The functions of the flavin contact residues, alphaArg249 and betaTyr16, in human electron transfer flavoprotein.
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Arg249 in the large (alpha) subunit of human electron transfer flavoprotein (ETF) heterodimer is absolutely conserved throughout the ETF superfamily. The guanidinium group of alphaArg249 is within van der Waals contact distance and lies perpendicular to the xylene subnucleus of the flavin ring, near the region proposed to be involved in electron transfer with medium chain acyl-CoA dehydrogenase. The backbone amide hydrogen of alphaArg249 is within hydrogen bonding distance of the carbonyl oxygen at the flavin C(2). alphaArg249 may modulate the potentials of the two flavin redox couples by hydrogen bonding the carbonyl oxygen at C(2) and by providing delocalized positive charge to neutralize the anionic semiquinone and anionic hydroquinone of the flavin. The potentials of the oxidized/semiquinone and semiquinone/hydroquinone couples decrease in an alphaR249K mutant ETF generated by site directed mutagenesis and expression in Escherichia coli, without major alterations of the flavin environment as judged by spectral criteria. The steady state turnover of medium chain acyl-CoA dehydrogenase and glutaryl-CoA dehydrogenase decrease greater than 90% as a result of the alphaR249Ks mutation. In contrast, the steady state turnover of short chain acyl-CoA dehydrogenase was decreased about 38% when alphaR249K ETF was the electron acceptor. Stopped flow absorbance measurements of the oxidation of reduced medium chain acyl-CoA dehydrogenase/octenoyl-CoA product complex by wild type human ETF at 3 degrees C are biphasic (t(1/2)=12 ms and 122 ms). The rate of oxidation of this reduced binary complex of the dehydrogenase by the alphaR249K mutant ETF is extremely slow and could not be reasonably estimated. alphaAsp253 is proposed to function with alphaArg249 in the electron transfer pathway from medium chain acyl-CoA dehydrogenase to ETF. The steady state kinetic constants of the dehydrogenase were not altered when ETF containing an alphaD253A mutant was the substrate. However, t(1/2) of the rapid phase of oxidation of the reduced medium chain acyl-CoA dehydrogenase/octenoyl-CoA charge transfer complex almost doubled. betaTyr16 lies on a loop near the C(8) methyl group, and is also near the proposed site for interflavin electron transfer with medium chain acyl-CoA dehydrogenase. The tyrosine residue makes van der Waals contact with the C(8) methyl group of the flavin in human ETF and Paracoccus denitrificans ETF (as betaTyr13) and lies at a 30 degrees C angle with the plane of the flavin. Human betaTyr16 was substituted with leucine and alanine residues to investigate the role of this residue in the modulation of the flavin redox potentials and in electron transfer to ETF. In betaY16L ETF, the potentials of the flavin were slightly reduced, and steady state kinetic constants were modestly altered. Substitution of an alanine residue for betaTyr16 yields an ETF with potentials very similar to the wild type but with steady state kinetic properties similar to betaY16L ETF. It is unlikely that the beta methyl group of the alanine residue interacts with the flavin C(8) methyl. Neither substitution of betaTyr16 had a large effect on the fast phase of ETF reduction by medium chain acyl-CoA dehydrogenase. (+info)
Mutation analysis in glutaric aciduria type I.
