Pyridoxal Phosphate Binding Site Research Articles

Identification of the pyridoxal 5'-phosphate allosteric site in human pyridox(am)ine 5'-phosphate oxidase.

Adequate levels of pyridoxal 5'-phosphate (PLP), the catalytically active form of vitamin B6 , and its proper distribution in the body are essential for human health. The PLP recycling pathway plays a crucial role in these processes and its defects cause severe neurological diseases. The enzyme pyridox(am)ine 5'-phosphate oxidase (PNPO), whose catalytic action yields PLP, is one of the key players in this pathway. Mutations in the gene encoding PNPO are responsible for a severe form of neonatal epilepsy. Recently, PNPO has also been described as a potential target for chemotherapeutic agents. Our laboratory has highlighted the crucial role of PNPO in the regulation of PLP levels in the cell, which occurs via a feedback inhibition mechanism of the enzyme, exerted by binding of PLP at an allosteric site. Through docking analyses and site-directed mutagenesis experiments, here we identified the allosteric PLP binding site of human PNPO. This site is located in the same protein region as the allosteric site we previously identified in the Escherichia coli enzyme homologue. However, the identity and arrangement of the amino acid residues involved in PLP binding are completely different and resemble those of the active site of PLP-dependent enzymes. The identification of the PLP allosteric site of human PNPO paves the way for the rational design of enzyme inhibitors as potential anti-cancer compounds.

Protein-protein interfaces as druggable targets: a common motif of the pyridoxal-5′-phosphate-dependent enzymes to receive the coenzyme from its producer

Pyridoxal-5′-phosphate (PLP), a phosphorylated form of vitamin B6, acts as a coenzyme for numerous reactions, including those changed in cancer and/or associated with the disease prognosis. Since highly reactive PLP may modify cellular proteins, it is hypothesized to be directly transferred from its donors to acceptors. Our goal is to validate the hypothesis by finding common motif(s) in a multitude of the PLP-dependent enzymes for binding the limited number of the PLP donors, namely pyridoxal kinase (PdxK), pyridox(am)in-5′-phosphate oxidase (PNPO) and the PLP-binding protein (PLPBP). Experimentally confirmed interactions between the PLP donors and acceptors reveal that PdxK and PNPO interact with the PLP acceptors of folds I and II, while PLPBP - with those of folds III and V. Aligning the sequences and 3D structures of the identified interactors of PdxK and PNPO, we have found a common motif in the PLP-dependent enzymes of folds I and II. The motif extends from the enzyme surface to the neighborhood of the PLP binding site, represented by an exposed alfa-helix, a partially buried beta-strand and residual loops. Pathogenicity of mutations in human PLP-dependent enzymes within or in the vicinity of the motif, but outside of the active sites, supports functional significance of the motif that may provide an interface for the direct transfer of PLP from the sites of its synthesis to those of the coenzyme binding. The enzyme-specific amino acid residues of the common motif may be used to develop selective inhibitors blocking PLP delivery to the PLP-dependent enzymes critical for proliferation of malignant cells.

Protein–Protein Interfaces as Druggable Targets: A Common Motif of the Pyridoxal-5′-Phosphate-Dependent Enzymes to Receive the Coenzyme from Its Producers

