Analysis of the roles of amino acid residues in the flavoprotein tryptophan 2-monooxygenase modified by 2-oxo-3-pentynoate: characterization of His338, Cys339, and Cys511 mutant enzymes

The flavoprotein tryptophan 2-monooxygenase catalyzes the oxidative decarboxylation of tryptophan to indoleacetamide. His338, Cys339, and Cys511 of the Pseudomonas savastanoi enzyme were previously identified as possible active-site residues by modification with 2-oxo-3-pentynoate ([G. Gadda, L.J. Dangott, W.H. Johnson Jr., C.P. Whitman, P.F. Fitzpatrick, Biochemistry 38 (1999) 5822-5828]). The H338N, C339A, and C511S enzymes have been characterized to determine the roles of these residues in catalysis. The steady-state kinetic parameters with both tryptophan and methionine decrease only slightly in the case of the H338N and C339A enzymes; the decrease in activity is greater for the C511S enzyme. Only in the case of the C511S enzyme do deuterium kinetic isotope effects on kinetic parameters indicate a significant change in catalytic rates. The structural bases for the effects of the mutations can be interpreted by identification of L-amino acid oxidase and tryptophan monooxygenase as homologous proteins.     

Identification of Tyr413 as an active site residue in the flavoprotein tryptophan 2-monooxygenase and analysis of its contribution to catalysis

The flavoenzyme tryptophan 2-monooxygenase catalyzes the oxidation of tryptophan to indoleacetamide, carbon dioxide, and water. The enzyme is a homologue of l-amino acid oxidase. In the structure of l-amino acid oxidase complexed with aminobenzoate, Tyr372 hydrogen bonds with the carboxylate of the inhibitor in the active site. All 10 conserved tyrosine residues in tryptophan 2-monooxygenase were mutated to phenylalanine; steady state kinetic characterization of the purified proteins identified Tyr413 as the residue homologous to Tyr372 of l-amino acid oxidase. Y413F and Y413A tryptophan 2-monooxygenase were characterized more completely with tryptophan as the substrate to probe the contribution of this residue to catalysis. Mutation of Tyr413 to phenylalanine results in a decrease in the value of the first-order rate constant for reduction of 35-fold and a decrease in the rate constant for oxidation of 11-fold. Mutation to alanine decreases the rate constant for reduction by 200-fold and that for oxidation by 33-fold. Both mutations increase the K(d) value for tryptophan and the K(i) values for the competitive inhibitors indoleacetamide and indole pyruvate by 5-10-fold. Both mutations convert the enzyme to an oxidase, in that the products of the catalytic reactions of both are indolepyruvate and hydrogen peroxide. The V/K(trp)-pH profiles for the Tyr413 mutant enzymes no longer show the pK(a) value of 9.9 seen in that for the wild-type enzyme, allowing identification of Tyr413 as the active site residue in the wild-type enzyme which must be protonated for catalysis. Substitution of Tyr413 abolishes the formation of the long wavelength charge transfer species observed in the wild-type enzyme. The data are consistent with the main role of Tyr413 being to maintain the correct orientation of tryptophan for effective hydride transfer and imino acid decarboxylation.     

Probing oxygen activation sites in two flavoprotein oxidases using chloride as an oxygen surrogate

