthase: implications into the mechanism of the reaction specificity.

Threonine synthase catalyzes the most complex reaction among the pyridoxal-5'-phosphate (PLP)-dependent enzymes. The important step is the addition of a water molecule to the Cß-Cα double bond of the PLP-α-aminocrotonate aldimine intermediate. Transaldimination of this intermediate with Lys61 as a side reaction to form α-ketobutyrate competes with the normal addition reaction. We previously found that the phosphate ion released from the O-phospho-l-homoserine substrate plays a critical role in specifically promoting the normal reaction. In order to elucidate the detailed mechanism of this "product-assisted catalysis", we performed comparative QM/MM calculations with an exhaustive search for the lowest-energy-barrier reaction pathways starting from PLP-α-aminocrotonate aldimine intermediate. Satisfactory agreements with the experiment were obtained for the free energy profile and the UV/vis spectra when the PLP pyridine N1 was unprotonated and the phosphate ion was monoprotonated. Contrary to an earlier proposal, the base that abstracts a proton from the attacking water was the ε-amino group of Lys61 rather than the phosphate ion. Nevertheless, the phosphate ion is important for stabilizing the transition state of the normal transaldimination to form l-threonine by making a hydrogen bond with the hydroxy group of the l-threonine moiety. The absence of this interaction may account for the higher energy barrier of the side reaction, and explains the mechanism of the reaction specificity afforded by the phosphate ion product. Additionally, a new mechanism, in which a proton temporarily resides at the phenolate O3' of PLP, was proposed for the transaldimination process, a prerequisite step for the catalysis of all the PLP enzymes.

Boundary of the nucleotide-binding domain of Streptococcus ComA based on functional and structural analysis.

The ATP-binding cassette (ABC) transporter ComA is a key molecule essential for the first step of the quorum-sensing system of Streptococcus. The nucleotide binding domains (NBD) of Streptococcus mutans ComA with different N termini, NBD1 (amino acid residues 495-760), NBD2 (517-760), and NBD3 (528-760), were expressed, purified, and characterized. The shortest NBD3 corresponds to the region commonly defined as NBD in the database searches of ABC transporters. A kinetic analysis showed that the extra N-terminal region conferred a significantly higher ATP hydrolytic activity on the NBD at a neutral pH. Gel-filtration, X-ray crystallography, and mutational analyses suggest that at least four to five residues beyond the N-terminal boundary of NBD3 indeed participate in stabilizing the protein scaffold of the domain structure, thereby facilitating the ATP-dependent dimerization of NBD which is a prerequisite to the catalysis. These findings, together with the presence of a highly conserved glycine residue in this region, support the redefinition of the N-terminal boundary of the NBD of these types of ABC exporters.

High-resolution crystal structure of copper amine oxidase from Arthrobacter globiformis: assignment of bound diatomic molecules as O2.

The crystal structure of a copper amine oxidase from Arthrobacter globiformis was determined at 1.08 Šresolution with the use of low-molecular-weight polyethylene glycol (LMW PEG; average molecular weight ∼200) as a cryoprotectant. The final crystallographic R factor and Rfree were 13.0 and 15.0%, respectively. Several molecules of LMW PEG were found to occupy cavities in the protein interior, including the active site, which resulted in a marked reduction in the overall B factor and consequently led to a subatomic resolution structure for a relatively large protein with a monomer molecular weight of ∼70,000. About 40% of the presumed H atoms were observed as clear electron densities in the Fo - Fc difference map. Multiple minor conformers were also identified for many residues. Anisotropic displacement fluctuations were evaluated in the active site, which contains a post-translationally derived quinone cofactor and a Cu atom. Furthermore, diatomic molecules, most likely to be molecular oxygen, are bound to the protein, one of which is located in a region that had previously been proposed as an entry route for the dioxygen substrate from the central cavity of the dimer interface to the active site.

Catalytic mechanism of the zinc-dependent MutL endonuclease reaction.

