Amine oxidases utilize a proton abstraction mechanism following binding of the amine substrate to the C5 position of the cofactor, the quinone form of trihydroxyphenylalanine (TPQ). Previous work [Wilmot, C. M., et al. (1997) Biochemistry 36, 1608-1620] has shown that Asp383 in Escherichia coliamine oxidase (ECAO) is the catalytic base which performs the key step of proton abstraction. This paper explores in more depth this and other roles of Asp383. The crystal structures of three mutational variants are presented together with their catalytic properties, visible spectra, and binding properties for a substrate-like inhibitor, 2-hydrazinopyridine (2-HP), in comparison to those of the wild type enzyme. In wild type ECAO, the TPQ is located in a wedge-shaped pocket which allows more freedom of movement at the substrate binding position (C5) than for TPQ ring carbons C1-C4. A role of Asp383, whose carboxylate is located close to O5, is to stabilize the TPQ in its major conformation in the pocket. Replacement of Asp383 with the isostructural, but chemically distinct, Asn383 does not affect the location or dynamics of the TPQ cofactor significantly, but eliminates catalytic activity and drastically reduces the affinity for 2-HP. Removal of the side chain carboxyl moiety, as in Ala383, additionally allows the TPQ the greater conformational flexibility to coordinate to the copper, which demonstrates that Asp383 helps maintain the active site structure by preventing TPQ from migrating to the copper. Glu383 has a greatly decreased catalytic activity, as well as a decreased affinity for 2-HP relative to that of wild type ECAO. The electron density reveals that the longer side chain of Glu prevents the pivotal motion of the TPQ by hindering its movement within the wedge-shaped active site pocket. The results show that Asp383 performs multiple roles in the catalytic mechanism of ECAO, not only in acting as the active site base at different stages of the catalytic cycle but also in regulating the mobility of the TPQ that is essential to catalysis.

Copper amine oxidases are homodimeric enzymes that catalyze two reactions: first, a self-processing reaction to generate the 2,4,5-trihydroxyphenylalanine (TPQ) cofactor from an active site tyrosine by a single turnover mechanism; second, the oxidative deamination of primary amine substrates with the production of aldehyde, hydrogen peroxide, and ammonia catalyzed by the mature enzyme. The importance of active site residues in both of these processes has been investigated by structural studies and site-directed mutagenesis in enzymes from various organisms. One conserved residue is a tyrosine, Tyr369 in the Escherichia coli enzyme, whose hydroxyl is hydrogen bonded to the O4 of TPQ. To explore the importance of this site, we have studied a mutant enzyme in which Tyr369 has been mutated to a phenylalanine. We have determined the X-ray crystal structure of this variant enzyme to 2.1 A resolution, which reveals that TPQ adopts a predominant nonproductive conformation in the resting enzyme. Reaction of the enzyme with the irreversible inhibitor 2-hydrazinopyridine (2-HP) reveals differences in the reactivity of Y369F compared with wild type with more efficient formation of an adduct (lambda(max) = 525 nm) perhaps reflecting increased mobility of the TPQ adduct within the active site of Y369F. Titration with 2-HP also reveals that both wild type and Y369F contain one TPQ per monomer, indicating that Tyr369 is not essential for TPQ formation, although we have not measured the rate of TPQ biogenesis. The UV-vis spectrum of the Y369F protein shows a broader peak and red-shifted lambda(max) at 496 nm compared with wild type (480 nm), consistent with an altered electronic structure of TPQ. Steady-state kinetic measurements reveal that Y369F has decreased catalytic activity particularly below pH 6.5 while the K(M) for substrate beta-phenethylamine increases significantly, apparently due to an elevated pK(a) (5.75-6.5) for the catalytic base, Asp383, that should be deprotonated for efficient binding of protonated substrate. At pH 7.0, the K(M) for wild type and Y369F are similar at 1.2 and 1.5 microM, respectively, while k(cat) is decreased from 15 s(-1) in wild type to 0.38 s(-1), resulting in a 50-fold decrease in k(cat)/K(M) for Y369F. Transient kinetics experiments indicate that while the initial stages of enzyme reduction are slower in the variant, these do not represent the rate-limiting step. Previous structural and solution studies have implicated Tyr369 as a component of a proton shuttle from TPQ to dioxygen. The moderate changes in kinetic parameters observed for the Y369F variant indicate that if this is the case, then the absence of the Tyr369 hydroxyl can be compensated for efficiently within the active site.