A single basic residue above the si-face of the flavin ring is the site of oxygen activation in glucose oxidase (GOX) (His516) and monomeric sarcosine oxidase (MSOX) (Lys265). Crystal structures of both flavoenzymes exhibit a small pocket at the oxygen activation site that might provide a preorganized binding site for superoxide anion, an obligatory intermediate in the two-electron reduction of oxygen. Chloride binds at these polar oxygen activation sites, as judged by solution and structural studies. First, chloride forms spectrally detectable complexes with GOX and MSOX. The protonated form of His516 is required for tight binding of chloride to oxidized GOX and for rapid reaction of reduced GOX with oxygen. Formation of a binary MSOX·chloride complex requires Lys265 and is not observed with Lys265Met. Binding of chloride to MSOX does not affect the binding of a sarcosine analogue (MTA, methylthioactetate) above the re-face of the flavin ring. Definitive evidence is provided by crystal structures determined for a binary MSOX·chloride complex and a ternary MSOX·chloride·MTA complex. Chloride binds in the small pocket at a position otherwise occupied by a water molecule and forms hydrogen bonds to four ligands that are arranged in approximate tetrahedral geometry: Lys265:NZ, Arg49:NH1, and two water molecules, one of which is hydrogen bonded to FAD:N5. The results show that chloride (i) acts as an oxygen surrogate, (ii) is an effective probe of polar oxygen activation sites, and (iii) provides a valuable complementary tool to the xenon gas method that is used to map nonpolar oxygen-binding cavities.     

Catalytic mechanism of 2-hydroxybiphenyl 3-monooxygenase, a flavoprotein from Pseudomonas azelaica HBP1

2-Hydroxybiphenyl 3-monooxygenase (EC 1.14.13.44) from Pseudomonas azelaica HBP1 is an FAD-dependent aromatic hydroxylase that catalyzes the conversion of 2-hydroxybiphenyl to 2, 3-dihydroxybiphenyl in the presence of NADH and oxygen. The catalytic mechanism of this three-substrate reaction was investigated at 7 degrees C by stopped-flow absorption spectroscopy. Various individual steps associated with catalysis were readily observed at pH 7.5, the optimum pH for enzyme turnover. Anaerobic reduction of the free enzyme by NADH is a biphasic process, most likely reflecting the presence of two distinct enzyme forms. Binding of 2-hydroxybiphenyl stimulated the rate of enzyme reduction by NADH by 2 orders of magnitude. The anaerobic reduction of the enzyme-substrate complex involved the formation of a transient charge-transfer complex between the reduced flavin and NAD(+). A similar transient intermediate was formed when the enzyme was complexed with the substrate analog 2-sec-butylphenol or with the non-substrate effector 2,3-dihydroxybiphenyl. Excess NAD(+) strongly stabilized the charge-transfer complexes but did not give rise to the appearance of any intermediate during the reduction of uncomplexed enzyme. Free reduced 2-hydroxybiphenyl 3-monooxygenase reacted rapidly with oxygen to form oxidized enzyme with no appearance of intermediates during this reaction. In the presence of 2-hydroxybiphenyl, two consecutive spectral intermediates were observed which were assigned to the flavin C(4a)-hydroperoxide and the flavin C(4a)-hydroxide, respectively. No oxygenated flavin intermediates were observed when the enzyme was in complex with 2, 3-dihydroxybiphenyl. Monovalent anions retarded the dehydration of the flavin C(4a)-hydroxide without stabilization of additional intermediates. The kinetic data for 2-hydroxybiphenyl 3-monooxygenase are consistent with a ternary complex mechanism in which the aromatic substrate has strict control in both the reductive and oxidative half-reaction in a way that reactions leading to substrate hydroxylation are favored over those leading to the futile formation of hydrogen peroxide. NAD(+) release from the reduced enzyme-substrate complex is the slowest step in catalysis.     

