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At least 973 records · Page 54Linked to original sources
PubMed1986

Pyruvate dehydrogenase and 3-fluoropyruvate: chemical competence of 2-acetylthiamin pyrophosphate as an acetyl group donor to dihydrolipoamide.

The pyruvate dehydrogenase component (E1) of the pyruvate dehydrogenase complex catalyzes the decomposition of 3-fluoropyruvate to CO2, fluoride anion, and acetate. Acetylthiamin pyrophosphate (acetyl-TPP) is an intermediate in this reaction. Incubation of the pyruvate dehydrogenase complex with 3-fluoro[1,2-14C]pyruvate, TPP, coenzyme A (CoASH), and either NADH or pyruvate as reducing systems leads to the formation of [14C]acetyl-CoA. In this reaction the acetyl group of acetyl-TPP is partitioned by transfer to both CoASH (87 +/- 2%) and water (13 +/- 2%). When the E1 component is incubated with 3-fluoro[1,2-14C]pyruvate, TPP, and dihydrolipoamide, [14C]acetyldihydrolipoamide is produced. The formation of [14C]acetyldihydrolipoamide was examined as a function of dihydrolipoamide concentration (0.25-16 mM). A plot of the extent of acetyl group partitioning to dihydrolipoamide as a function of 1/[dihydrolipoamide] showed 95 +/- 2% acetyl group transfer to dihydrolipoamide when dihydrolipoamide concentration was extrapolated to infinity. It is concluded that acetyl-TPP is chemically competent as an intermediate for the pyruvate dehydrogenase complex catalyzed oxidative decarboxylation of pyruvate.

Carbon Radioisotopes↗
PubMed1989

2-Acetylthiamin pyrophosphate (acetyl-TPP) pH-rate profile for hydrolysis of acetyl-TPP and isolation of acetyl-TPP as a transient species in pyruvate dehydrogenase catalyzed reactions.

Rate constants for the hydrolysis of acetyl-TPP were measured between pH values of 2.5 and 7.5 and plotted as log kobs versus pH. The pH-rate profile defined two legs, each with a slope of +1 but separated by a region of decreased slope between pH 4 and pH 6. The rates were insensitive to buffer concentrations. Each leg of the profile reflected specific-base-catalyzed hydrolysis of acetyl-TPP, analogous to the hydrolysis of 2-acetyl-3,4-dimethylthiazolium ion [Lienhard, G.E. (1966) J. Am. Chem. Soc. 88, 5642-5649]. The separation of the two legs of this profile has been shown to be caused by the ionization of a group exhibiting a pKa of 4.73 within acetyl-TPP that is remote from the acetyl group, the amino-pyrimidine ring, which is protonated below pH 4.73. The protonation level of this ring has been shown to control the equilibrium partitioning of acetyl-TPP among its carbinolamine, keto, and hydrate forms. The differential partitioning of these species is a major factor causing the separation between the two legs of the pH-rate profile. The characteristic pH-rate profile and the availability of synthetic acetyl-TPP [Gruys, K.J., Halkides, C.J., & Frey, P.A. (1987) Biochemistry 26, 7575-7585] have facilitated the isolation and identification of [1-14C]acetyl-TPP from acid-quenched enzymatic reaction mixtures at steady states. [1-14C]Acetyl-TPP was identified as a transient species in reactions catalyzed by the PDH complex or the pyruvate dehydrogenase component of the complex (E1). The pH-rate profile for hydrolysis of [1-14C]-acetyl-TPP isolated from enzymatic reactions was found to be indistinguishable from that for authentic acetyl-TPP, which constituted positive identification of the 14C-labeled enzymic species.

Hydrogen-Ion Concentration↗
PubMed2000

Isolation and characterization of Saccharomyces cerevisiae mutants with derepressed thiamine gene expression.

Using a THI4-lacZ reporter gene, mutant strains have been isolated that display constitutive expression of thiamine genes in the presence of normally repressing levels of exogenous thiamine. In total, eight strains were isolated in which this derepressed expression on thiamine (Det(-)) phenotype was the result of single gene mutations. The Det(-) mutations of three of these strains were partially dominant in a heterozygous diploid configuration, whereas the other five were recessive. The partially dominant mutants DET1, DET12 and DET13, and the recessive mutant det2, all showed derepressed THI4-lacZ expression levels comparable to those of a fully induced normal strain. Use of other promoter-lacZ gene fusions revealed that these four mutants were pleiotropic; expression levels of all thiamine-regulated genes tested were also derepressed. Genetic analysis of the four mutants suggested that det2 and DET13 were allelic, whereas the others were at different loci; these four mutations therefore represent three different genes. None of the mutations were allelic with THI80, mutations of which have previously been shown to confer derepression on thiamine-regulated genes. Also, intracellular thiamine levels were close to normal and none of the four mutants excreted thiamine into the growth medium. All mutant strains were found to be prototrophic for thiamine and none of those tested were compromised for thiamine uptake. It is possible that some may be alleles of, or interact with, the activator gene THI3. Taken together, these results imply that DET1, det2, DET12 and DET13 represent new genes encoding negative regulators of thiamine-repressed genes.