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Glutaric aciduria type 1 (GA1), resulting from the genetic deficiency of glutaryl-CoA dehydrogenase (GDH), is a relatively common cause of acute metabolic brain damage in infants. Encephalopathic crises may be prevented by carnitine supplementation and diet, but diagnosis can be difficult as some patients do not show the typical excretion of large amounts of glutaric and 3-hydroxyglutaric acids in the urine. We present a rapid and efficient denaturing gradient gel electrophoresis (DGGE) method for the identification of mutations in the glutaryl-CoA dehydrogenase (GCDH) gene that may be used for the molecular diagnosis of GA1 in a routine setting. Using this technique, we identified mutations on both alleles in 48 patients with confirmed GDH deficiency, while no mutations were detected in other patients with clinical suspicion of GA1 but normal enzyme studies. There was a total of 38 different mutations; 27 mutations were found in single patients only, and 21 mutations have not been previously reported. Fourteen mutations involved hypermutable CpG sites. The commonest GA1 mutation in Europeans is R402W, which accounts for almost 40% of alleles in patients of German origin. GCDH gene haplotypes were determined through the analysis of polymorphic markers in all families, and three CpG mutations were associated with different haplotypes, possibly reflecting independent recurrence. The high sensitivity of the DGGE method allows the rapid and cost efficient diagnosis of GA1 in instances where enzyme analyses are not available or feasible, despite the marked heterogeneity of the disease. (+info)
The function of Arg-94 in the oxidation and decarboxylation of glutaryl-CoA by human glutaryl-CoA dehydrogenase.
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Glutaryl-CoA dehydrogenase catalyzes the oxidation and decarboxylation of glutaryl-CoA to crotonyl-CoA and CO(2). Inherited defects in the protein cause glutaric acidemia type I, a fatal neurologic disease. Glutaryl-CoA dehydrogenase is the only member of the acyl-CoA dehydrogenase family with a cationic residue, Arg-94, situated in the binding site of the acyl moiety of the substrate. Crystallographic investigations suggest that Arg-94 is within hydrogen bonding distance of the gamma-carboxylate of glutaryl-CoA. Substitution of Arg-94 by glycine, a disease-causing mutation, and by glutamine, which is sterically more closely related to arginine, reduced k(cat) of the mutant dehydrogenases to 2-3% of k(cat) of the wild type enzyme. K(m) of these mutant dehydrogenases for glutaryl-CoA increases 10- to 16-fold. The steady-state kinetic constants of alternative substrates, hexanoyl-CoA and glutaramyl-CoA, which are not decarboxylated, are modestly affected by the mutations. The latter changes are probably due to steric and polar effects. The dissociation constants of the non-oxidizable substrate analogs, 3-thiaglutaryl-CoA and acetoacetyl-CoA, are not altered by the mutations. However, abstraction of a alpha-proton from 3-thiaglutaryl-CoA, to yield a charge transfer complex with the oxidized flavin, is severely limited. In contrast, abstraction of the alpha-proton of acetoacetyl-CoA by Arg-94 --> Gln mutant dehydrogenase is unaffected, and the resulting enolate forms a charge transfer complex with the oxidized flavin. These experiments indicate that Arg-94 does not make a major contribution to glutaryl-CoA binding. However, the electric field of Arg-94 may stabilize the dianions resulting from abstraction of the alpha-proton of glutaryl-CoA and 3-thiaglutaryl-CoA, both of which contain gamma-carboxylates. It is also possible that Arg-94 may orient glutaryl-CoA and 3-thiaglutaryl-CoA for abstraction of an alpha-proton. (+info)
Biochemical, pathologic and behavioral analysis of a mouse model of glutaric acidemia type I.
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Glutaric acidemia type I (GA-I) is an autosomal recessive disorder of amino acid metabolism resulting from a deficiency of glutaryl-CoA dehydrogenase (GCDH). Patients accumulate glutaric acid (GA) and 3-OH glutaric acid (3-OHGA) in their blood, urine and CSF. Clinically, GA-I is characterized by macrocephaly, progressive dystonia and dyskinesia. Degeneration of the caudate and putamen of the basal ganglia, widening of the Sylvian fissures, fronto-temporal atrophy and severe spongiform change in the white matter are also commonly observed. In this report we describe the phenotype of a mouse model of GA-I generated via targeted deletion of the Gcdh gene in embryonic stem cells. The Gcdh-/- mice have a biochemical phenotype very similar to human GA-I patients, including elevations of GA and 3-OHGA at levels similar to those seen in GA-I patients. The affected mice have a mild motor deficit but do not develop the progressive dystonia seen in human patients. Pathologically, the Gcdh-/- mice have a diffuse spongiform myelinopathy similar to that seen in GA-I patients. However, unlike in human patients, there is no evidence of neuron loss or astrogliosis in the striatum. Subjecting the Gcdh-/- mice to a metabolic stress, which often precipitates an encephalopathic crisis and the development of dystonia in GA-I patients, failed to have any neurologic effect on the mice. We hypothesize that the lack of similarity in regards to the neurologic phenotype and striatal pathology of GA-I patients, as compared with the Gcdh-/- mice, is due to intrinsic differences between the striata of mice and men. (+info)
Mechanism-based inactivation of human glutaryl-CoA dehydrogenase by 2-pentynoyl-CoA: rationale for enhanced reactivity.