Tyrosine replacing tryptophan as an anchor in GWALP peptides

Synthetic model peptides have proven useful for examining fundamental peptide-lipid interactions. A frequently employed peptide design consists of a hydrophobic core of Leu-Ala residues with polar or aromatic amino acids flanking each side at the interfacial positions, which serve to "anchor" a specific transmembrane orientation. For example, WALP family peptides (acetyl-GWW(LA)(n)LWWA-[ethanol]amide), anchored by four Trp residues, have received particular attention in both experimental and theoretical studies. A recent modification proved successful in reducing the number of Trp anchors to only one near each end of the peptide. The resulting GWALP23 (acetyl-GGALW(5)(LA)(6)LW(19)LAGA-[ethanol]amide) displays reduced dynamics and greater sensitivity to lipid-peptide hydrophobic mismatch than traditional WALP peptides. We have further modified GWALP23 to incorporate a single tyrosine, replacing W(5) with Y(5). The resulting peptide, Y(5)GWALP23 (acetyl-GGALY(5)(LA)(6)LW(19)LAGA-amide), has a single Trp residue that is sensitive to fluorescence experiments. By incorporating specific (2)H and (15)N labels in the core sequence of Y(5)GWALP23, we were able to use solid-state NMR spectroscopy to examine the peptide orientation in hydrated lipid bilayer membranes. The peptide orients well in membranes and gives well-defined (2)H quadrupolar splittings and (15)N/(1)H dipolar couplings throughout the core helical sequence between the aromatic residues. The substitution of Y(5) for W(5) has remarkably little influence on the tilt or dynamics of GWALP23 in bilayer membranes of the phospholipids DOPC, DMPC, or DLPC. A second analogue of the peptide with one Trp and two Tyr anchors, Y(4,5)GWALP23, is generally less responsive to the bilayer thickness and exhibits lower apparent tilt angles with evidence of more extensive dynamics. In general, the peptide behavior with multiple Tyr anchors appears to be quite similar to the situation when multiple Trp anchors are present, as in the original WALP series of model peptides.     

Analysis of the role of the active site residue Arg98 in the flavoprotein tryptophan 2-monooxygenase, a member of the L-amino oxidase family

The flavoprotein tryptophan 2-monooxygenase catalyzes the oxidative decarboxylation of tryptophan to indoleacetamide. We have previously identified tryptophan 2-monooxygenase as a homologue of L-amino acid oxidase [Sobrado, P., and Fitzpatrick, P. F. (2002) Arch. Biochem. Biophys. 402, 24-30]. On the basis of the sequence comparisons of the different LAAO family members, Arg98 of tryptophan 2-monooxygenase can be identified as an active site residue which interacts with the carboxylate of the amino acid substrate. The catalytic properties of R98K and R98A tryptophan 2-monooxygenase have been characterized to evaluate the role of this residue. Mutation of Arg98 to lysine decreases the first-order rate constant for flavin reduction by 180-fold and the second-order rate constant for flavin oxidation by 26-fold, has no significant effect on the K(d) value for tryptophan or the K(i) value for the competitive inhibitor indoleacetamide, and increases the K(i) value for indolepyruvate less than 2-fold. Mutation of this residue to alanine decreases the rate constants for reduction and oxidation an additional 5- and 2-fold, respectively, and increases the K(d) value for tryptophan and the K(i) value for indolepyruvate by 31- and 17-fold, respectively, while having an only 2-fold effect on the K(i) value for indoleacetamide. Both mutations increase the value of the primary deuterium isotope effect with tryptophan as a substrate, consistent with a later transition state. Both mutant enzymes catalyze a simple oxidase reaction, producing indolepyruvate and hydrogen peroxide. The pH dependences of the V/K(trp) values for the mutant enzymes show that the anionic form of the substrate is preferred but that the zwitterionic form is a substrate. The results are consistent with the interaction between Arg98 and the carboxylate of the amino acid substrate being critical for correct positioning of the substrate in the active site for efficient catalysis.     