2-Hydrazinopyridine (2HP) is an irreversible inhibitor of copper amine oxidases (CAOs). 2HP reacts directly at the C5 position of the TPQ cofactor, yielding an intense chromophore with lambda(max) approximately 430 nm (adduct I) in Escherichia coli amine oxidase (ECAO). The adduct I form of wild type (WT-ECAO) was assigned as a hydrazone on the basis of the X-ray crystal structure. The hydrazone adduct appears to be stabilized by two key hydrogen-bonding interactions between the TPQ-2HP moiety and two active site residues: the catalytic base (D383) and the conserved tyrosine residue (Y369). In this work, we have synthesized a model compound (2) for adduct I from the reaction of a TPQ model compound (1) and 2HP. NMR spectroscopy and X-ray crystallography show that 2 exists predominantly as the azo form (lambda(max) at 414 nm). Comparison of the UV-vis and resonance Raman spectra of 2 with adduct I in WT, D383E, D383N, and Y369F forms of ECAO revealed that adduct I in WT and D383N is a tautomeric mixture where the hydrazone form is favored. In D383E adduct I, the equilibrium is further shifted in favor of the hydrazone form. UV-vis spectroscopic pH titrations of adduct I in WT, D383N, D383E, and 2 confirmed that D383 in WT adduct I is protonated at pH 7 and stabilizes the hydrazone tautomer by a short hydrogen-bonding interaction. The deprotonation of D383 (pKa approximately 9.7) in adduct I resulted in conversion of adduct I to the azo tautomer with a blue shift of the lambda(max) to 420 nm, close to that of 2. In contrast, adduct I in D383N and D383E is stable and did not show any pH-dependent spectral changes. In Y369F, adduct I was not stable and gradually converted into a new species with lambda(max) at approximately 530 nm (adduct II). A detailed mechanism for the adduct I formation in WT has been proposed that is consistent with the mechanism proposed for the oxidation of substrate by CAOs but addresses some key differences in the active site chemistry of hydrazine inhibitors and substrate amines.

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Reactions (25 7C) of galactose oxidase, GOase ox from Fusarium NRRL 2903 with five different primary-alcohol-containing substrates RCH 2 OH:-D-galactose (I) and 2-deoxy-D-galactose (II) (monosaccharides); methyl-b-D-galactopyranoside (III) (glycoside); D-raffinose (IV) (trisaccharide); and dihydroxyacetone (V) have been studied in the presence of O 2 . The GOase ox state has a tyrosyl radical coordinated at a square-pyramidal Cu II active site, and is a two-equivalent oxidant. Reactant concentrations were [GOase ox ] (0.8-10 mM), RCH 2 OH (1.0-6.0 mM), and O 2 (0.14-0.29 mM), with Ip0.100 M (NaCl). The reactions, monitored at 450 nm by stopped-flow spectrophotometry, terminated with depletion of the O 2 . Each trace was fitted to the competing reactions GOase ox cRCH 2 OH ] GOase red H 2 cRCHO (k 1 ), and GOase red H 2 cO 2 ] GOase ox cH 2 O 2 (k 2 ), with GOase red H 2 written as the doubly protonated two-electron-reduced Cu I product. It was necessary to avoid auto-redox interconversion of GOase ox and GOase semi . Information obtained at pH 7.5 indicates a 5 : 95 (ox : semi) "native" mix equilibration complete in F3 h. At pH `7.5, rate constants 10 P4 k 1 /M P1 s P1 for the reactions of GOase ox with (I) (1.19), (II) (1.07), (III) (1.29), (IV) (1.81), (V) (2.94) were determined. On decreasing the pH to 5.5, k 1 values decreased by factors of up to a half, and acid dissociation pK a s in the range 6.6-6.9 were obtained. UVVis spectrophotometric studies on GOase ox gave an independently determined pK a of 6.7. No corresponding reactions of the Tyr495Phe variant were observed, and there are no similar UV-Vis absorbance changes for this variant. The pK a is therefore assigned to protonation of Tyr-495 which is a ligand to the Cu. The rate constant k 2 (1.01!10 7 M P1 s P1 ) is independent of pH in the range 5.5-9.0 investigated, suggesting that H c (or H-atoms) for the O 2 ] H 2 O 2 change are provided by the active site of GOase red . The Cu I of GOase red is less extensively complexed, and a coordination number of three is likely.