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Insights into the mechanism of oxidative deamination catalyzed by Dopa decarboxylase
No full textThe unusual oxygen-consuming oxidative deamination reaction catalyzed by the pyridoxal
5′-phosphate (PLP) enzyme DOPA decarboxylase (DDC) was here investigated. Either wild-type or Y332F
DDC variant is able to perform such oxidation toward aromatic amines or aromatic L-amino acids,
respectively, without the aid of any cofactor related to oxygen chemistry. Oxidative deamination produces,
in equivalent amounts, a carbonyl compound and ammonia, accompanied by dioxygen consumption in a
1:2 molar ratio with respect to the products. Kinetic studies either in the pre-steady or in the steady state,
together with HPLC analyses of reaction mixtures under varying experimental conditions, revealed that
a ketimine accumulates during the linear phase of product formation. This species is reactive since it is
converted back to PLP when the substrate is consumed. Rapid-mixing chemical quench studies provide
evidence that the ketimine is indeed an intermediate formed during the first catalytic cycle. Moreover,
superoxide anion and hydrogen peroxide are both generated during the catalytic cycles. On this basis, a
mechanism of oxidative deamination consistent with the present data is proposed. Furthermore, the catalytic
properties of the T246A DDC mutant together with those previously obtained with H192Q mutant allow
us to propose that the Thr246-His192 dyad could act as a general base in promoting the first step of the
oxidative deamination of aromatic amines
Tat-mediated delivery of human alanine:glyoxylate aminotransferase in a cellular model of Primary Hyperoxaluria Type I
No full textDefects in liver peroxisomal alanine:glyoxylate aminotransferase (AGT), as a consequence of inherited mutations on the AGXT gene, lead to primary hyperoxaluria type I (PH1), a rare metabolic disorder characterized by the formation of calcium oxalate stones at first in the urinary tract and then in the whole body. The curative treatments currently available for PH1 are pyridoxine therapy, effective in only 10–30 % of the patients, and liver transplantation, an invasive procedure with potentially serious complications. A valid therapeutic option for PH1 patients would be the development of an enzyme administration therapy. However, the exogenous administration of the missing AGT would require the crossing of the plasma membrane to deliver the protein to liver peroxisomes. In this study, we constructed, purified and characterized the fusion protein of AGT with the membrane-penetrating Tat peptide (Tat-AGT). Although Tat-AGT shows subtle active site conformational changes as compared with untagged AGT, it retains a significant transaminase activity. Western-blot analyses, enzymatic assays and immunofluorescence studies show that active Tat-AGT can be successfully delivered to a mammalian cellular model of PH1 consisting of chinese hamster ovary cells expressing glycolate oxidase (CHO-GO), whereas untagged AGT cannot. Moreover, the intracellular transduced Tat-AGT makes CHO-GO cells able to detoxify endogenously produced glyoxylate to an extent similar to that of CHO-GO cells stably expressing AGT. Altogether, these results show that the Tat peptide is capable of delivering a functional AGT to mammalian cells, thus paving the way for the possibility to use Tat-AGT as an enzyme replacement therapy to counteract PH1
Reactions of human liver peroxisomal alanine:glyoxylate aminotransferase with beta-chloro-L-alanine and L-cysteine: Spectroscopic and kinetic analysis
No full textIn addition to the main transaminase reaction, the pyridoxal 5'-phosphate-dependent enzyme human liver peroxisomal alanine:glyoxylate aminotransferase (AGT) is able to catalyze the alpha,beta-elimination of beta-chloro-L-alanine with a catalytic efficiency similar to that of the physiological transaminase reaction with L-alanine. On the other hand, during the reaction of AGT with L-cysteine, changes in the coenzyme forms and analysis of the products reveal the occurrence of both beta-elimination and half-transamination of L-cysteine together with the pyruvate transamination. A mechanism in which a ketimine species is the common intermediate of half-transamination and beta-elimination of L-cysteine is proposed. L-cysteine partitions between these two reactions with a ratio of approximately 2.5. Rapid scanning stopped-flow and quench flow experiments permit the identification of reaction intermediates and the measurements of the kinetic parameters of L-cysteine half-transamination. The k(cat) of this reaction is 200- or 60-fold lower than that of L-alanine and L-serine, respectively. Conversely, L-cysteine binds to AGT with a binding affinity 30- and 200-fold higher than that of L-alanine and L-serine, respectively. This appears to be consistent with the calculated interaction energies of the L-cysteine, L-alanine and L-serine docked at the active site of AGT
Construction, purification and characterization of untagged human liver alanine-glyoxylate aminotransferase expressed in Escherichia coli
No full textHis-tagging is commonly used to aid and expedite the purification of recombinant proteins. It is commonly assumed, though less frequently tested, that the His-tag affects neither the structure nor the stability of the protein. Alanine:glyoxylate aminotransferase (AGT) is a peroxisomal pyridoxal 5′-phosphate (PLP) dependent enzyme which catalyzes the transamination of alanine and glyoxylate to pyruvate and glycine. AGT is a clinically relevant enzyme whose deficiency causes an inherited rare metabolic disorder named primary hyperoxaluria type I. Until now, the structure and function of this enzyme have been studied using recombinant wild-type AGT and variants purified using a hexahistidine tag. However, the study of the functional roles of the N- and C-termini in the dimerization process and on the import into the peroxisome, respectively, requires the preparation of human liver AGT without histidine tags. We report for the first time the expression of untagged AGT together with a new rapid protocol for its purification. In addition, the kinetic parameters for the forward and reverse transamination catalyzed by untagged AGT as well as the spectroscopic features, the KD(PLP), the pH and thermal stability of the enzyme in the holo- and apo-form have been determined. This investigation will be the starting point for a detailed understanding of the contributions of the N- and C-termini on the dimerization and folding of AGT, and on its import into the peroxisome. This is prerequisite to understand how pathological mutations affect the proper native quaternary and tertiary structure, stability, and targeting of the enzyme. © 2008 Bentham Science Publishers Ltd
S81 L and G170R mutations causing Primary Hyperoxaluria Type I in homozygosis and heterozygosis: an example of positive interallelic complementation.