DNA mismatch repair endonuclease MutL binds two zinc ions. However, the endonuclease activity of MutL is drastically enhanced by other divalent metals such as manganese, implying that MutL binds another catalytic metal at some site other than the zinc-binding sites. Here, we solved the crystal structure of the endonuclease domain of Aquifex aeolicus MutL in the manganese- or cadmium-bound form, revealing that these metals compete with zinc at the same sites. Mass spectrometry revealed that the MutL yielded 5'-phosphate and 3'-OH products, which is characteristic of the two-metal-ion mechanism. Crystallographic analyses also showed that the position and flexibility of a highly conserved Arg of A. aeolicus MutL altered depending on the presence of zinc/manganese or the specific inhibitor cadmium. Site-directed mutagenesis revealed that the Arg was critical for the catalysis. We propose that zinc ion and its binding sites are physiologically of catalytic importance and that the two-metal-ion mechanism works in the reaction, where the Arg plays a catalytic role. Our results also provide a mechanistic insight into the inhibitory effect of a mutagen/carcinogen, cadmium, on MutL.

Product-assisted catalysis as the basis of the reaction specificity of threonine synthase.

Threonine synthase (TS), which is a pyridoxal 5'-phosphate (PLP)-dependent enzyme, catalyzes the elimination of the γ-phosphate group from O-phospho-L-homoserine (OPHS) and the subsequent addition of water at Cß to form L-threonine. The catalytic course of TS is the most complex among the PLP enzymes, and it is an intriguing problem how the elementary steps are controlled in TS to carry out selective reactions. When L-vinylglycine was added to Thermus thermophilus HB8 TS in the presence of phosphate, L-threonine was formed with k(cat) and reaction specificity comparable with those when OPHS was used as the substrate. However, in the absence of phosphate or when sulfate was used in place of phosphate, only the side reaction product, α-ketobutyrate, was formed. Global analysis of the spectral changes in the reaction of TS with L-threonine showed that compared with the more acidic sulfate ion, the phosphate ion decreased the energy levels of the transition states of the addition of water at the Cß of the PLP-α-aminocrotonate aldimine (AC) and the transaldimination to form L-threonine. The x-ray crystallographic analysis of TS complexed with an analog for AC gave a distinct electron density assigned to the phosphate ion derived from the solvent near the Cß of the analog. These results indicated that the phosphate ion released from OPHS by γ-elimination acts as the base catalyst for the addition of water at Cß of AC, thereby providing the basis of the reaction specificity. The phosphate ion is also considered to accelerate the protonation/deprotonation at Cγ.

Re-evaluation of protein neutron crystallography with and without X-ray/neutron joint refinement.

Protein neutron crystallography is a powerful technique to determine the positions of H atoms, providing crucial biochemical information such as the protonation states of catalytic groups and the geometry of hydrogen bonds. Recently, the crystal structure of a bacterial copper amine oxidase was determined by joint refinement using X-ray and neutron diffraction data sets at resolutions of 1.14 and 1.72 Å, respectively [Murakawa et al. (2020 ▸). Proc. Natl Acad. Sci. USA, 117, 10818-10824]. While joint refinement is effective for the determination of the accurate positions of heavy atoms on the basis of the electron density, the structural information on light atoms (hydrogen and deuterium) derived from the neutron diffraction data might be affected by the X-ray data. To unravel the information included in the neutron diffraction data, the structure determination was conducted again using only the neutron diffraction data at 1.72 Šresolution and the results were compared with those obtained in the previous study. Most H and D atoms were identified at essentially the same positions in both the neutron-only and the X-ray/neutron joint refinements. Nevertheless, neutron-only refinement was found to be less effective than joint refinement in providing very accurate heavy-atom coordinates that lead to significant improvement of the neutron scattering length density map, especially for the active-site cofactor. Consequently, it was confirmed that X-ray/neutron joint refinement is crucial for determination of the real chemical structure of the catalytic site of the enzyme.

Structural and functional insights into the mechanism by which MutS2 recognizes a DNA junction.

MutS family proteins are classified into MutS-I and -II lineages: MutS-I recognizes mismatched DNA and initiates mismatch repair, whereas MutS-II recognizes DNA junctions to modulate recombination. MutS-I forms dimeric clamp-like structures enclosing the mismatched DNA, and its composite ATPase sites regulate DNA-binding modes. Meanwhile, the structures of MutS-II have not been determined; accordingly, it remains unknown how MutS-II recognizes DNA junctions and how nucleotides control DNA binding. Here, we solved the ligand-free and ADP-bound crystal structures of bacterial MutS2 belonging to MutS-II. MutS2 also formed a dimeric clamp-like structure with composite ATPase sites. The ADP-bound MutS2 was more flexible compared to the ligand-free form and could be more suitable for DNA entry. The inner hole of the MutS2 clamp was two times larger than that of MutS-I, and site-directed mutagenesis analyses revealed DNA-binding sites at the inner hole. Based on these, a model is proposed that describes how MutS2 recognizes DNA junctions.