Pyridoxal-5′-phosphate (PLP), a phosphorylated form of vitamin B6, acts as a coenzyme for numerous reactions, including those changed in cancer and/or associated with the disease prognosis. Since highly reactive PLP can modify cellular proteins, it is hypothesized to be directly transferred from its donors to acceptors. Our goal is to validate the hypothesis by finding common motif(s) in the multitude of PLP-dependent enzymes for binding the limited number of PLP donors, namely pyridoxal kinase (PdxK), pyridox(am)in-5′-phosphate oxidase (PNPO), and PLP-binding protein (PLPBP). Experimentally confirmed interactions between the PLP donors and acceptors reveal that PdxK and PNPO interact with the most abundant PLP acceptors belonging to structural folds I and II, while PLPBP – with those belonging to folds III and V. Aligning sequences and 3D structures of the identified interactors of PdxK and PNPO, we have identified a common motif in the PLP-dependent enzymes of folds I and II. The motif extends from the enzyme surface to the neighborhood of the PLP binding site, represented by an exposed alfa-helix, a partially buried beta-strand, and residual loops. Pathogenicity of mutations in the human PLP-dependent enzymes within or in the vicinity of the motif, but outside of the active sites, supports functional significance of the motif that may provide an interface for the direct transfer of PLP from the sites of its synthesis to those of coenzyme binding. The enzyme-specific amino acid residues of the common motif may be useful to develop selective inhibitors blocking PLP delivery to the PLP-dependent enzymes critical for proliferation of malignant cells.

Biochemical and Bioinformatic Studies of Mutations of Residues at the Monomer-Monomer Interface of Human Ornithine Aminotransferase Leading to Gyrate Atrophy of Choroid and Retina.

Deficit of human ornithine aminotransferase (hOAT), a mitochondrial tetrameric pyridoxal-5'-phosphate (PLP) enzyme, leads to gyrate atrophy of the choroid and retina (GA). Although 70 pathogenic mutations have been identified, only few enzymatic phenotypes are known. Here, we report biochemical and bioinformatic analyses of the G51D, G121D, R154L, Y158S, T181M, and P199Q pathogenic variants involving residues located at the monomer-monomer interface. All mutations cause a shift toward a dimeric structure, and changes in tertiary structure, thermal stability, and PLP microenvironment. The impact on these features is less pronounced for the mutations of Gly51 and Gly121 mapping to the N-terminal segment of the enzyme than those of Arg154, Tyr158, Thr181, and Pro199 belonging to the large domain. These data, together with the predicted ΔΔG values of monomer-monomer binding for the variants, suggest that the proper monomer-monomer interactions seem to be correlated with the thermal stability, the PLP binding site and the tetrameric structure of hOAT. The different impact of these mutations on the catalytic activity was also reported and discussed on the basis of the computational information. Together, these results allow the identification of the molecular defects of these variants, thus extending the knowledge of enzymatic phenotypes of GA patients.

Inhibiting Pyridoxal Kinase of Entamoeba histolytica Is Lethal for This Pathogen.

Pyridoxal 5’-phosphate (PLP) functions as a cofactor for hundreds of different enzymes that are crucial to the survival of microorganisms. PLP-dependent enzymes have been extensively characterized and proposed as drug targets in Entamoeba histolytica. This pathogen is unable to synthesize vitamin B6 via de-novo pathway and relies on the uptake of vitamin B6 vitamers from the host which are then phosphorylated by the enzyme pyridoxal kinase to produce PLP, the active form of vitamin B6. Previous studies from our lab shows that EhPLK is essential for the survival and growth of this protozoan parasite and its active site differs significantly with respect to its human homologue making it a potential drug target. In-silico screening of EhPLK against small molecule libraries were performed and top five ranked molecules were shortlisted on the basis of docking scores. These compounds dock into the PLP binding site of the enzyme such that binding of these compounds hinders the binding of substrate. Of these five compounds, two compounds showed inhibitory activity with IC50 values between 100-250 μM when tested in-vitro. The effect of these compounds proved to be extremely lethal for Entamoeba trophozoites in cultured cells as the growth was hampered by 91.5% and 89.5% when grown in the presence of these compounds over the period of 72 hours.

