Glutamate decarboxylase side reactions catalyzed by the enzyme

Oxygen reactivity with pyridoxal 5'-phosphate enzymes: biochemical implications and functional relevance

The versatility of reactions catalyzed by pyridoxal 5'-phosphate (PLP) enzymes is largely due to the chemistry of their extraordinary catalyst. PLP is necessary for many reactions involving amino acids. Reaction specificity is controlled by the orientation of the external aldimine intermediate that is formed upon addition of the amino acidic substrate to the coenzyme. The breakage of a specific bond of the external aldimine gives rise to a carbanionic intermediate. From this point, the different reaction pathways diverge leading to multiple activities: transamination, decarboxylation, racemization, elimination, and synthesis. A significant novelty appeared approximately 30 years ago when it was reported that some PLP-dependent decarboxylases are able to consume molecular oxygen transforming an amino acid into a carbonyl compound. These side paracatalytic reactions could be particularly relevant for human health, also considering that some of these enzymes are responsible for the synthesis of important neurotransmitters such as γ-aminobutyric acid, dopamine, and serotonin, whose dysregulation under oxidative conditions could have important implications in neurodegenerative states. However, the reactivity of PLP enzymes with dioxygen is not confined to mammals/animals. In fact, some plant PLP decarboxylases have been reported to catalyze oxidative reactions producing carbonyl compounds. Moreover, other recent reports revealed the existence of new oxidase activities catalyzed by new PLP enzymes, MppP, RohP, Ind4, CcbF, PvdN, Cap15, and CuaB. These PLP enzymes belong to the bacterial and fungal kingdoms and are present in organisms synthesizing bioactive compounds. These new PLP activities are not paracatalytic and could only scratch the surface on a wider and unexpected catalytic capability of PLP enzymes.

Emergence of oxygen- and pyridoxal phosphate-dependent reactions

Pyridoxal 5'-phosphate (PLP) is an organic cofactor employed by ~ 4% of enzymes. The structure of the PLP cofactor allows for the stabilization of carbanions through resonance. A small number of PLP-dependent enzymes employ molecular oxygen as a cosubstrate. Here, we review the biological roles and possible mechanisms of these enzymes, and we observe that these enzymes are found in multiple protein families, suggesting that reaction with oxygen might have emerged de novo in several protein families and thus could be directed to emerge again through laboratory evolution experiments.

Studies on the inhibition of sphingosine-1-phosphate lyase by stabilized reaction intermediates and stereodefined azido phosphates

Two kinds of inhibitors of the PLP-dependent enzyme sphingosine-1-phosphate lyase have been designed and tested on the bacterial (StS1PL) and the human (hS1PL) enzymes. Amino phosphates 1, 12, and 32, mimicking the intermediate aldimines of the catalytic process, were weak inhibitors on both enzyme sources. On the other hand, a series of stereodefined azido phosphates, resulting from the replacement of the amino group of the natural substrates with an azido group, afforded competitive inhibitors in the low micromolar range on both enzyme sources. This similar behavior represents an experimental evidence of the reported structural similarities for both enzymes at their active site level. Interestingly, the anti-isomers of the non-natural enantiomeric series where the most potent inhibitors on hS1PL.

Enzyme-Catalyzed Side Reactions with Molecular Oxygen may Contribute to Cell Signaling and Neurodegenerative Diseases

A link between neurodegeneration and well-characterized enzymatic and non-enzymatic reactions that produce reactive oxygen species (ROS) from O(2) is well established. Several enzymes that contain pyridoxal 5'-phosphate (PLP) or thiamine diphosphate (ThDP) catalyze side reactions (paracatalytic reactions) in the presence of ambient O(2). These side reactions produce oxidants such as hydrogen peroxide [H(2)O(2)] or extremely reactive peracids [RC(O)OOH]. We hypothesize that although these enzymes normally produce oxidants at low or undetectable levels, changes in substrate levels or disease-induced structural alterations may enhance interactions with O(2), thereby generating higher levels of reactive oxidants. These oxidants may damage the enzymes producing them, alter nearby macromolecules and/or destroy important metabolites/coenzymes. We propose that paracatalytic reactions with O(2) catalyzed by PLP-dependent decarboxylases and by ThDP-dependent enzymes within the alpha-keto acid dehydrogenase complexes may contribute to normal cellular signaling and to cellular damage in neurodegenerative diseases.