Alleles↗
PubMed2002

Thiamin-diphosphate-dependent enzymes: new aspects of asymmetric C-C bond formation.

Starting from a thorough investigation of mechanistic aspects of ThDP-dependent (ThDP = thiamin diphosphate) enzymes in combination with mutagenesis studies and a detailed substrate screening, new general synthetic methods have been developed based on Umpolung reactions by thiamin catalysis. A selective donor-acceptor concept was established leading to the first asymmetric cross-benzoin condensation, and a kinetic racemic resolution through C-C bond cleavage was developed. With these tools and in combination with protein engineering, we approached the synthesis of new chiral building blocks on a preparative scale. An outlook is given with respect to the potential of other ThDP-dependent enzymes as catalysts in asymmetric synthesis.

Aldehyde-Lyases↗
PubMed1978

Leigh's disease in an adult with evidence of "inhibitor factor" in family members.

A 21-year-old man with a longstanding history of impaired visual acuity, strabismus, broad-based gait, and below-average intellectual capacity developed respiratory difficulties and intermittent generalized weakness at age 19. He subsequently showed signs of massive brainstem dysfunction and died. Postmortem examination demonstrated changes compatible with subacute necrotizing encephalomyelopathy of Leigh involving portions of the diencephalon, midbrain, pons, and medulla as well as portions of the spinal cord and optic nerves. Evaluation of family members for presence of the urinary inhibitor factor for thiamine diphosphate phosphoryl transferase revealed abnormal levels in a brother, a maternal uncle, and the maternal grandfather of the patient.

Adult↗
PubMed2006

Factors mediating activity, selectivity, and substrate specificity for the thiamin diphosphate-dependent enzymes benzaldehyde lyase and benzoylformate decarboxylase.

Benzaldehyde lyase from Pseudomonas fluorescens and benzoylformate decarboxylase from Pseudomonas putida are homologous thiamin diphosphate-dependent enzymes that catalyze carboligase and carbolyase reactions. Both enzymes catalyze the formation of chiral 2-hydroxy ketones from aldehydes. However, the reverse reaction has only been observed with benzaldehyde lyase. Whereas benzaldehyde lyase is strictly R specific, the stereoselectivity of benzoylformate decarboxylase from P. putida is dependent on the structure and orientation of the substrate aldehydes. In this study, the binding sites of both enzymes were investigated by using molecular modelling studies to explain the experimentally observed differences in the activity, stereo- and enantioselectivity and substrate specificity of both enzymes. We designed a detailed illustration that describes the shape of the binding site of both enzymes and sufficiently explains the experimental effects observed with the wild-type enzymes and different variants. These findings demonstrate that steric reasons are predominantly responsible for the differences observed in the (R)-benzoin cleavage and in the formation of chiral 2-hydroxy ketones.

Aldehyde-Lyases↗
PubMed1992

Three-dimensional structure of transketolase, a thiamine diphosphate dependent enzyme, at 2.5 A resolution.

The crystal structure of Saccharomyces cerevisiae transketolase, a thiamine diphosphate dependent enzyme, has been determined to 2.5 A resolution. The enzyme is a dimer with the active sites located at the interface between the two identical subunits. The cofactor, vitamin B1 derived thiamine diphosphate, is bound at the interface between the two subunits. The enzyme subunit is built up of three domains of the alpha/beta type. The diphosphate moiety of thiamine diphosphate is bound to the enzyme at the carboxyl end of the parallel beta-sheet of the N-terminal domain and interacts with the protein through a Ca2+ ion. The thiazolium ring interacts with residues from both subunits, whereas the pyrimidine ring is buried in a hydrophobic pocket of the enzyme, formed by the loops at the carboxyl end of the beta-sheet in the middle domain in the second subunit. The structure analysis identifies amino acids critical for cofactor binding and provides mechanistic insights into thiamine catalysis.