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2-Pentynoyl-CoA inactivates glutaryl-CoA dehydrogenase at a rate that considerably exceeds the rates of inactivation of short chain and medium chain acyl-CoA dehydrogenases by this inhibitor and related 2-alkynoyl-CoAs. To determine the rate of inactivation by 2-pentynoyl-CoA, we investigated the inactivation in the presence of a non-oxidizable analog, 3-thiaglutaryl-CoA, which competes for the binding site. The enhanced rate of inactivation does not reflect an alteration in specificity for the acyl group, nor does it reflect the covalent modification of a residue other than the active site glutamate. In addition to determining the inactivation of catalytic activity a spectral intermediate was detected by stopped-flow spectrophotometry, and the rate constants of formation and decay of this charge transfer complex (lambdamax approximately 790 nm) were determined by global analysis. Although the rate-limiting step in the inactivation of the other acyl-CoA dehydrogenases can involve the abstraction of a proton at C-4, this is not the case with glutaryl-CoA dehydrogenase. Glutaryl-CoA dehydrogenase is also differentiated from other acyl-CoA dehydrogenases in that the catalytic base must access both C-2 and C-4 in the normal catalytic pathway. Access to C-4 is not obligatory for the other dehydrogenases. Analysis of the distance from the closest carboxylate oxygen of the glutamate base catalyst to C-4 of a bound acyl-CoA ligand for medium chain, short chain, and isovaleryl-CoA dehydrogenases suggests that the increased rate of inactivation reflects the carboxylate oxygen to ligand C-4 distance in the binary complexes. This distance for wild type glutaryl-CoA dehydrogenase is not known. Comparison of the rate constants of inactivation and formation of a spectral species between wild type glutaryl-CoA dehydrogenase and a E370D mutant are consistent with the idea that this distance in glutaryl-CoA dehydrogenase contributes to the enhanced rate of inactivation and the 1,3-prototropic shift catalyzed by the enzyme. (+info)
Neuropathological, biochemical and molecular findings in a glutaric acidemia type 1 cohort.
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Glutaric acidemia type 1 (GA-1) is an autosomal recessive disorder characterized by a deficiency of glutaryl-CoA dehydrogenase (GCDH) activity. GA-1 is often associated with an acute encephalopathy between 6 and 18 months of age that causes striatal damage resulting in a severe dystonic movement disorder. Ten autopsy cases have been previously described. Our goal is to understand the disorder better so that treatments can be designed. Therefore, we present the neuropathological features of six additional cases (8 months-40 years), all North American aboriginals with the identical homozygous mutation. This cohort displays similar pathological characteristics to those previously described. Four had macroencephaly. All had striatal atrophy with severe loss of medium-sized neurons. We present several novel findings. This natural time course study allows us to conclude that neuron loss occurs shortly after the encephalopathical crisis and does not progress. In addition, we demonstrate mild loss of large striatal neurons, spongiform changes restricted to brainstem white matter and a mild lymphocytic infiltrate in the early stages. Reverse transcriptase-PCR to detect the GCDH mRNA revealed normal and truncated transcripts similar to those in fibroblasts. All brain regions demonstrated markedly elevated concentrations of GA (3770-21 200 nmol/g protein) and 3-OH-GA (280-740 nmol/g protein), with no evidence of striatal specificity or age dependency. The role of organic acids as toxic agents and as osmolytes is discussed. The pathogenesis of selective neuronal loss cannot be explained on the basis of regional genetic and/or metabolic differences. A suitable animal model for GA-1 is needed. (+info)
Bioenergetics in glutaryl-coenzyme A dehydrogenase deficiency: a role for glutaryl-coenzyme A.