Oxygen activation in flavoprotein oxidases: the importance of being positive

The oxidation of flavin hydroquinones by O(2) in solution is slow, with second-order rate constants of ~250 M(-1) s(-1). This is due to the obligatory, single-electron transfer that initiates the reaction being thermodynamically unfavored and poorly catalyzed. Notwithstanding considerations of O(2) accessibility to the reaction site, its desolvation and geometry and other factors that can also contribute to further rate acceleration, flavoprotein oxidases must activate O(2) for reaction with flavin hydroquinones to be able to achieve the 100-1000-fold rate enhancements typically observed. Protein positive charges have been identified in glucose oxidase, monomeric sarcosine oxidase, N-methyltryptophan oxidase and fructosamine oxidase that electrostatically stabilize the transition state for the initial single electron transfer that generates the O(2)(-?)/flavin semiquinone radical pair. In choline oxidase despite the presence of three histidines in the active site, the trimethylammonium group of the reaction product provides such an electrostatic stabilization. A nonpolar site proximal to the flavin C(4a) atom in choline oxidase has also been identified, which contributes to the geometry and desolvation of the O(2) reaction site. The relevance of O(2) activation by product charges to other flavoprotein oxidases, such as for example those catalyzing amine oxidations, is discussed in this review. A nonpolar site close to the flavin C(4a) atom and a positive charge is identified through structural analysis in several flavoprotein oxidases. Mutagenesis has disclosed nonpolar sites in O(2)-reducing enzymes that utilize copper/TPQ or iron. It is predicted that classes of O(2)-reducing enzymes utilizing other cofactors also contain a similar catalytic motif.     

Helix switching of a key active-site residue in the cytochrome cbb3 oxidases

In the respiratory chains of mitochondria and many aerobic prokaryotes, heme-copper oxidases are the terminal enzymes that couple the reduction of molecular oxygen to proton pumping, contributing to the protonmotive force. The cbb(3) oxidases belong to the superfamily of enzymes that includes all of the heme-copper oxidases. Sequence analysis indicates that the cbb(3) oxidases are missing an active-site tyrosine residue that is absolutely conserved in all other known heme-copper oxidases. In the other heme-copper oxidases, this tyrosine is known to be subject to an unusual post-translational modification and to play a critical role in the catalytic mechanism. The absence of this tyrosine in the cbb(3) oxidases raises the possibility that the cbb(3) oxidases utilize a different catalytic mechanism from that of the other members of the superfamily. Using homology modeling, quantum chemistry, and molecular dynamics, a model of the structure of subunit I of a cbb(3) oxidase (Vibrio cholerae) was constructed. The model predicts that a tyrosine residue structurally analogous to the active-site tyrosine in other oxidases is present in the cbb(3) oxidases but that the tyrosine originates from a different transmembrane helix within the protein. The predicted active-site tyrosine is conserved in the sequences of all of the known cbb(3) oxidases. Mutagenesis of the tyrosine to phenylalanine in the V. cholerae oxidase resulted in a fully assembled enzyme with nativelike structure but lacking catalytic activity. These findings strongly suggest that all of the heme-copper oxidases utilize the same catalytic mechanism and provide an unusual example in which a critical active-site residue originates from different places within the primary sequence for different members of the same superfamily.     

Regulation of 3-indoleacetic acid production in Pseudomonas syringae pv. savastanoi. Purification and properties of tryptophan 2-monooxygenase

The oxidative decarboxylation of L-tryptophan to yield 3-indoleacetamide, catalyzed by tryptophan 2-monooxygenase, represents a controlling reaction in the synthesis of indoleacetic acid by Pseudomonas savastanoi (Pseudomonas syringae pv. savastanoi), a gall-forming pathogen of olive (Olea europea L.) and oleander (Nerium oleander L.). Production of indoleacetic acid is essential for virulence of the bacterium in its hosts. Tryptophan 2-monooxygenase was characterized to determine its role in indoleacetic acid metabolism in the bacterium. The enzyme was purified to apparent homogeneity from Escherichia coli cells containing the genetic locus for this enzyme obtained from P. savastanoi. The preparation contained a single polypeptide with a mass of 62,000 that cross-reacted immunologically with a homologous protein in P. savastanoi. The holoenzyme contained one FAD moiety/subunit with properties consistent with a catalytic function. The enzyme preparation catalyzed an L-tryptophan-dependent O2 uptake and yielded 3-indoleacetamide as a product. Enzyme activity fit simple Michaelis Menten kinetics with a Km for L-tryptophan of 50 microM. 3-Indoleacetamide and 3-indoleacetic acid were identified as regulatory effectors. The apparent Ki for 3-indoleacetamide was 7 microM; that for indoleacetic acid was 225 microM. At Km concentrations of tryptophan, enzyme activity was inhibited 50% by 25 microM 3-indoleacetamide. In contrast, 230 microM indoleacetic acid was required to effect a similar inhibition. Phenylalanine and tyrosine were ineffective as regulatory metabolites. These results indicate that IAA synthesis in P. savastanoi is regulated by limiting tryptophan and by feedback inhibition from indoleacetamide and indoleacetic acid.     