No full textPrimary Hyperoxaluria type I (PH1) is a rare disease due to the deficit of peroxisomal alanine:glyoxylate aminotransferase (AGT), a homodimeric pyridoxal-5'-phosphate (PLP) enzyme present in humans as major (Ma) and minor (Mi) allele. PH1-causing mutations are mostly missense identified in both homozygous and compound heterozygous patients. Until now, the pathogenesis of PH1 has been only studied by approaches mimicking homozygous patients, whereas the molecular aspects of the genotype-enzymatic-clinical phenotype relationship in compound heterozygous patients are completely unknown. Here, for the first time, we elucidate the enzymatic phenotype linked to the S81L mutation on AGT-Ma, relative to a PLP-binding residue, and how it changes when the most common mutation G170R on AGT-Mi, known to cause AGT mistargeting without affecting the enzyme functionality, is present in the second allele. By using a bicistronic eukaryotic expression vector, we demonstrate that (i) S81L-Ma is mainly in its apo-form and has a significant peroxisomal localization and (ii) S81L and G170R monomers interact giving rise to the G170R-Mi/S81L-Ma holo-form, which is imported into peroxisomes and exhibits an enhanced functionality with respect to the parental enzymes. These data, integrated with the biochemical features of the heterodimer and the homodimeric counterparts in their purified recombinant form, (i) highlight the molecular basis of the pathogenicity of S81L-Ma and (ii) provide evidence for a positive interallelic complementation between the S81L and G170R monomers. Our study represents a valid approach to investigate the molecular pathogenesis of PH1 in compound heterozygous patients
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Dimerization and folding processes of Treponema denticola cystalysin: the role of pyridoxal 5'-phosphate.
No full textCystalysin, the key virulence factor in the bacterium Treponema denticola responsible for periodontitis, is a homodimeric pyridoxal 5'-phosphate (PLP)-C-S lyase. The dimerization process and the urea-induced unfolding equilibrium of holocystalysin were compared with those of the apo form. The presence of PLP decreases approximately 4 times the monomer-dimer equilibrium dissociation constant. By using a variety of spectroscopic and analytical procedures, we demonstrated a difference in their unfolding profiles. Upon the monomerization of apocystalysin, occurring between 1 and 2 M urea, a self-associated equilibrium intermediate with a very high beta-sheet content is stabilized over the 2.5-4 M urea range, giving rise to a fully unfolded monomer at higher urea concentrations. On the other hand, highly destabilizing conditions, accompanied by the formation of a significant amount of insoluble aggregates, are required for PLP release and monomerization. Refolding studies, together with analysis of the dissociation/association process of cystalysin, shed light on how the protein concentration and the presence or absence of PLP under refolding conditions could affect the recovery of the active dimeric enzyme and the production of insoluble aggregates. When the protein is completely denatured, the best reactivation yield found was approximately 50% and 25% for holo and apocystalysin, respectively. The dimerization and folding processes of cystalysin have been compared with those of another PLP C-S lyase, MalY from E. coli, and the possible relevance of their PLP binding mode in these processes has been discussed
Crystal structure of the apo form of human aromatic L-amino acid decarboxylase: an open conformation unveils novel insights into the mechanism of PLP addition to the apoenzymes of group II decarboxylases
No full textCrystal structure of Human aromatic L-amino acid decarboxylase (AADC) in the apo form
No full textMethod: X-RAY DIFFRACTION
Resolution: 3.24 Å
R-Value Free: 0.294
R-Value Work: 0.236
R-Value Observed: 0.23
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