Binding Sites↗
PubMed1978

[Multiphasic growth of microorganisms: modeling and computer simulation of linear growth phases].

A phenomenological and a more causal model are developed for the multiphasic discontinuous growth. The first model distinguished between the lag-phase, the exponential phase, the transient phase from the exponential to the linear phase, the linear phase, the transient phase from the linear to the stationary phase, and the stationary phase. The parameters are rate constants, critical values of biomass, and time constants. The parameters are estimated for experimental data of growth of Candida lipolytica under limitation of thiamine (Müller et al. 1978). These data are fitted also by a more causal model. This second model is in agreement with Monod's idea that a linear growth phase may be due to an enzyme or enzyme system which has a constant activity. In the analysed case of limitation of thiamine the constancy of the dehydrogenase activity is caused by a constant level of the coenzymethiaminepyrophosphate. Thus, when such a thiamine requiring enzymatic step becomes to the "bottle-neck", bacterial culture switches over from exponential to linear growth. The end of linear phase is discussed more hypothetically by the high cooperativity of activity of dehydrogenases and the existence of a mimimal specific growth rate. The results of modeling and parameter estimation are compared with experimental data of C. lipolytica. These two models are able to interpret the growth kinetic of these multiphasic growth.

Candida↗
PubMed1996

Brain levels of thiamine and its phosphate esters in Friedreich's ataxia and spinocerebellar ataxia type 1.

Decreased blood and cerebrospinal fluid levels of thiamine have been reported in patients with spinocerebellar ataxia disorders. To determine whether a thiamine deficiency is present in the brain, we measured levels of thiamine and its phosphate esters thiamine monophosphate (TMP) and thiamine diphosphate (TDP), in postmortem cerebellar and cerebral cortices of patients with Friedreich's ataxia (FA) and spinocerebellar ataxia type 1 (SCA1). Brain levels of free (nonphosphorylated) thiamine, TMP, TDP, and total thiamine in FA and SCA1 were, on average, not significantly different from control values. However, a nonsignificant trend was observed for slightly reduced levels of TDP and total thiamine in cerebellar cortex of the SCA1 patients, a finding that might be related to the severe neuronal damage in this brain area. We conclude that in FA, brain thiamine concentrations are normal, whereas in SCA1 the levels are, at most, only slightly reduced.

Adolescent↗
PubMed1992

Stability and reconstitution of pyruvate oxidase from Lactobacillus plantarum: dissection of the stabilizing effects of coenzyme binding and subunit interaction.

Pyruvate oxidase from Lactobacillus plantarum is a homotetrameric flavoprotein with strong binding sites for FAD, TPP, and a divalent cation. Treatment with acid ammonium sulfate in the presence of 1.5 M KBr leads to the release of the cofactors, yielding the stable apoenzyme. In the present study, the effects of FAD, TPP, and Mn2+ on the structural properties of the apoenzyme and the reconstitution of the active holoenzyme from its constituents have been investigated. As shown by circular dichroism and fluorescence emission, as well as by Nile red binding, the secondary and tertiary structures of the apoenzyme and the holoenzyme do not exhibit marked differences. The quaternary structure is stabilized significantly in the presence of the cofactors. Size-exclusion high-performance liquid chromatography and analytical ultracentrifugation demonstrate that the holoenzyme retains its tetrameric state down to 20 micrograms/mL, whereas the apoenzyme shows stepwise tetramer-dimer-monomer dissociation, with the monomer as the major component, at a protein concentration of < 20 micrograms/mL. In the presence of divalent cations, the coenzymes FAD and TPP bind to the apoenzyme, forming the inactive binary FAD or TPP complexes. Both FAD and TPP affect the quaternary structure by shifting the equilibrium of association toward the dimer or tetramer. High FAD concentrations exert significant stabilization against urea and heat denaturation, whereas excess TPP has no effect. Reconstitution of the holoenzyme from its components yields full reactivation. The kinetic analysis reveals a compulsory sequential mechanism of cofactor binding and quaternary structure formation, with TPP binding as the first step. The binary TPP complex (in the presence of 1 mM Mn2+/TPP) is characterized by a dimer-tetramer equilibrium transition with an association constant of Ka = 2 x 10(7) M-1. The apoenzyme TPP complex dimer associates with the tetrameric holoenzyme in the presence of 10 microM FAD. This association step obeys second-order kinetics with an association rate constant k = 7.4 x 10(3) M-1 s-1 at 20 degrees C. FAD binding to the tetrameric binary TPP complex is too fast to be resolved by manual mixing.

Apoenzymes↗