Characterization of 4-methyl-2-oxo-1,2-dihydroquinolin-6-yl acetate as an effective antiplatelet agent

We have studied earlier a membrane bound novel enzyme Acetoxy Drug: protein transacetylase identified as Calreticulin Transacetylase (CRTAase) that catalyzes the transfer of acetyl groups from polyphenolic acetates (PAs) to the receptor proteins and thus modulating their biological activities. In this communication, we have reported for the first time that acetoxy quinolones are endowed with antiplatelet action by virtue of causing CRTAase catalyzed activation of platelet Nitric Oxide Synthase (NOS) by way of acetylation leading to the inhibition of ADP/Arachidonic acid (AA)-dependent platelet aggregation. The correlation of specificity of platelet CRTAase to various analogues of acetoxy quinolones with intracellular NO and consequent effect on inhibition of platelet aggregation was considered crucial. Among acetoxy quinolones screened, 6-AQ (4-methyl-2-oxo-1,2-dihydroquinolin-6-yl acetate/6-acetoxyquinolin-2-one, 22) was found to be the superior substrate to platelet CRTAase and emerged as the most active entity to produce antiplatelet action both in vitro and in vivo. 6-AQ caused the inhibition of cyclooxygenase-1 (Cox-1) resulting in the down regulation of thromboxane A2 (TxA2) and the inhibition of platelet aggregation. Structural modification of acetoxy quinolones positively correlated with enhancement of intracellular NO and antiplatelet action.     

Phosphorylation-dependent interaction of tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein (YWHA) with PADI6 following oocyte maturation in mice

Proteins in the tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein family (YWHA; also known as 14-3-3) are involved in the regulation of many intracellular processes. We have examined the interaction of YWHA with peptidylarginine deiminase type VI (PADI6), an abundant protein in mammalian oocytes, eggs, and early embryos. Peptidylarginine deiminases catalyze the posttranslational modification of peptidylarginine to citrulline. PADI6 is associated with oocyte cytoplasmic sheets, and PADI6-deficient mice are infertile because of disruption of development beyond the two-cell stage. We found that PADI6 undergoes a dramatic developmental change in phosphorylation during oocyte maturation. This change in phosphorylation is linked to an interaction of PADI6 with YWHA in the mature egg. Recombinant glutathione S-transferase YWHA pull-down experiments and transgenic tandem affinity purification with liquid chromatography-mass spectrometry demonstrate a binding interaction between YWHA and PADI6 in mature eggs. YWHA proteins modulate or complement intracellular events involving phosphorylation-dependent switching or protein modification. These results indicate that phosphorylation and/or YWHA binding may serve as a means of intracellular PADI6 regulation.     

On the oxygen reactivity of flavoprotein oxidases: an oxygen access tunnel and gate in brevibacterium sterolicum cholesterol oxidase

The flavoprotein cholesterol oxidase from Brevibacterium sterolicum (BCO) possesses a narrow channel that links the active center containing the flavin to the outside solvent. This channel has been proposed to serve for the access of dioxygen; it contains at its "bottom" a Glu-Arg pair (Glu-475-Arg-477) that was found by crystallographic studies to exist in two forms named "open" and "closed," which in turn was suggested to constitute a gate functioning in the control of oxygen access. Most mutations of residues that flank the channel have minor effects on the oxygen reactivity. Mutations of Glu-311, however, cause a switch in the basic kinetic mechanism of the reaction of reduced BCO with dioxygen; wild-type BCO and most mutants show a saturation behavior with increasing oxygen concentration, whereas for Glu-311 mutants a linear dependence is found that is assumed to reflect a "simple" second order process. This is taken as support for the assumption that residue Glu-311 finely tunes the Glu-475-Arg-477 pair, forming a gate that functions in modulating the access/reactivity of dioxygen.     