Structural characteristics of brain glutamate decarboxylase in relation to its interaction and activation

The conformation, stability, cofactor interactions, and activation of a recombinant 65-kDa form of rat brain glutamate decarboxylase (GAD65) were investigated by using UV/visible spectrophotometry, fluorescence spectroscopy, circular dichroism, and differential scanning microcalorimetry. The enzyme was prepared from Sf9 insect cells infected with a recombinant baculovirus containing the entire GAD65 coding region. The UV/visible absorption spectrum of purified holoenzyme (holoGAD) exhibits two peaks in the range of 300-450 nm, which are due to the formation of a Schiff base when pyridoxal phosphate (pyridoxal-P) binds to GAD. Fluorescence emission intensity (excited at 295 or 280 nm) was substantially enhanced when pyridoxal-P was removed from holoGAD and quenched when pyridoxal-P was added to the apoenzyme (apoGAD). These observations implied that a significant enzyme conformational change occurs during the formation of holoGAD. Circular dichroism provided additional evidence for a conformational change, as the ellipticity of both negative (202-242 nm) and positive (188-202 nm) bands decreased when pyridoxal-P was removed from holoGAD. Secondary structure determination estimated that holoGAD contains a higher content of alpha-helix (34% versus 24%) and a lower content of beta-sheet (18% versus 30%) than apo-GAD. Differential scanning microcalorimetry indicated that holoGAD exhibits a much larger enthalpy and a 3 degrees C higher temperature of thermal unfolding than apoGAD, suggesting that holoGAD has a much tighter conformation and greater stability than apoGAD. A model describing the interaction of pyridoxal-P with GAD is presented, which proposes that an intermediate complex involving ionic interaction between the phosphate group of pyridoxal-P and the positive, charged residues in the active site of GAD maintains the pyridoxal-P molecule in an appropriate position in the active center. Simultaneously, this complex formation is accompanied by a moderate enzyme conformational change, providing a favorable configuration that enables the epsilon-amino of the active-site lysine to react with the aldehyde group of pyridoxal-P. The formation of active holoGAD involves a large enzyme conformational change, which leads to increased stability.

3-alkyl GABA and 3-alkylglutamic acid analogues: two new classes of anticonvulsant agents

Recently we showed that 3-alkyl-4-aminobutanoic acids are in vitro activators of brain L-glutamic acid decarboxylase (GAD) that show anticonvulsant activity. Since activation of GAD leads to increased concentrations of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in vitro, these compounds could represent a new class of anticonvulsant agents. Here it is shown that 3-alkylglutamic acid analogues also activate GAD and that all of the compounds in both series are active anticonvulsant agents against low intensity electroshock in mice. The most active compound, 3-isobutyl GABA, was tested further against maximal electroshock in mice and was shown to be very potent after both intravenous and oral administration without causing ataxia. It is not known if brain GABA levels are elevated in vivo by administration of these compounds or if the mechanism of anticonvulsant activity is related to their ability to activate GAD.

L-methionine decarboxylase from Dryopteris filix-mas: purification, characterization, substrate specificity, abortive transamination of the coenzyme, and stereochemical courses of substrate decarboxylation and coenzyme transamination