Molecular mechanism of a large conformational change of the quinone cofactor in the semiquinone intermediate of bacterial copper amine oxidase.

Copper amine oxidase from Arthrobacter globiformis (AGAO) catalyses the oxidative deamination of primary amines via a large conformational change of a topaquinone (TPQ) cofactor during the semiquinone formation step. This conformational change of TPQ occurs in the presence of strong hydrogen bonds and neighboring bulky amino acids, especially the conserved Asn381, which restricts TPQ conformational changes over the catalytic cycle. Whether such a semiquinone intermediate is catalytically active or inert has been a matter of debate in copper amine oxidases. Here, we show that the reaction rate of the Asn381Ala mutant decreases 160-fold, and the X-ray crystal structures of the mutant reveals a TPQ-flipped conformation in both the oxidized and reduced states, preceding semiquinone formation. Our hybrid quantum mechanics/molecular mechanics (QM/MM) simulations show that the TPQ conformational change is realized through the sequential steps of the TPQ ring-rotation and slide. We determine that the bulky side chain of Asn381 hinders the undesired TPQ ring-rotation in the oxidized form, favoring the TPQ ring-rotation in reduced TPQ by a further stabilization leading to the TPQ semiquinone form. The acquired conformational flexibility of TPQ semiquinone promotes a high reactivity of Cu(i) to O2, suggesting that the semiquinone form is catalytically active for the subsequent oxidative half-reaction in AGAO. The ingenious molecular mechanism exerted by TPQ to achieve the "state-specific" reaction sheds new light on a drastic environmental transformation around the catalytic center.

Serial femtosecond X-ray crystallography of an anaerobically formed catalytic intermediate of copper amine oxidase.

The mechanisms by which enzymes promote catalytic reactions efficiently through their structural changes remain to be fully elucidated. Recent progress in serial femtosecond X-ray crystallography (SFX) using X-ray free-electron lasers (XFELs) has made it possible to address these issues. In particular, mix-and-inject serial crystallography (MISC) is promising for the direct observation of structural changes associated with ongoing enzymic reactions. In this study, SFX measurements using a liquid-jet system were performed on microcrystals of bacterial copper amine oxidase anaerobically premixed with a substrate amine solution. The structure determined at 1.94 Šresolution indicated that the peptidyl quinone cofactor is in equilibrium between the aminoresorcinol and semiquinone radical intermediates, which accumulate only under anaerobic single-turnover conditions. These results show that anaerobic conditions were well maintained throughout the liquid-jet SFX measurements, preventing the catalytic intermediates from reacting with dioxygen. These results also provide a necessary framework for performing time-resolved MISC to study enzymic reaction mechanisms under anaerobic conditions.

Microcrystal preparation for serial femtosecond X-ray crystallography of bacterial copper amine oxidase.

Recent advances in serial femtosecond X-ray crystallography (SFX) using X-ray free-electron lasers have paved the way for determining radiation-damage-free protein structures under nonfreezing conditions. However, the large-scale preparation of high-quality microcrystals of uniform size is a prerequisite for SFX, and this has been a barrier to its widespread application. Here, a convenient method for preparing high-quality microcrystals of a bacterial quinoprotein enzyme, copper amine oxidase from Arthrobacter globiformis, is reported. The method consists of the mechanical crushing of large crystals (5-15 mm3), seeding the crushed crystals into the enzyme solution and standing for 1 h at an ambient temperature of ∼26°C, leading to the rapid formation of microcrystals with a uniform size of 3-5 µm. The microcrystals diffracted X-rays to a resolution beyond 2.0 Šin SFX measurements at the SPring-8 Angstrom Compact Free Electron Laser facility. The damage-free structure determined at 2.2 Šresolution was essentially identical to that determined previously by cryogenic crystallography using synchrotron X-ray radiation.