A new activator protein that activates tryptophan 5-monooxygenase and tyrosine 3-monooxygenase in the presence of Ca2+-, calmodulin-dependent protein kinase. Purification and characterization

Rat brain tryptophan 5-monooxygenase was activated by incubation with ATP, Mg2+, calmodulin, and micromolar concentrations of Ca2+. The activating activity was resolved into two distinct peaks upon gel filtration on Sepharose CL-6B: one, Ca2+-, calmodulin-dependent protein kinase, and the other, a heat-labile activator protein. The activator protein was purified to apparent homogeneity from rat brain by a procedure involving calmodulin-Sepharose 4B, Sephadex G-150, and phenyl-Sepharose CL-4B column chromatography. The molecular weight of the activator protein was determined to be 70,000 by sedimentation equilibrium and by gel filtration on Sephadex G-150. The protein gave a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the molecular weight of which was estimated to be 35,000, indicating that the protein might be composed of two identical subunits. Analysis of cross-linked activator protein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis also suggested that the protein might be a dimer of identical subunits. Some other molecular properties of the activator protein were: sedimentation coefficient, 4.3 S; Stokes radius, 3.6 nm; diffusion coefficient, 6.0 x 10(-7) cm2/s; frictional ratio, 1.32; and partial specific volume, 0.73 cm3/g. The activator protein activated tyrosine 5-monooxygenase as well as tryptophan 5-monooxygenase in the presence of ATP, Mg2+, Ca2+, calmodulin, and Ca2+-, calmodulin-dependent protein kinase.     

Inactivation of Escherichia coli 2-amino-3-ketobutyrate CoA ligase by phenylglyoxal and identification of an active-site arginine peptide.

Treatment of homogeneous preparations of 2-amino-3-ketobutyrate CoA ligase from Escherichia coli, a pyridoxal 5'-phosphate-dependent enzyme, with phenylglyoxal, 4-(oxyacetyl)phenoxyacetic acid, 2,3-butanedione, or 1,2-cyclohexanedione results in a time- and concentration-dependent loss of enzymatic activity. Phenylglyoxal in 50 mM phosphate buffer (pH 7.0) is the most effective modifier, causing > 95% inactivation within 20 min at 25 degrees C. Controls establish that this inactivation is not due to modifier-induced dissociation or photoinduced nonspecific alteration of the ligase. The substrate, acetyl CoA, or the coenzyme, pyridoxal 5'-phosphate, gives > 50% protection against inactivation. Enzyme partially inactivated by phenylglyoxal has the same Km value for glycine but the Vmax decreases in proportion to the observed level of inactivation. Whereas the native apoligase shows good recovery of activity with time in parallel with an increase in 428-nm absorptivity when incubated with pyridoxal 5'-phosphate, no such effects are seen with the phenylglyoxal-modified apoligase. Reaction of the enzyme with [14C]phenylglyoxal allowed for the isolation of a peptide which, by amino acid composition and sequencing data, was found to correspond to residues 349-378 in the intact enzyme. These results indicate that arginine residue-366 and/or residue-368 in the primary structure of E. coli 2-amino-3-ketobutyrate ligase is at the active site.     

