Spectroscopic characterization of true enzyme-substrate intermediates of aspartate aminotransferase trapped at subzero temperatures

Study of DNA binding and bending by Bacillus subtilis GabR, a PLP-dependent transcription factor

BACKGROUND

GabR is a transcriptional regulator belonging to the MocR/GabR family, characterized by a N-terminal wHTH DNA-binding domain and a C-terminal effector binding and/or oligomerization domain, structurally homologous to aminotransferases (ATs). In the presence of γ-aminobutyrate (GABA) and pyridoxal 5'-phosphate (PLP), GabR activates the transcription of gabT and gabD genes involved in GABA metabolism.

METHODS

Here we report a biochemical and atomic force microscopy characterization of Bacillus subtilis GabR in complex with DNA. Complexes were assembled in vitro to study their stoichiometry, stability and conformation.

RESULTS

The fractional occupancy of the GabR cognate site suggests that GabR binds as a dimer with Kd of 10nM. Upon binding GabR bends the DNA by 80° as measured by anomalous electrophoretic mobility. With GABA we observed a decrease in affinity and conformational rearrangements compatible with a less compact nucleo-protein complex but no changes of the DNA bending angle. By employing promoter and GabR mutants we found that basic residues of the positively charged groove on the surface of the AT domain affect DNA affinity.

CONCLUSIONS

The present data extend current understanding of the GabR-DNA interaction and the effect of GABA and PLP. A model for the GabR-DNA complex, corroborated by a docking simulation, is proposed.

GENERAL SIGNIFICANCE

Characterization of the GabR DNA binding mode highlights the key role of DNA bending and interactions with bases outside the canonical direct repeats, and might be of general relevance for the action mechanism of MocR transcription factors.

Kinetic characterization of the human O-phosphoethanolamine phospho-lyase reveals unconventional features of this specialized pyridoxal phosphate-dependent lyase

Human O-phosphoethanolamine (PEA) phospho-lyase is a pyridoxal 5'-phosphate (PLP) dependent enzyme that catalyzes the degradation of PEA to acetaldehyde, phosphate and ammonia. Physiologically, the enzyme is involved in phospholipid metabolism and is expressed mainly in the brain, where its expression becomes dysregulated in the course of neuropsychiatric diseases. Mechanistically, PEA phospho-lyase shows a remarkable substrate selectivity, strongly discriminating against other amino compounds structurally similar to PEA. Herein, we studied the enzyme under steady-state and pre-steady-state conditions, analyzing its kinetic features and getting insights into the factors that contribute to its specificity. The pH dependence of the catalytic parameters and the pattern of inhibition by the product phosphate and by other anionic compounds suggest that the active site of PEA phospho-lyase is optimized to bind dianionic groups and that this is a prime determinant of the enzyme specificity towards PEA. Single- and multiple-wavelength stopped-flow studies show that upon reaction with PEA the main absorption band of PLP (λmax  = 412 nm) rapidly blue-shifts to ~ 400 nm. Further experiments suggest that the newly formed and rather stable 400-nm species most probably represents a Michaelis (noncovalent) complex of PEA with the enzyme. Accumulation of such an early intermediate during turnover is unusual for PLP-dependent enzymes and appears counterproductive for absolute catalytic performance, but it can contribute to optimize substrate specificity. PEA phospho-lyase may hence represent a case of selectivity-efficiency tradeoff. In turn, the strict specificity of the enzyme seems important to prevent inactivation by other amines, structurally resembling PEA, that occur in the brain.

The imine-pyridine torsion of the pyridoxal 5'-phosphate Schiff base of aspartate aminotransferase lowers its pKa in the unliganded enzyme and is crucial for the successive increase in the pKa during catalysis