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Inherited deficiency of glutaryl-CoA dehydrogenase results in an accumulation of glutaryl-CoA, glutaric, and 3-hydroxyglutaric acids. If untreated, most patients suffer an acute encephalopathic crisis and, subsequently, acute striatal damage being precipitated by febrile infectious diseases during a vulnerable period of brain development (age 3 and 36 months). It has been suggested before that some of these organic acids may induce excitotoxic cell damage, however, the relevance of bioenergetic impairment is not yet understood. The major aim of our study was to investigate respiratory chain, tricarboxylic acid cycle, and fatty acid oxidation in this disease using purified single enzymes and tissue homogenates from Gcdh-deficient and wild-type mice. In purified enzymes, glutaryl-CoA but not glutaric or 3-hydroxyglutaric induced an uncompetitive inhibition of alpha-ketoglutarate dehydrogenase complex activity. Notably, reduced activity of alpha-ketoglutarate dehydrogenase activity has recently been demonstrated in other neurodegenerative diseases, such as Alzheimer, Parkinson, and Huntington diseases. In contrast to alpha-ketoglutarate dehydrogenase complex, no direct inhibition of glutaryl-CoA, glutaric acid, and 3-hydroxyglutaric acid was found in other enzymes tested. In Gcdh-deficient mice, respiratory chain and tricarboxylic acid activities remained widely unaffected, virtually excluding regulatory changes in these enzymes. However, hepatic activity of very long-chain acyl-CoA dehydrogenase was decreased and concentrations of long-chain acylcarnitines increased in the bile of these mice, which suggested disturbed oxidation of long-chain fatty acids. In conclusion, our results demonstrate that bioenergetic impairment may play an important role in the pathomechanisms underlying neurodegenerative changes in glutaryl-CoA dehydrogenase deficiency. (+info)
Neurophysiologic features in glutaric aciduria type I.
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Neurophysiologic abnormalities are frequently seen in organic acidemias, but knowledge of the specific changes in the different types of organic acidemias is lacking. We studied electroencephalogram (EEG), visual evoked potential (VEP) and brain-stem auditory evoked response (BAER) in seven children with glutaric aciduria type I (GA1) to assess the neurophysiologic features in this rare inborn error of metabolism. Age at the time of the diagnosis ranged between 3 months and 36 months. Age at the time of neurophysiologic evaluation ranged between 11 months and 36 months. At the time of neurophysiologic evaluation, severe global developmental delay was seen in four patients, dystonia in four patients, motor delay in two patients, and axial hypotonia in two patients; macrocephaly, spasticity, moderate mental retardation and borderline intelligence were each seen in one patient. One patient had autistic features characterized by lack of language and social skills, poor eye contact and stereotypical behavior. Three of seven patients showed abnormal EEG findings. Two patients showed asymmetry with intermittent occipital delta slowing in one hemisphere. This finding probably indicates underlying cerebral dysfunction, and is not a specific feature. However, it suggests that these patients may develop abnormal EEG features during the course of the disease, and thus a baseline EEG may be useful for comparison over time. One patient showed high amplitude bursts of beta in the occipital regions with left predominance while on clonazepam and baclofen. We believe this finding was due to medication effect, and that what we observed was an exaggarated response to benzodiazepine. The clinical significance of this finding is unclear. VEP and BAER were available in four patients, and we found abnormalities in three of them. Neurophysiologic evaluation may be helpful in patients with GA1 as in other types of organic acidemias to help detect subtle changes that are not reflected by neurological examination or neuroimaging studies, and it may guide future treatment plans. Detailed neurophysiologic analysis in a large series of GA1 may yield further information regarding the extent of cerebral dysfunction. (+info)