Binding, hydration, and decarboxylation of the reaction intermediate glutaconyl-coenzyme A by human glutaryl-CoA dehydrogenase

Glutaconyl-coenzyme A (CoA) is the presumed enzyme-bound intermediate in the oxidative decarboxylation of glutaryl-CoA that is catalyzed by glutaryl-CoA dehydrogenase. We demonstrated glutaconyl-CoA bound to glutaryl-CoA dehydrogenase after anaerobic reduction of the dehydrogenase with glutaryl-CoA. Glutaryl-CoA dehydrogenase also has intrinsic enoyl-CoA hydratase activity, a property of other members of the acyl-CoA dehydrogenase family. The enzyme rapidly hydrates glutaconyl-CoA at pH 7.6 with a k(cat) of 2.7 s(-1). The k(cat) in the overall oxidation-decarboxylation reaction at pH 7.6 is about 9 s(-1). The binding of glutaconyl-CoA was quantitatively assessed from the K(m) in the hydratase reaction, 3 microM, and the K(i), 1.0 microM, as a competitive inhibitor of the dehydrogenase. These values compare with K(m) and K(i) of 4.0 and 12.9 microM, respectively, for crotonyl-CoA. Glu370 is the general base catalyst in the dehydrogenase that abstracts an alpha-proton of the substrate to initiate the catalytic pathway. The mutant dehydrogenase, Glu370Gln, is inactive in the dehydrogenation and the hydratase reactions. However, this mutant dehydrogenase decarboxylates glutaconyl-CoA to crotonyl-CoA without oxidation-reduction reactions of the dehydrogenase flavin. Addition of glutaconyl-CoA to this mutant dehydrogenase results in a rapid, transient increase in long-wavelength absorbance (lambda(max) approximately 725 nm), and crotonyl-CoA is found as the sole product. We propose that this 725 nm-absorbing species is the delocalized crotonyl-CoA anion that follows decarboxylation and that the decay is the result of slow protonation of the anion in the absence of the general acid catalyst, Glu370(H(+)). In the absence of detectable oxidation-reduction, the data indicate that oxidation-reduction of the dehydrogenase flavin is not essential for decarboxylation of glutaconyl-CoA.

Prenatal diagnosis of multiple acyl-CoA dehydrogenase deficiency: association with elevated alpha-fetoprotein and cystic renal changes

We report the occurrence of multiple acyl-CoA dehydrogenase deficiency (MADD) in two consecutive pregnancies in a young, Caucasian, non-consanguineous couple. In the first pregnancy, the maternal serum alpha-fetoprotein was elevated. A sonogram showed growth delay, cystic renal disease, and oligohydramnios; the parents decided to terminate the pregnancy. Postmortem examination confirmed the cystic renal disease and showed hepatic steatosis, raising the suspicion of a metabolic disorder. The diagnosis of MADD was made by immunoblot studies on cultured fibroblasts. In the subsequent pregnancy, a sonogram at 15 weeks' gestation showed an early growth delay but normal kidneys. The maternal serum and amniotic fluid concentrations of alpha-fetoprotein were elevated, and the amniotic fluid acylcarnitine profile was consistent with MADD. In vitro metabolic studies on cultured amniocytes confirmed the diagnosis. A follow-up sonogram showed cystic renal changes. These cases provide additional information regarding the evolution of renal changes in affected fetuses and show a relationship with elevated alpha-fetoprotein, which may be useful in counseling the couple at risk. MADD should be considered in the differential diagnosis of elevated alpha-fetoprotein and cystic renal disease. Early growth delay may be an additional feature.

Protein dynamics enhance electronic coupling in electron transfer complexes

Electron-transferring flavoproteins (ETFs) from human and Paracoccus denitrificans have been analyzed by small angle x-ray scattering, showing that neither molecule exists in a rigid conformation in solution. Both ETFs sample a range of conformations corresponding to a large rotation of domain II with respect to domains I and III. A model of the human ETF.medium chain acyl-CoA dehydrogenase complex, consistent with x-ray scattering data, indicates that optimal electron transfer requires domain II of ETF to rotate by approximately 30 to 50 degrees toward domain I relative to its position in the x-ray structure. Domain motion establishes a new "robust engineering principle" for electron transfer complexes, tolerating multiple configurations of the complex while retaining efficient electron transfer.

The function of Arg-94 in the oxidation and decarboxylation of glutaryl-CoA by human glutaryl-CoA dehydrogenase

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.