Unique protonation states of aspartate and topaquinone in the active site of copper amine oxidase.

The oxidative deamination of biogenic amines, crucial in the metabolism of a wealth of living organisms, is catalyzed by copper amine oxidases (CAOs). In this work, on the ground of accurate molecular modeling, we provide a clear insight into the unique protonation states of the key catalytic aspartate residue Asp298 and the prosthetic group of topaquinone (TPQ) in the CAO of Arthrobacter globiformis (AGAO). This provides both extensions and complementary information to the crystal structure determined by our recent neutron diffraction (ND) experiment. The hybrid quantum mechanics/molecular mechanics (QM/MM) simulations suggest that the ND structure closely resembles a state in which Asp298 is protonated and the TPQ takes an enolate form. The TPQ keto form can coexist in the fully protonated state. The energetic and structural analyses indicate that the active site structure of the AGAO crystal is not a single state but rather a mixture of the different protonation and conformational states identified in this work.

Reaction of threonine synthase with the substrate analogue 2-amino-5-phosphonopentanoate: implications into the proton transfer at the active site.

Threonine synthase catalyses the conversion of O-phospho-l-homoserine and a water molecule to l-threonine and has the most complex catalytic mechanism among the pyridoxal 5'-phosphate-dependent enzymes. In order to study the less-characterized earlier stage of the catalytic reaction, we studied the reaction of threonine synthase with 2-amino-5-phosphonopentanoate, which stops the catalytic reaction at the enamine intermediate. The global kinetic analysis of the triphasic spectral changes showed that, in addition to the theoretically expected pathway, the carbanion is rapidly reprotonated at Cα to form an aldimine distinct from the external aldimine directly formed from the Michaelis complex. The Kd for the binding of inhibitor to the enzyme decreased with increasing pH, showing that the 2-amino-group-unprotonated form of the ligand binds to the enzyme. On the other hand, the rate constants for the proton migration steps within the active site are independent of the solvent pH, indicating that protons are shared by the active dissociative groups and are not exchanged with the solvent during the course of catalysis. This gives an insight into the role of the phosphate group of the substrate, which may increase the basicity of the ε-amino group of the catalytic lysine residue in the active site.

Further insight into the mechanism of stereoselective proton abstraction by bacterial copper amine oxidase.

During the catalytic reaction of copper amine oxidase, one of the two prochiral hydrogen atoms at the C1 position of substrate amine is stereoselectively abstracted by a conserved Asp residue serving as a general base. Using stereospecifically deuterium-labeled enantiomers of 2-phenylethylamine, we previously showed that the pro-S alpha-proton is abstracted by the enzyme from Arthrobacter globiformis (AGAO) [Uchida, M., et al. (2003) Biosci. Biotechnol. Biochem. 67, 2664-2667]. More recently, we have also demonstrated that the pro-S selectivity of alpha-proton abstraction is fully retained even in the reaction of a mutant AGAO lacking the catalytic base [Chiu, Y.-C., et al. (2006) Biochemistry 45, 4105-4120]. On the basis of these findings, we have proposed that the stereoselectivity of alpha-proton abstraction is primarily determined by the conformation of the Schiff base intermediate formed between the substrate and the topa quinone cofactor (TPQ), stabilized by the binding of the distal part of the substrate to a hydrophobic pocket of the enzyme. In this conformation, the pro-S hydrogen atom to be abstracted is nearly perpendicular to the plane of the Schiff base-TPQ conjugate system, achieving the maximum overlap of sigma- and pi-orbitals. To further elucidate the stereochemical details, we have synthesized stereospecifically deuterium-labeled enantiomers of ethylamine, a very poor substrate for AGAO, in addition to those structurally related to the preferred substrate, 2-phenylethylamine. In marked contrast to the nearly complete pro-S selectivity of alpha-proton abstraction for most substrates that have been examined, the stereoselectivity for ethylamine decreased significantly to as little as 88%. The crystal structure of AGAO soaked with ethylamine showed very poor electron densities for the substrate Schiff base intermediate, showing that its conformation is not defined uniquely. Thus, the stereoselectivity of alpha-proton abstraction during the copper amine oxidase reaction is closely associated with the conformational flexibility of the substrate Schiff base intermediate.