Characterization and functional insights into the Entamoeba histolytica pyridoxal kinase, an enzyme essential for its survival

Pyridoxal 5′-phosphate (PLP) is the active form of vitamin B6 and a cofactor for more than 140 enzymes. This coenzyme plays a pivotal role in catalysis of various enzymatic reactions that are critical for the survival of organisms. Entamoeba histolytica depends on the uptake of pyridoxal (PL), a B6 vitamer from the external environment which is then phosphorylated by pyridoxal kinase (EhPLK) to form PLP via the salvage pathway. E. histolytica cannot synthesise vitamin B6de-novo, and also lacks pyridoxine 5′-phosphate oxidase, a salvage pathway enzyme required to produce PLP from pyridoxine phosphate (PNP) and pyridoxamine phosphate (PMP). Analysing the importance of PLK in E. histolytica, we have determined the high-resolution crystal structures of the dimeric pyridoxal kinase in apo, ADP-bound, and PLP-bound states. These structures provided a snapshot of the transition state and help in understanding the reaction mechanism in greater detail. The EhPLK structure significantly differed from the human homologue at its PLP binding site, and the phylogenetic study also revealed its divergence from human PLK. Further, gene regulation of EhPLK using sense and antisense RNA showed that any change in optimal level is harmful to the pathogen. Biochemical and in vivo studies unveiled EhPLK to be essential for this pathogen, while the molecular differences with human PLK structure can be exploited for the structure-guided design of EhPLK inhibitors.

Lysine Decarboxylase with an Enhanced Affinity for Pyridoxal 5-Phosphate by Disulfide Bond-Mediated Spatial Reconstitution.

Lysine decarboxylase (LDC) catalyzes the decarboxylation of l-lysine to produce cadaverine, an important industrial platform chemical for bio-based polyamides. However, due to high flexibility at the pyridoxal 5-phosphate (PLP) binding site, use of the enzyme for cadaverine production requires continuous supplement of large amounts of PLP. In order to develop an LDC enzyme from Selenomonas ruminantium (SrLDC) with an enhanced affinity for PLP, we introduced an internal disulfide bond between Ala225 and Thr302 residues with a desire to retain the PLP binding site in a closed conformation. The SrLDCA225C/T302C mutant showed a yellow color and the characteristic UV/Vis absorption peaks for enzymes with bound PLP, and exhibited three-fold enhanced PLP affinity compared with the wild-type SrLDC. The mutant also exhibited a dramatically enhanced LDC activity and cadaverine conversion particularly under no or low PLP concentrations. Moreover, introduction of the disulfide bond rendered SrLDC more resistant to high pH and temperature. The formation of the introduced disulfide bond and the maintenance of the PLP binding site in the closed conformation were confirmed by determination of the crystal structure of the mutant. This study shows that disulfide bond-mediated spatial reconstitution can be a platform technology for development of enzymes with enhanced PLP affinity.

Cystathionine β-Lyase-Like Protein with Pyridoxal Binding Domain Characterized in Leishmania major by Comparative Sequence Analysis and Homology Modelling

Cystathionine β-lyase-like protein (CBLP), one of the key enzymes involved in methionine biosynthesis utilising pyridoxal phosphate (PLP) as a cofactor, has recently been reported in Leishmania major. Its presence in the parasite and absence in humans warrant its full characterisation and fruition as a potent, selective, and inevitable druggable target. Due to the unavailability of X-ray 3D structure of CBLP, a homology model for this protein was developed for the first time. The model was evaluated for PLP binding site and various conserve domain residues of the protein recommended by comparative sequence analyses by different protein analysis tools. The model was validated and discovered to be robust and statistically significant. The final model was superimposed on template of Arabidopsis thaliana (PDB ID: 1IBJ) and RMSD was found to be 0.486. The PLP binding site residues of both the proteins were ensued to be highly conserved indicated by Gly71, Met72, Tyr95, Asp169, and Ser193 as well as formation of aldimine bond with Lys194. This was further verified through molecular simulation of PLP into the cofactor binding site of the modelled protein. The present study may therefore play a directing role in the designing of novel, potential, and selective antileishmanial agents.