Proton abstraction reaction, steady-state kinetics, and oxidation-reduction potential of human glutaryl-CoA dehydrogenase

Glutaryl-CoA dehydrogenase catalyzes the oxidation of glutaryl-CoA to crotonyl-CoA and CO(2) in the mitochondrial degradation of lysine, hydroxylysine, and tryptophan. We have characterized the human enzyme that was expressed in Escherichia coli. Anaerobic reduction of the enzyme with sodium dithionite or substrate yields no detectable semiquinone; however, like other acyl-CoA dehydrogenases, the human enzyme stabilizes an anionic semiquinone upon reduction of the complex between the enzyme and 2,3-enoyl-CoA product. The flavin potential of the free enzyme determined by the xanthine-xanthine oxidase method is -0.132 V at pH 7.0, slightly more negative than that of related flavoprotein dehydrogenases. A single equivalent of substrate reduces 26% of the dehydrogenase flavin, suggesting that the redox equilibrium on the enzyme between substrate and product and oxidized and reduced flavin is not as favorable as that observed with other acyl-CoA dehydrogenases. This equilibrium is, however, similar to that observed in isovaleryl-CoA dehydrogenase. Comparison of steady-state kinetic constants of glutaryl-CoA dehydrogenase with glutaryl-CoA and the alternative substrates, pentanoyl-CoA and hexanoyl-CoA, suggests that the gamma-carboxyl group of glutaryl-CoA stabilizes the enzyme-substrate complex by at least 5.7 kJ/mol, perhaps by interaction with Arg94 or Ser98. Glu370 is positioned to function as the catalytic base, and previous studies indicate that the conjugate acid of Glu370 also protonates the transient crotonyl-CoA anion following decarboxylation [Gomes, B., Fendrich, G. , and Abeles, R. H. (1981) Biochemistry 20, 3154-3160]. Glu370Asp and Glu370Gln mutants of glutaryl-CoA dehydrogenase exhibit 7% and 0. 04% residual activity, respectively, with human electron-transfer flavoprotein; these mutations do not grossly affect the flavin redox potentials of the mutant enzymes. The reduced catalytic activities of these mutants can be attributed to reduced extent and rate of substrate deprotonation based on experiments with the nonoxidizable substrate analogue, 3-thiaglutaryl-CoA, and kinetic experiments. Determination of these fundamental properties of the human enzyme will serve as the basis for future studies of the decarboxylation reaction which is unique among the acyl-CoA dehydrogenases.

The intraflavin hydrogen bond in human electron transfer flavoprotein modulates redox potentials and may participate in electron transfer

Electron-transfer flavoprotein (ETF) serves as an intermediate electron carrier between primary flavoprotein dehydrogenases and terminal respiratory chains in mitochondria and prokaryotic cells. The three-dimensional structures of human and Paracoccus denitrificans ETFs determined by X-ray crystallography indicate that the 4'-hydroxyl of the ribityl side chain of FAD is hydrogen bonded to N(1) of the flavin ring. We have substituted 4'-deoxy-FAD for the native FAD and investigated the analog-containing ETF to determine the role of this rare intra-cofactor hydrogen bond. The binding constants for 4'-deoxy-FAD and FAD with the apoprotein are very similar, and the energy of binding differs by only 2 kJ/mol. The overall two-electron oxidation-reduction potential of 4'-deoxy-FAD in solution is identical to that of FAD. However, the potential of the oxidized/semiquinone couple of the ETF containing 4'-deoxy-FAD is 0.116 V less than the oxidized/semiquinone couple of the native protein. These data suggest that the 4'-hydoxyl-N(1) hydrogen bond stabilizes the anionic semiquinone in which negative charge is delocalized over the N(1)-C(2)O region. Transfer of the second electron to 4'-deoxy-FAD reconstituted ETF is extremely slow, and it was very difficult to achieve complete reduction of the flavin semiquinone to the hydroquinone. The turnover of medium chain acyl-CoA dehydrogenase with native ETF and ETF containing the 4'-deoxy analogue was essentially identical when the reduced ETF was recycled by reduction of 2,6-dichlorophenolindophenol. However, the steady-state turnover of the dehydrogenase with 4'-deoxy-FAD was only 23% of the turnover with native ETF when ETF semiquinone formation was assayed directly under anaerobic conditions. This is consistent with the decreased potential of the oxidized semiquinone couple of the analog-containing ETF. ETF containing 4'-deoxy-FAD neither donates to nor accepts electrons from electron-transfer flavoprotein ubiquinone oxidoreductase (ETF-QO) at significant rates (</=0.5% the wild-type rates). These results indicate that the 4'-hydroxyl-N(1) hydrogen bond plays a major role in the stabilization of the anionic semiquinone and anionic hydroquinone oxidation states of ETF and that this hydrogen bond may provide a pathway for electron transfer between the ETF flavin and the flavin of ETF-QO.