Mechanistic studies of the flavoenzyme tryptophan 2-monooxygenase: deuterium and 15N kinetic isotope effects on alanine oxidation by an L-amino acid oxidase

Tryptophan 2-monooxygenase (TMO) from Pseudomonas savastanoi catalyzes the oxidative decarboxylation of l-tryptophan during the biosynthesis of indoleacetic acid. Structurally and mechanistically, the enzyme is a member of the family of l-amino acid oxidases. Deuterium and 15N kinetic isotope effects were used to probe the chemical mechanism of l-alanine oxidation by TMO. The primary deuterium kinetic isotope effect was pH independent over the pH range 6.5-10, with an average value of 6.0 +/- 0.5, consistent with this being the intrinsic value. The deuterium isotope effect on the rate constant for flavin reduction by alanine was 6.3 +/- 0.9; no intermediate flavin species were observed during flavin reduction. The kcat/Kala value was 1.0145 +/- 0.0007 at pH 8. NMR analyses gave an equilibrium 15N isotope effect for deprotonation of the alanine amino group of 1.0233 +/- 0.0004, allowing calculation of the 15N isotope effect on the CH bond cleavage step of 0.9917 +/- 0.0006. The results are consistent with TMO oxidation of alanine occurring through a hydride transfer mechanism.     

Characterization of lysyl residues of NADH-cytochrome b5 reductase implicated in charge-pairing with active-site carboxyl residues of cytochrome b5 by site-directed mutagenesis of an expression vector for the flavoprotein.

An expression vector for bovine NADH-cytochrome b5 reductase was constructed from two DNA fragments that were derived from beef liver poly(A+) RNA using the polymerase chain reaction. Site-directed mutagenesis of the 3 lysine residues of the reductase, previously implicated in the formation of active-site charge pairs with carboxylate residues of cytochrome b5, was then used to obtain the purified catalytic domains of flavoproteins modified at each of these sites. The observed marked decreases in catalytic efficiencies of substitutions of a negative charge at the normally positively charged residues with the catalytic domain of cytochrome b5 are consistent with their participation in the formation of charge pairs with carboxylate groups of the hemeprotein to optimize rapid electron transfer from the reductase flavin to the heme of the cytochrome.     

Mechanism of 3-methylaspartase probed using deuterium and solvent isotope effects and active-site directed reagents: identification of an essential cysteine residue.

The mechanism of the L-threo-3-methylaspartate ammonia-lyase (EC 4.3.1.2) reaction has been probed using deuterium and solvent isotope effects with three different substrates, (2S,3S)-3-methylaspartic acid, (2S)-aspartic acid and (2S,3R)-3-methylaspartic acid. Each substrate appears to form a covalent adduct with the enzyme through the amination of a dehydroalanine (DehydAla-173) residue. The true substrates are N-protonated and at low pH, the alkylammonium groups are deprotonated internally in a closed solvent-excluded pocket after K+ ion, an essential cofactor, has become bound to the enzyme. At high pH, the amino groups of the substrates are able to react with the dehydroalanine residue prior to K+ ion binding. This property of the system gives rise to complex kinetics at pH 9.0 or greater and causes the formation of dead-end complexes which lack Mg2+ ion, another essential cofactor. The enzyme-substrate adduct is subsequently deaminated in two elimination processes. Hydrazines act as alternative substrates in the reverse reaction direction in the presence of fumaric acid derivatives, but cause irreversible inhibition in their absence. Borohydride and cyanide are not inhibitors. N-Ethylmaleimide also irreversibly inactivates the enzyme and labels residue Cys-361. The inactivation process is enhanced in the presence of cofactor Mg2+ ions and Cys-361 appears to serve as a base for the removal of the C-3 proton from the natural substrate, (2S,3S)-3-methylaspartic acid. The dehydroalanine residue appears to be protected in the resting form of the enzyme by generation of an internal thioether cross-link. The binding of the substrate and K+ ion appear to cause a conformational change which requires hydroxide ion. This is linked to reversal of the thioether protection step and generation of the base for substrate deprotonation at C-3. The deamination reaction displays high reverse reaction commitments and independent evidence from primary deuterium isotope effect data indicates that a thiolate acts as the base for deprotonation at C-3.