Crystal Structures of Lysine-Preferred Racemases, the Non-Antibiotic Selectable Markers for Transgenic Plants

Lysine racemase, a pyridoxal 5′-phosphate (PLP)-dependent amino acid racemase that catalyzes the interconversion of lysine enantiomers, is valuable to serve as a novel non-antibiotic selectable marker in the generation of transgenic plants. Here, we have determined the first crystal structure of a lysine racemase (Lyr) from Proteus mirabilis BCRC10725, which shows the highest activity toward lysine and weaker activity towards arginine. In addition, we establish the first broad-specificity amino acid racemase (Bar) structure from Pseudomonas putida DSM84, which presents not only the highest activity toward lysine but also remarkably broad substrate specificity. A complex structure of Bar-lysine is also established here. These structures demonstrate the similar fold of alanine racemase, which is a head-to-tail homodimer with each protomer containing an N-terminal (α/β)8 barrel and a C-terminal β-stranded domain. The active-site residues are located at the protomer interface that is a funnel-like cavity with two catalytic bases, one from each protomer, and the PLP binding site is at the bottom of this cavity. Structural comparisons, site-directed mutagenesis, kinetic, and modeling studies identify a conserved arginine and an adjacent conserved asparagine that fix the orientation of the PLP O3 atom in both structures and assist in the enzyme activity. Furthermore, side chains of two residues in α-helix 10 have been discovered to point toward the cavity and define the substrate specificity. Our results provide a structural foundation for the design of racemases with pre-determined substrate specificity and for the development of the non-antibiotic selection system in transgenic plants.

Structure of Soybean β-Cyanoalanine Synthase and the Molecular Basis for Cyanide Detoxification in Plants

Plants produce cyanide (CN-) during ethylene biosynthesis in the mitochondria and require β-cyanoalanine synthase (CAS) for CN- detoxification. Recent studies show that CAS is a member of the β-substituted alanine synthase (BSAS) family, which also includes the Cys biosynthesis enzyme O-acetylserine sulfhydrylase (OASS), but how the BSAS evolved distinct metabolic functions is not understood. Here we show that soybean (Glycine max) CAS and OASS form α-aminoacrylate reaction intermediates from Cys and O-acetylserine, respectively. To understand the molecular evolution of CAS and OASS in the BSAS enzyme family, the crystal structures of Gm-CAS and the Gm-CAS K95A mutant with a linked pyridoxal phosphate (PLP)-Cys molecule in the active site were determined. These structures establish a common fold for the plant BSAS family and reveal a substrate-induced conformational change that encloses the active site for catalysis. Comparison of CAS and OASS identified residues that covary in the PLP binding site. The Gm-OASS T81M, S181M, and T185S mutants altered the ratio of OASS:CAS activity but did not convert substrate preference to that of a CAS. Generation of a triple mutant Gm-OASS successfully switched reaction chemistry to that of a CAS. This study provides new molecular insight into the evolution of diverse enzyme functions across the BSAS family in plants.

Sideroblastic anemia, iron overload, andALAS2R452S in African-American males: Phenotype and genotype features of five unrelated patients

Sideroblastic anemia, iron overload, and<i>ALAS2</i>R452S in African-American males: Phenotype and genotype features of five unrelated patients

Human liver peroxisomal alanine:glyoxylate aminotransferase: Different stability under chemical stress of the major allele, the minor allele, and its pathogenic G170R variant

The sensitivity to denaturant stress of the major (AGT-Ma) and the minor (AGT-Mi) allele of alanine:glyoxylate aminotransferase and P11L mutant has been examined by studying their urea-induced equilibrium unfolding processes with various spectroscopic and analytical techniques. AGT-Ma loses pyridoxal 5′-phosphate (PLP) and unfolds completely without exposing significant hydrophobic clusters through a two-state model ( C m ∼ 6.9 M urea). Instead, the unfolding of AGT-Mi and P11L variant proceeds in two steps. The first transition ( C m ∼ 4.6 M urea) involves PLP release, dimer dissociation and exposure of hydrophobic patches leading to a self-associated intermediate which is converted to an unfolded monomer in the second step. The unfolding pathways of apoAGT-Mi and apoP11L are similar to each other, but different from that of apoAGT-Ma. Notably, the monomerization step in apoAGT-Mi and apoP11L occurs with a C m value (∼1.6 M urea) lower than in apoAGT-Ma (∼2.4 M urea). These data indicate that Pro11 is relevant for the stability of both the dimeric structure and the PLP binding site of AGT. Moreover, to understand the pathogenic consequences of G170R mutation on AGT-Mi at the protein level, G170R-Mi has been characterized. HoloG170R-Mi exhibits spectroscopic and catalytic features and urea unfolding profiles comparable to those of AGT-Mi, while the apo form monomerizes with a C m of ∼1.1 M urea. These biochemical results are discussed in the light of the characteristics of the enzymatic phenotype of PH1 patients bearing G170R mutation in AGT-Mi and the positive response of these patients to pyridoxine treatment.