Crystal structure of Paracoccus denitrificans electron transfer flavoprotein: structural and electrostatic analysis of a conserved flavin binding domain

The crystal structure of electron transfer flavoprotein (ETF) from Paracoccus denitrificans was determined and refined to an R-factor of 19.3% at 2.6 A resolution. The overall fold is identical to that of the human enzyme, with the exception of a single loop region. Like the human structure, the structure of the P. denitrificans ETF is comprised of three distinct domains, two contributed by the alpha-subunit and the third from the beta-subunit. Close analysis of the structure reveals that the loop containing betaI63 is in part responsible for conferring the high specificity of AMP binding by the ETF protein. Using the sequence and structures of the human and P. denitrificans enzymes as models, a detailed sequence alignment has been constructed for several members of the ETF family, including sequences derived for the putative FixA and FixB proteins. From this alignment, it is evident that in all members of the ETF family the residues located in the immediate vicinity of the FAD cofactor are identical, with the exception of the substitution of serine and leucine residues in the W3A1 ETF protein for the human residues alphaT266 and betaY16, respectively. Mapping of ionic differences between the human and P. denitrificans ETF onto the structure identifies a surface that is electrostatically very similar between the two proteins, thus supporting a previous docking model between human ETF and pig medium-chain acyl-CoA dehydrogenase (MCAD). Analysis of the ionic strength dependence of the electron transfer reaction between either human or P. denitrificans ETF and MCAD demonstrates that the human ETF functions optimally at low ( approximately 10 mequiv) ionic strength, while P. denitrificans ETF is a better electron acceptor at higher (>75 mequiv) ionic strength. This suggests that the electrostatic surface potential of the two proteins is very different and is consistent with the difference in isoelectric points between the proteins. Analysis of the electrostatic potentials of the human and P. denitrificans ETFs reveals that the P. denitrificans ETF is more negatively charged. This excess negative charge may contribute to the difference in redox potentials between the two ETF flavoproteins and suggests an explanation for the opposing ionic strength dependencies for the reaction of MCAD with the two ETFs. Furthermore, by analysis of a model of the previously described human-P. denitrificans chimeric ETF protein, it is possible to identify one region of ETF that participates in docking with ETF-ubiquinone oxidoreductase, the physiological electron acceptor for ETF.

31P-NMR spectroscopy of human and Paracoccus denitrificans electron transfer flavoproteins, and 13C- and 15N-NMR spectroscopy of human electron transfer flavoprotein in the oxidised and reduced states

Human and Paracoccus denitrificans wild-type electron transfer flavoproteins have been investigated by 31P-NMR in the oxidised and reduced states. The 31P chemical shifts of the diphosphate moiety of the protein-bound FAD were similar in the proteins and were independent of the redox state. The chemical shifts were remarkably similar to those of ferredoxin-NADP+ reductase and, to a lesser degree, with those of NADPH-cytochrome P-450 reductase. The wild-type human electron transfer apoprotein was reconstituted with [2,4a-13C2]FAD, [4,10a-13C2]FAD, or [U-15N4]FAD. The reconstituted proteins were studied by 13C- and 15N-NMR techniques in the oxidised and reduced states. The chemical shifts were compared with those of free flavin in aqueous solution or in chloroform, and those of flavoproteins published in the literature. In the oxidised state, strong hydrogen bonds exist between residues of the apoprotein and C(2)O and N(5) of FAD. The N(1) atom is also hydrogen bonded and, as shown by X-ray data, involves the C'(4)-OH group of FAD. The sp2 hybridisation of N(10) is small compared to other flavoproteins. In the reduced state, there are strong hydrogen bonds involving C(2)O and N(5) of FAD. The N(1) atom is ionised as observed also in other flavoproteins when investigated by NMR. The intramolecular hydrogen bond between the C'(4)-OH group and the N(1) atom of FAD is maintained in the reduced state, suggesting an involvement in the stabilisation of a certain configuration of the diphosphate group of protein-bound FAD in both redox states. The N(10) atom in the reduced protein is highly sp3 hybridised in comparison to those of other flavoproteins.