In aspartate aminotransferase, pyridoxal 5'-phosphate (PLP) forms a Schiff base with the epsilon-amino group of Lys258 (internal aldimine). The internal aldimine has a pKa value of 6.8. Binding of a substrate amino acid to the enzyme yields the Michaelis complex, in which PLP still forms the internal aldimine with Lys258. This is followed by a transaldimination process to form a Schiff base of PLP with the alpha-amino group of substrates (external aldimine). Kinetic analysis of the spectral changes during the reaction of the enzyme with a substrate analogue 2-methylaspartate showed that the aldimine is 6.4-8.6% protonated in the Michaelis complex and 32-43% in the external aldimine. The bases that accept protons from the aldimines are considered to be the substrate alpha-amino group in the Michaelis complex and the epsilon-amino group of Lys258 in the external aldimine. Therefore, the intrinsic pKa value of the aldimine is expected to increase over a range of 3 during transformation from the unliganded enzyme (pKa = 6.8) to the Michaelis complex (pKa = 8.8) and the external aldimine (pKa > 10). When the Lys258 side chain of the internal aldimine was "cleaved" by the construction of an enzyme in which Lys258 was replaced by Ala and the aldimine was reconstituted with methylamine, the pKa of the internal aldimine was increased to 9.6. This indicates that the low pKa value of the internal aldimine of the unliganded enzyme is provided by the side chain of Lys258 which destabilizes the planar conformation of the aldimine suitable for protonation. This strained conformation is partially relaxed in the Michaelis complex, and the pKa is moderately increased. On formation of the external aldimine, Lys258 is released and the aldimine is fixed to a near planar conformation and has a high pKa value. Thus, the aldimine pKa is modulated by a mechanism that exploits the conformational differences between the intermediate structures. The strain of the protonated internal aldimine is interpreted to enhance the catalytic ability of the enzyme by increasing the energy level of the free enzyme plus substrate at neutral pH relative to the transition state.

Transient-state kinetic analysis of Synechococcus glutamate 1-semialdehyde aminotransferase

We report a transient-state kinetic analysis relating to the mechanism of glutamate 1-semialdehyde aminotransferase (GSAT). Multiple-wavelength spectral kinetic data were collected by micro-stopped-flow spectrophotometry. Time resolved spectral sketches resulting from reactions with glutamate 1-semialdehyde (GSA), 4,5-diaminovalerate (DAVA), and 5-aminolevulinate (ALA) indicated various transient chromophoric intermediates. On the basis of the generally accepted mechanism of other aminotransferases and absorbance characteristics of associated intermediates, these transient chromophores are likely associated with Schiff base formation, ketimine/aldimine tautomerization, and transimidation etc. Spectral kinetic changes associated with these putative intermediates were, in general, concentration dependent. Various experimental evidence, including reactions with the GSAT lys272ile mutant, suggested rapid equilibrium of isomeric aldimines and geminal diamines. With this and related simplifying assumptions, a minimal mechanism was derived which provided a means for transient-state spectral kinetic analysis of reactions with GSA, DAVA, and ALA, all of which lead to the formation of the same putative central enzyme complex. Resulting kinetic constants were internally consistent, in general agreement with steady-state and equilibrium data (KM, kcat, and Keq), and provided the basis for a reasonable computer simulation of the original data set (variance approximately 4 x 10(-5)). Reequilibration of enzyme intermediates following an apparent pseudoequilibrium indicated thermodynamically driven dissociation of the central aldiminic enzyme complex. This is consistent with previous observations and the minimal mechanism used in this kinetic analysis and suggests a plausible regulatory mechanism of GSAT.

Structural basis for the catalytic activity of aspartate aminotransferase K258H lacking the pyridoxal 5'-phosphate-binding lysine residue

Chicken mitochondrial and Escherichia coli aspartate aminotransferases K258H, in which the active site lysine residue has been exchanged for a histidine residue, retain partial catalytic competence [Ziak et al. (1993) Eur. J. Biochem. 211, 475-484]. Mutant PLP and PMP holoenzymes and the complexes of the latter (E. coli enzyme) with sulfate and 2-oxoglutarate, as well as complexes of the mitochondrial apoenzyme with N-(5'-phosphopyridoxyl)-L-aspartate or N-(5'-phosphopyridoxyl)-L-glutamate, were crystallized and analyzed by means of X-ray crystallography in order to examine how the side chain of histidine 258 can substitute as a general acid/base catalyst of the aldimine-ketimine tautomerization in enzymic transamination. The structures have been solved and refined at resolutions between 2.1 and 2.8 A. Both the closed and the open conformations, identical to those of the wild-type enzyme, were observed, indicating that the mutant enzymes of both species exhibit the same conformational flexibility as the wild-type enzymes, although in AspAT K258H the equilibrium is somewhat shifted toward the open conformation. The replacement of the active site K258 by a histidine residue resulted only in local structural adaptations necessary to accommodate the imidazole ring. The catalytic competence of the mutant enzyme, which in the forward half-reaction is 0.1% of that of the wild-type enzyme, suggests that the imidazole group is involved in the aldimine-ketimine tautomerization. However, the imidazole ring of H258 is too far away from C alpha and C4' of the coenzyme-substrate adduct for direct proton transfer, suggesting that the 1,3-prototropic shift is mediated by a water molecule. Although there is enough space for a water molecule in this area, it has not been detected. Dynamic fluctuations of the protein matrix might transiently open a channel, giving a water molecule fleeting access to the active site.