L-Methionine decarboxylase from the male fern Dryopteris filix-mas has been purified 256-fold from acetone powder extracts to very near homogeneity. The enzyme is membrane-associated and requires detergent for solubilization during the initial extraction. The enzyme is a homodimer of subunit Mr 57,000 and shows a pH optimum at approximately 5.0 with 20 mM (2S)-methionine as substrate. The specific activity, kcat, for methionine is approximately 50 mol s(-1) (mol of active site)(-1) at pH 4.5 and below. A wide range of straight- and branched-chain (2S)-alkylamino acids are substrates for the enzyme. The values for the rate of decarboxylation, Vmax, and for the apparent Michaelis constant, Km, however, vary with structure and with the chirality at C-3. The pH dependence of V and V/K has been examined for three substrates: (2S)-methionine, valine, and leucine. Pyridoxal 5'-phosphate (PLP) is required for activity, and in the absence of excess PLP, the activity of the enzyme in incubations reduced with respect to time. The addition of PLP fully restores the activity, indicating that an abortive decarboxylation-transamination accompanies the normal decarboxylation reaction. The occurrence of the abortive reaction was confirmed by showing that [35S]methionine is converted to labeled 3-(methylthio)propionaldehyde while [4'-3H]PLP is converted to labeled pyridoxamine 5'-phosphate (PMP). The decarboxylation of (2S)-methionine gave 3-(methylthio)-1-aminopropane. Preparation of the N-camphanamide derivative of the amine allowed the C-1 methylene protons to be distinguished by 1H NMR spectroscopy. Synthetic samples of the camphanamide were prepared in which each of the C-1 methylene protons was replaced by deuterium. When (2S)-methionine and the C-2 deuteriated isotopomer were incubated with the enzyme in deuterium oxide and protium oxide, respectively, and the products were converted to their camphanamide derivatives and analyzed by 1H NMR spectroscopy, it was evident that decarboxylation occurred with retention of configuration at C-2. When the decarboxylation of six other substrates was studied, examination of the N-camphanamide derivatives of the amines indicated that decarboxylation occurred stereospecifically and, by analogy, with retention of configuration at C-2. When tritiated pyridoxal phosphate was incubated with the enzyme, tritiated pyridoxamine phosphate was formed. Analysis of the chirality of the methylene group at C-4' indicated that, during abortive transamination, protonation occurred from the 4'-si face of the coenzyme, the same stereochemical result as that obtained for several bona fide transaminase enzymes.(ABSTRACT TRUNCATED AT 400 WORDS)

Fern L-methionine decarboxylase: kinetics and mechanism of decarboxylation and abortive transamination

L-Methionine decarboxylase from Dryopteris filix-mas catalyzes the decarboxylation of L-methionine and a range of straight- and branched-chain L-amino acids to give the corresponding amine products. The deuterium solvent isotope effects for the decarboxylation of (2S)-methionine are D(V/K) = 6.5 and DV = 2.3, for (2S)-valine are D(V/K) = 1.9 and DV = 2.6, and for (2S)-leucine are D(V/K) = 2.5 and DV = 1.0 at pL 5.5. At pL 6.0 and above, where the value of kcat for all of the substrates is low, the solvent isotope effects on Vmax for methionine are 1.1-1.2 whereas the effects on V/K remain unchanged, indicating that the solvent-sensitive transition state occurs before the first irreversible step, carbon dioxide desorption. The enzyme also catalyzes an abortive decarboxylation-transamination reaction in which the coenzyme is converted to pyridoxamine phosphate [Stevenson, D. E., Akhtar, M., & Gani, D. (1990a) Biochemistry (first paper of three in this issue)]. At very high concentration, the product amine can promote transamination of the coenzyme. However, the reaction occurs infrequently and does not influence the partitioning between decarboxylation and substrate-mediated abortive transamination under steady-state turnover conditions. The partition ratio, normal catalytic versus abortive events, can be determined from the amount of substrate consumed by a known amount of enzyme at infinite time, and the rate of inactivation can be determined by measuring the decrease in enzyme activity with respect to time. For methionine, the values of Km as determined from double-reciprocal plots of concentration versus inactivation rate are the same as those calculated from initial catalytic (decarboxylation) rate data, indicating that a single common intermediate partitions between product formation and slow transamination. The partition ratio is sensitive to changes in pH and is also dependent upon the structure of the substrate; methionine causes less frequent inactivation than either valine or leucine. The pH dependence of the partition ratio with methionine as substrate is very similar to that for V/K. Both curves show a sharp increase at approximately pH 6.25, indicating that a catalytic group on the enzyme simultaneously suppresses the abortive reaction and enhances physiological reaction in its unprotonated state. Experiments conducted in deuterium oxide allowed the solvent isotope effects for the partition ratio and the abortive reaction to be determined.(ABSTRACT TRUNCATED AT 400 WORDS)