Expression and characterization of two pathogenic mutations in human electron transfer flavoprotein

Defects in electron transfer flavoprotein (ETF) or its electron acceptor, electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO), cause the human inherited metabolic disease glutaric acidemia type II. In this disease, electron transfer from nine primary flavoprotein dehydrogenases to the main respiratory chain is impaired. Among these dehydrogenases are the four chain length-specific flavoprotein dehydrogenases of fatty acid beta-oxidation. In this investigation, two mutations in the alpha subunit that have been identified in patients were expressed in Escherichia coli. Of the two mutant alleles, alphaT266M and alphaG116R, the former is the most frequent mutation found in patients with ETF deficiency. The crystal structure of human ETF shows that alphaG116 lies in a hydrophobic pocket, under a contact residue of the alpha/beta subunit interface, and that the hydroxyl hydrogen of alphaT266 is hydrogen-bonded to N(5) of the FAD; the amide backbone hydrogen of alphaT266 is hydrogen-bonded to C(4)-O of the flavin prosthetic group (Roberts, D. L., Frerman, F. E. and Kim, J-J. P. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 14355-14360). Stable expression of the alphaG116R ETF required coexpression of the chaperonins, GroEL and GroES. alphaG116R ETF folds into a conformation different from the wild type, and is catalytically inactive in crude extracts. It is unstable and could not be extensively purified. The alphaT266M ETF was purified and characterized after stabilization to proteolysis in crude extracts. Although the global structure of this mutant protein is unchanged, its flavin environment is altered as indicated by absorption and circular dichroism spectroscopy and the kinetics of flavin release from the oxidized and reduced protein. The loss of the hydrogen bond at N(5) of the flavin and the altered flavin binding increase the thermodynamic stability of the flavin semiquinone by 10-fold relative to the semiquinone of wild type ETF. The mutation has relatively little effect on the reductive half-reaction of ETF catalyzed by sarcosine and medium chain acyl-CoA dehydrogenases which reduce the flavin to the semiquinone. However, kcat/Km of ETF-QO in a coupled acyl-CoA:ubiquinone reductase assay with oxidized alphaT266M ETF as substrate is reduced 33-fold; this decrease is due in largest part to a decrease in the rate of disproportionation of the alphaT266M ETF semiquinone catalyzed by ETF-QO.

alphaT244M mutation affects the redox, kinetic, and in vitro folding properties of Paracoccus denitrificans electron transfer flavoprotein

Threonine 244 in the alpha subunit of Paracoccus denitrificans transfer flavoprotein (ETF) lies seven residues to the amino terminus of a proposed dinucleotide binding motif for the ADP moiety of the FAD prosthetic group. This residue is highly conserved in the alpha subunits of all known ETFs, and the most frequent pathogenic mutation in human ETF encodes a methionine substitution at the corresponding position, alphaT266. The X-ray crystal structures of human and P. denitrificans ETFs are very similar. The hydroxyl hydrogen and a backbone amide hydrogen of alphaT266 are hydrogen bonded to N(5) and C(4)O of the flavin, respectively, and the corresponding alphaT244 has the same structural role in P. denitrificans ETF. We substituted a methionine for T244 in the alpha subunit of P. denitrificans ETF and expressed the mutant ETF in Escherichia coli. The mutant protein was purified, characterized, and compared with wild type P. denitrificans ETF. The mutation has no significant effect on the global structure of the protein as inferred from visible and near-ultraviolet absorption and circular dichroism spectra, far-ultraviolet circular dichroism spectra, and infrared spectra in 1H2O and 2H2O. Intrinsic fluorescence due to tryptophan of the mutant protein is 60% greater than that of the wild type ETF. This increased tryptophan fluorescence is probably due to a change in the environment of the nearby W239. Tyrosine fluorescence is unchanged in the mutant protein, although two tyrosine residues are close to the site of the mutation. These results indicate that a change in structure is minor and localized. Kinetic constants of the reductive half-reaction of ETF with porcine medium chain acyl-CoA dehydrogenase are unaltered when alphaT244M ETF serves as the substrate; however, the mutant ETF fails to exhibit saturation kinetics when the semiquinone form of the protein is used as the substrate in the disproportionation reaction catalyzed by P. denitrificans electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO). The redox behavior of the mutant ETF was also altered as determined from the equilibrium constant of the disproportionation reaction. The separation of flavin redox potentials between the oxidized/semiquinone couple and semiquinone/hydroquinone couple are -6 mV in the wild type ETF and -27 mV in the mutant ETF. The mutation does not alter the AMP content of the protein, although the extent and fidelity of AMP-dependent, in vitro renaturation of the mutant AMP-free apoETF is reduced by 57% compared to renaturation of wild type apoETF, likely due to the absence of the potential hydrogen bond donor T244.