Decrypting the Biochemical Function of an Essential Gene from Streptococcus pneumoniae Using ThermoFluor® Technology

The protein product of an essential gene of unknown function from Streptococcus pneumoniae was expressed and purified for screening in the ThermoFluor affinity screening assay. This assay can detect ligand binding to proteins of unknown function. The recombinant protein was found to be in a dimeric, native-like folded state and to unfold cooperatively. ThermoFluor was used to screen the protein against a library of 3000 compounds that were specifically selected to provide information about possible biological functions. The results of this screen identified pyridoxal phosphate and pyridoxamine phosphate as equilibrium binding ligands (K(d) approximately 50 pM, K(d) approximately 2.5 microM, respectively), consistent with an enzymatic cofactor function. Several nucleotides and nucleotide sugars were also identified as ligands of this protein. Sequence comparison with two enzymes of known structure but relatively low overall sequence homology established that several key residues directly involved in pyridoxal phosphate binding were strictly conserved. Screening a collection of generic drugs and natural products identified the antifungal compound canescin A as an irreversible covalent modifier of the enzyme. Our investigation of this protein indicates that its probable biological role is that of a nucleoside diphospho-keto-sugar aminotransferase, although the preferred keto-sugar substrate remains unknown. These experiments demonstrate the utility of a generic affinity-based ligand binding technology in decrypting possible biological functions of a protein, an approach that is both independent of and complementary to existing genomic and proteomic technologies.

Characterization of the human gene encoding α-aminoadipate aminotransferase (AADAT)

In mammals, the conversion of α-aminoadipate to α-ketoadipate by α-aminoadipate aminotransferase (AADAT) is an intermediate step in lysine degradation. A gene encoding for α-aminoadipate aminotransferase and kynurenine aminotransferase activites had been previously identified in the rat ( KAT/AadAT). We identified the human gene ( AADAT) encoding for AADAT. It has a 2329 bp cDNA, a 1278 bp open-reading frame, and is predicted to encode 425 amino acids with a mitochondrial cleavage signal and a pyridoxal-phosphate binding site. AADAT is 73% and 72% identical to the mouse and rat orthologs, respectively. The genomic structure spans 30 kb and consists of 13 exons. FISH studies localized the gene to 4q32.2. Two transcripts (∼2.9 and ∼4.7 kb ) were identified, with expression highest in liver. Bacterial expression studies confirm that the gene encodes for AADAT activity. The availability of the DNA sequence and enzyme assay will allow further evaluation of individuals suspected to have defects in this enzyme.

Identification of the Enterococcus faecalis tyrosine decarboxylase operon involved in tyramine production.

Screening of a library of Enterococcus faecalis insertional mutants allowed isolation of a mutant affected in tyramine production. The growth of this mutant was similar to that of the wild-type E. faecalis JH2-2 strain in Maijala broth, whereas high-performance liquid chromatography analyses showed that tyramine production, which reached 1,000 microg ml(-1) for the wild-type strain, was completely abolished. Genetic analysis of the insertion locus revealed a gene encoding a decarboxylase with similarity to eukaryotic tyrosine decarboxylases. Sequence analysis revealed a pyridoxal phosphate binding site, indicating that this enzyme belongs to the family of amino acid decarboxylases using this cofactor. Reverse transcription-PCR analyses demonstrated that the gene (tdc) encoding the putative tyrosine decarboxylase of E. faecalis JH2-2 is cotranscribed with the downstream gene encoding a putative tyrosine-tyramine antiporter and with the upstream tyrosyl-tRNA synthetase gene. This study is the first description of a tyrosine decarboxylase gene in prokaryotes.