Evolutionary and biosynthetic aspects of aspartate aminotransferase isoenzymes and other aminotransferases

The mitochondrial and cytosolic isoenzymes of aspartate aminotransferase are homologous proteins. Both are encoded by nuclear DNA and synthesized on free polysomes. The organization of their genes is very similar, five out of a total of eight introns are located at the same nucleotide position. A variant consensus sequence was observed at the 3' splice site of introns of genes of imported mitochondrial proteins which may reflect the existence of splicing factors specific for the genes of this particular group of nuclear-encoded proteins. To date the amino acid sequences of 22 aminotransferases are known. A rigorous analysis yielded clear evidence that aspartate, tyrosine, and histidinol-phosphate aminotransferases are homologous proteins despite their low degree of sequence identity. The evolutionary relationship among the vitamin B6-dependent enzymes in general appears less clear. Conceivably, their common structural and mechanistic features are dictated by the chemical properties of pyridoxal 5'-phosphate rather than being due to a common ancestor of their protein moieties. In agreement with this notion, the ubiquitous active-site lysine residue that forms a Schiff base with the coenzyme can be replaced in the case of aspartate aminotransferase by a histidine residue without complete loss of catalytic competence.

The enantiomeric error frequency of aspartate aminotransferase

The enantiomeric error frequency of aspartate aminotransferase (mitochondrial isoenzyme from chicken) was assessed by adding the enzyme in high concentration (0.89 mM) to a mixture of L-glutamate and 2-oxoglutarate (12 and 1.2 mM, respectively, at pH 7.5 and 25 degrees C). The substrates continuously undergo the transamination cycle under these conditions. Thereby, L-glutamate is progressively racemized, a 1:1 ratio of two enantiomers being reached within 240 h. The enantiomeric error frequency, i.e. the ratio of the rate of D-glutamate production and the rate of the transamination reaction with glutamate and 2-oxoglutarate as substrates, is 1.5 x 10(-7). D-Glutamate is also converted to a 1:1 racemic mixture. The racemizing activity of a mixture of free pyridoxal 5'-phosphate and pyridoxamine 5'-phosphate is about two orders of magnitude lower than that of aspartate aminotransferase. The error frequency of the enzyme in the case of the C4 substrate pair aspartate and oxalacetate is 3.4 x 10(-8), i.e. 4 times lower than that with the C5 substrate pair.

Regulatory properties of brain glutamate decarboxylase

1. Glutamate decarboxylase is a focal point for controlling gamma-aminobutyric acid (GABA) synthesis in brain. Several factors that appear to be important in the regulation of GABA synthesis have been identified by relating studies of purified glutamate decarboxylase to conditions in vivo. 2. The interaction of glutamate decarboxylase with its cofactor, pyridoxal 5'-phosphate, is a regulated process and appears to be one of the major means of controlling enzyme activity. The enzyme is present in brain predominantly as apoenzyme (inactive enzyme without bound cofactor). Studies with purified enzyme indicate that the relative amounts of apo- and holoenzyme are determined by the balance in a cycle that continuously interconverts the two. 3. The cycle that interconverts apo- and holoenzyme is part of the normal catalytic mechanism of the enzyme and is strongly affected by several probable regulatory compounds including pyridoxal 5'-phosphate, ATP, inorganic phosphate, and the amino acids glutamate, GABA, and aspartate. ATP and the amino acids promote apoenzyme formation and pyridoxal 5'-phosphate and inorganic phosphate promote holoenzyme formation. 4. Numerous studies indicate that brain contains multiple molecular forms of glutamate decarboxylase. Multiple forms that differ markedly in kinetic properties including their interactions with the cofactor have been isolated and characterized. The kinetic differences among the forms suggest that they play a significant role in the regulation of GABA synthesis.