Three-dimensional structure of human electron transfer flavoprotein to 2.1-A resolution

Mammalian electron transfer flavoproteins (ETF) are heterodimers containing a single equivalent of flavin adenine dinucleotide (FAD). They function as electron shuttles between primary flavoprotein dehydrogenases involved in mitochondrial fatty acid and amino acid catabolism and the membrane-bound electron transfer flavoprotein ubiquinone oxidoreductase. The structure of human ETF solved to 2.1-A resolution reveals that the ETF molecule is comprised of three distinct domains: two domains are contributed by the alpha subunit and the third domain is made up entirely by the beta subunit. The N-terminal portion of the alpha subunit and the majority of the beta subunit have identical polypeptide folds, in the absence of any sequence homology. FAD lies in a cleft between the two subunits, with most of the FAD molecule residing in the C-terminal portion of the alpha subunit. Alignment of all the known sequences for the ETF alpha subunits together with the putative FixB gene product shows that the residues directly involved in FAD binding are conserved. A hydrogen bond is formed between the N5 of the FAD isoalloxazine ring and the hydroxyl side chain of alpha T266, suggesting why the pathogenic mutation, alpha T266M, affects ETF activity in patients with glutaric acidemia type II. Hydrogen bonds between the 4'-hydroxyl of the ribityl chain of FAD and N1 of the isoalloxazine ring, and between alpha H286 and the C2-carbonyl oxygen of the isoalloxazine ring, may play a role in the stabilization of the anionic semiquinone. With the known structure of medium chain acyl-CoA dehydrogenase, we hypothesize a possible structure for docking the two proteins.

Cloning of glutaryl-CoA dehydrogenase cDNA, and expression of wild type and mutant enzymes in Escherichia coli

We have cloned, sequenced, and expressed cDNAs encoding wild type human glutaryl-CoA dehydrogenase subunit, and have expressed a mutant enzyme found in a patient with glutaric acidemia type I. The mutant protein is expressed at the same level as the wild type in Escherichia coli, but has less than 1% of the activity of wild-type dehydrogenase. We also present evidence that the glutaryl-CoA dehydrogenase transcript is alternatively spliced in human fibroblasts and liver; the alternatively spliced mRNA, when expressed in E.coli, encodes a stable but inactive protein. Purified expressed human glutaryl-CoA dehydrogenase has kinetic constants similar to those of the previously purified porcine dehydrogenase. The primary translation product from in vitro transcribed glutaryl-CoA dehydrogenase mRNA is translocated into mitochondria and processed in the same manner as most other nuclear-encoded mitochondrial proteins. Human glutaryl-CoA dehydrogenase shows 53% sequence similarity to porcine medium chain acyl-CoA dehydrogenase, and these similarities were utilized to predict structure-function relationships in glutaryl-CoA dehydrogenase.

Cloning, structure, and chromosome localization of the mouse glutaryl-CoA dehydrogenase gene

Glutaryl-CoA dehydrogenase (GCDH) is a nuclear-encoded, mitochondrial matrix enzyme. In humans, deficiency of GCDH leads to glutaric acidemia type I, an inherited disorder of amino acid metabolism characterized by a progressive neurodegenerative disease. In this report we describe the cloning and structure of the mouse GCDH (Gcdh) gene and cDNA and its chromosomal localization. The mouse Gcdh cDNA is 1.75 kb long and contains an open reading frame of 438 amino acids. The amino acid sequences of mouse, human, and pig GCDH are highly conserved. The mouse Gcdh gene contains 11 exons and spans 7 kb of genomic DNA. Gcdh was mapped by backcross analysis to mouse chromosome 8 within a region that is homologous to a region of human chromosome 19, where the human gene was previously mapped.