Cloning two genes for nicotianamine aminotransferase, a critical enzyme in iron acquisition (Strategy II) in graminaceous plants.

Nicotianamine aminotransferase (NAAT), the key enzyme involved in the biosynthesis of mugineic acid family phytosiderophores (MAs), catalyzes the amino transfer of nicotianamine (NA). MAs are found only in graminaceous plants, although NA has been detected in every plant so far investigated. Therefore, this amino transfer reaction is the first step in the unique biosynthesis of MAs that has evolved in graminaceous plants. NAAT activity is dramatically induced by Fe deficiency and suppressed by Fe resupply. Based on the protein sequence of NAAT purified from Fe-deficient barley (Hordeum vulgare) roots, two distinct cDNA clones encoding NAAT, naat-A and naat-B, were identified. Their deduced amino acid sequences were homologous to several aminotransferases, and shared consensus sequences for the pyridoxal phosphate-binding site lysine residue and its surrounding residues. The expression of both naat-A and naat-B is increased in Fe-deficient barley roots, while naat-B has a low level of constitutive expression in Fe-sufficient barley roots. No detectable mRNA from either naat-A or naat-B was present in the leaves of either Fe-deficient or Fe-sufficient barley. One genomic clone with a tandem array of naat-B and naat-A in this order was identified. naat-B and naat-A each have six introns at the same locations. The isolation of NAAT genes will pave the way to understanding the mechanism of the response to Fe in graminaceous plants, and may lead to the development of cultivars tolerant to Fe deficiency that can grow in calcareous soils.

Tryptophan synthase mutations that alter cofactor chemistry lead to mechanism-based inactivation.

Mutations in the pyridoxal phosphate binding site of the tryptophan synthase beta subunit (S377D and S377E) alter cofactor chemistry [Jhee, K.-H., et al. (1998) J. Biol. Chem. 273, 11417-11422]. We now report that the S377D, S377E, and S377A beta2 subunits form alpha2 beta2 complexes with the alpha subunit and activate the alpha subunit-catalyzed cleavage of indole 3-glycerol phosphate. The apparent Kd for dissociation of the alpha and beta subunits is unaffected by the S377A mutation but is increased up to 500-fold by the S377D and S377E mutations. Although the three mutant alpha2 beta2 complexes exhibit very low activities in beta elimination and beta replacement reactions catalyzed at the beta site in the presence of Na+, the activities and spectroscopic properties of the S377A alpha2 beta2 complex are partially repaired by addition of Cs+. The S377D and S377E alpha2 beta2 complexes, unlike the wild-type and S377A alpha2 beta2 complexes and the mutant beta2 subunits, undergo irreversible substrate-induced inactivation by L-serine or by beta-chloro-L-alanine. The rates of inactivation (kinact) are similar to the rates of catalysis (kcat). The partition ratios are very low (kcat/kinact = 0.25-3) and are affected by alpha subunit ligands and monovalent cations. The inactivation product released by alkali was shown by HPLC and by fluorescence, absorption, and mass spectroscopy to be identical to a compound previously synthesized from pyridoxal phosphate and pyruvate. We suggest that alterations in the cofactor chemistry that result from the engineered Asp377 in the active site of the beta subunit may promote the mechanism-based inactivation.

Mutation of cysteine 111 in Dopa decarboxylase leads to active site perturbation.