Crystal structures of true enzymatic reaction intermediates: aspartate and glutamate ketimines in aspartate aminotransferase

The crystal structures of the stable, closed complexes of chicken mitochondrial aspartate aminotransferase with the natural substrates L-aspartate and L-glutamate have been solved and refined at 2.4- and 2.3-A resolution, respectively. In both cases, clear electron density at the substrate-coenzyme binding site unequivocally indicates the presence of a covalent intermediate. The crystallographically identical environments of the two subunits of the alpha 2 dimer allow a simple, direct correlation of the coenzyme absorption spectra of the crystalline enzyme with the diffraction results. Deconvolution of the spectra of the crystalline complexes using lognormal curves indicates that the ketimine intermediates constitute 76% and 83% of the total enzyme populations with L-aspartate and L-glutamate, respectively. The electron density maps accommodate the ketimine structures best in agreement with the independent spectral data. Crystalline enzyme has a much higher affinity for keto acid substrates compared to enzyme in solution. The increased affinity is interpreted in terms of a perturbation of the open/closed conformational equilibrium by the crystal lattice, with the closed form having greater affinity for substrate. The crystal lattice contacts provide energy required for domain closure normally supplied by the excess binding energy of the substrate. In solution, enzyme saturated with amino/keto acid substrate pairs has a greater total fraction of intermediates in the aldehyde oxidation state compared to crystalline enzyme. Assuming the only difference between the solution and crystalline enzymes is in conformational freedom, this difference suggests that one or more substantially populated, aldehydic intermediates in solution exist in the open conformation. Quantitative analyses of the spectra indicate that the value of the equilibrium constant for the open-closed conformational transition of the liganded, aldehydic enzyme in solution is near 1. The C4' pro-S proton in the ketimine models is oriented nearly perpendicularly to the plane of the pyridine ring, suggesting that the enzyme facilitates its removal by maximizing sigma-pi orbital overlap. The absence of a localized water molecule near Lys258 dictates that ketimine hydrolysis occurs via a transiently bound water molecule or from an alternative, possibly more open, structure in which water is appropriately bound. A prominent mechanistic role for flexibility of the Lys258 side chain is suggested by the absence of hydrogen bonds to the amino group in the aspartate structure and the relatively high temperature factors for these atoms in both structures.

Mutant aspartate aminotransferase (K258H) without pyridoxal-5'-phosphate-binding lysine residue. Structural and catalytic properties

If the pyridoxal-phosphate-binding lysine residue 258 of aspartate aminotransferase is exchanged for a histidine residue, the enzyme retains partial catalytic competence [Ziak, M., Jaussi, R., Gehring, H. and Christen, P. (1990) Eur. J. Biochem. 187, 329-333]. The three-dimensional structures of the mutant enzymes of both chicken mitochondria and Escherichia coli were determined at high resolution. The folding patterns of the polypeptide chains proved to be identical to those of the wild-type enzymes, small conformational differences being restricted to parts of the active site. If aspartate or glutamate was added to the pyridoxal form of the mutant enzyme [lambda max 392 nm and 330 nm (weak); negative CD at 420 nm, positive CD at 370 nm and 330 nm], the external aldimine (lambda max = 430 nm; negative CD at 360 nm and 430 nm) transiently accumulated. Upon addition of 2-oxoglutarate to the pyridoxamine form (lambda max 330 nm, positive CD), a putative ketamine intermediate could be detected; however, with oxalacetate, an equilibrium between external aldimine and the pyridoxal form, which was strongly in favour of the former, was established within seconds. The transamination cycle with glutamate and oxalacetate proceeds only three orders of magnitude more slowly than the overall reaction of the wild-type enzyme. The specific activity of the mutant enzyme is 0.1 U/mg at 25 degrees C and constant from pH 6.0 to 8.5. Reconstitution of the mutant apoenzyme with [4'-3H]pyridoxamine 5'-phosphate resulted in rapid release of 3H with a first-order rate constant kappa' = 5 x 10(-4) s-1 similar to that of the wild-type enzyme. Apparently, in aspartate aminotransferase, histidine can to some extent substitute for the active-site lysine residue. The imidazole ring of H258, however, seems too distant from C alpha and C4' to act efficiently as proton donor/acceptor in the aldimine-ketamine tautomerization, suggesting that the prototropic shift might be mediated by an intervening water molecule. Transmination of the internal to the external aldimine apparently can be replaced by de novo formation of the latter, and by its hydrolysis in the reverse direction.