Cysteine 111 in Dopa decarboxylase (DDC) has been replaced by alanine or serine by site-directed mutagenesis. Compared to the wild-type enzyme, the resultant C111A and C111S mutant enzymes exhibit Kcat values of about 50% and 15%, respectively, at pH 6.8, while the K(m) values remain relatively unaltered for L-3,4-dihydroxyphenylalanine (L-Dopa) and L-5-hydroxytryptophan (L-5-HTP). While a significant decrease of the 280 nm optically active band present in the wild type is observed in mutant DDCs, their visible co-enzyme absorption and CD spectra are similar to those of the wild type. With respect to the wild type, the Cys-111-->Ala mutant displays a reduced affinity for pyridoxal 5'-phosphate (PLP), slower kinetics of reconstitution to holoenzyme, a decreased ability to anchor the external aldimine formed between D-Dopa and the bound co-enzyme, and a decreased efficiency of energy transfer between tryptophan residue(s) and reduced PLP. Values of pKa and pKb for the groups involved in catalysis were determined for the wild-type and the C111A mutant enzymes. The mutant showed a decrease in both pK values by about 1 pH unit, resulting in a shift of the pH of the maximum velocity from 7.2 (wild-type) to 6.2 (mutant). This change in maximum velocity is mirrored by a similar shift in the spectrophotometrically determined pK value of the 420-->390 nm transition of the external aldimine. These results demonstrate that the sulfhydryl group of Cys-111 is catalytically nonessential and provide strong support for previous suggestion that this residue is located at or near the PLP binding site (Dominici P, Maras B, Mei G, Borri Voltattorni C. 1991. Eur J Biochem 201:393-397). Moreover, our findings provide evidence that Cys-111 has a structural role in PLP binding and suggest that this residue is required for maintenance of proper active-site conformation.

Substitutions of alanine for cysteine at a reactive thiol site and for lysine at a pyridoxal phosphate binding site of 1-aminocyclopropane-1-carboxylate deaminase.

1-Aminocyclopropane-1-carboxylate (ACC) deaminase catalyzes the cyclopropane ring fragmentation and deamination of ACC. Replacement of cysteine with alanine at a reactive thiol site, Cys-162, of ACC deaminase did not affect the enzyme activity, in spite of the previous result that modification of Cys-162 caused complete loss of the enzyme activity. Substitution of glycine or valine for the cysteine residue gave a higher Km for ACC without a significant change of the K0, indicating that changes of the amino acid side chain had structural effects on substrate binding. Replacement of lysine with alanine at the pyridoxal phosphate (PLP) binding site of the ACC deaminase caused a lower content of PLP and loss of detectable activity of ACC deamination. This mutant enzyme, K51A, showed absorption peaks at 330 nm and 405 nm. The peak at 405 nm was shifted to about 425 nm by the addition of ACC, D-, L-alanine, and D-, L-serine. The formation of aldimine complexes indicated by the spectral shift was reversible. It is suggested that lysine 51 affects the formation of holoenzyme and is important in catalysis.

Cloning and analysis of the gene for cystathionine gamma-synthase from Arabidopsis thaliana.

A cDNA clone, CGS1, encoding cystathionine gamma-synthase (CGS) from Arabidopsis thaliana was selected by complementation of CGS mutant strain of Escherichia coli (metB). Cells expressing CGS1 can grow on medium lacking Met and contain CGS enzyme activity. Genomic DNA blot analysis of A. thaliana revealed that there is a single gene homologous with CGS1. A genomic fragment carrying CGS1 was cloned and sequenced. Through combined analysis of the cDNA and genomic clone it was determined that the CGS1 coding sequence is 1692 bp, encodes a 563 amino acid, 60 kDa protein, and is interrupted by ten introns. A transcriptional initiation site was detected 260 bp 5' of the initiator codon. The predicted amino acid sequence of CGS1 contains a consensus pyridoxal phosphate-binding site and is similar to MetB of E. coli, with which it is 35 percent identical. The CGS1 product has a sequence at the amino terminus that resembles a transit peptide for localization to plastids. At least 160 amino acids from the amino terminus of the CGS1 enzyme are not essential for enzymatic activity.