Crystallization and preliminary X-ray analysis of electron transfer flavoproteins from human and Paracoccus denitrificans

Mammalian electron transfer flavoprotein (ETF) is a soluble, heterodimeric flavoprotein responsible for the oxidation of at least nine primary matrix flavoprotein dehydrogenases. Crystals have been obtained for the recombinant human electron transfer flavoprotein (ETFhum) by the sitting-drop vapor diffusion technique using polyethylene glycol (PEG) 1500 at pH 7.0 as the precipitating agent. ETFhum crystallizes in the monoclinic space group P2(1), with unit cell parameters a = 47.46 angstrum, b = 104.10 angstrum, c = 63.79 angstrum, and beta = 110.02 degrees. Based on the assumption of one alpha beta dimer per asymmetric unit, the Vm value is 2.69 angstrum 3/Da. A native data set has been collected to 2.1 angstrum resolution. One heavy-atom derivative has also been obtained by soaking a preformed crystal of ETFhum in 2 mM thimerosal solution for 2h at 19 degrees C. Patterson analysis indicates one major site. The analogous electron transfer flavoprotein from Paracoccus denitrificans (ETFpar) has also been crystallized using PEG 8000 at pH 5.5 as the precipitating agent. ETFpar crystallizes in the orthorhombic space group P2(1)2(1)2(1), with unit cell parameters a = 79.98 angstrum, b = 182.90 angstrum, and c = 70.07 angstrum. The Vm value of 2.33 angstrum 3/Da is consistent with two alpha beta dimers per asymmetric unit. A native data set has been collected to 2.5 angstrum resolution.

Characterization of a mutation that abolishes quinone reduction by electron transfer flavoprotein-ubiquinone oxidoreductase

Two mutant alleles of the gene encoding electron transfer flavoprotein-ubiquinone oxidoreductase were identified and characterized in fibroblasts from a patient with glutaric acidemia type II. One of these alleles is a C-T transition in the donor site of an intron that causes skipping of a 222 bp exon. Included in the missing 74 amino acids is C561, which is predicted to be one of the four cysteine ligands of the 4Fe4S cluster. This mutant allele does not encode a stable ETF-QO in human fibroblasts but, when expressed in Saccharomyces cerevisiae, the mutant ETF-QO is relatively stable and properly targeted to and processed by mitochondria. The mutant protein lacks ubiquinone reductase activity, but does accept electrons from ETF in the catalyzed disproportionation of ETF semiquinone. These data suggest that in the normal protein the flavin center accepts electrons from ETF and that the 4Fe4S cluster reduces ubiquinone. Deleting the 74 amino acids also alters the association between the protein and membrane such that the mutant ETF-QO cannot be extracted from the membrane using the same conditions used for wild type ETF-QO. A site directed mutant that contains only the single amino acid substitution, C561A, exhibits the same catalytic behavior as the deletion mutant, supporting the hypothesis regarding the specific functions of the two redox centers. It is, however, solubilized by the same conditions as wild type ETF-QO.

Expression and characterization of human and chimeric human-Paracoccus denitrificans electron transfer flavoproteins

Electron transfer flavoprotein (ETF) is a heterodimer that contains a single equivalent of FAD and accepts electrons from nine flavoprotein dehydrogenases in the mitochondrial matrix. Human ETF was expressed in Escherichia coli using the expression vector previously employed to express Paracoccus denitrificans ETF (Bedzyk, L. A., Escudero, K. W., Gill, R. E., Griffin, K. J., and Frerman, F. E. (1993) J. Biol. Chem. 268, 20211-20217). cDNAs encoding the beta and alpha subunits of the human protein were inserted into the vector, mimicking the arrangement of the P. denitrificans genes in which coding sequences are joined by overlapping termination and initiation codons. A human ETF containing 30% P. denitrificans sequence at the amino terminus of the beta subunit was also expressed and purified. This chimeric ETF has 64% sequence identity with the human sequence in the substituted region. Kinetic constants of medium chain and short chain acyl-CoA dehydrogenases for the chimeric ETFs were slightly changed from those of human ETF; but, there are marked differences in the kinetic constants of sarcosine dehydrogenase and electron transfer flavoprotein-ubiquinone oxidoreductase with the two ETFs. Absorption spectra of the three redox states of human, chimeric, and P. denitrificans ETF flavins are identical. However, the flavin circular dichroism spectra of the three ETFs are characteristic for each species. The spectrum of the chimeric ETF has both human and P. denitrificans ETF features. The amplitude of the 436 nm band is identical to that of the of the human ETF flavin, but the amplitude of the 375 nm band is identical to that of the P. denitrificans ETF flavin. Thus, flavin in the chimeric ETF appears to be exposed to dipoles in the protein framework provided by human and bacterial sequences. These spectral data indicate that the flavin is located in the vicinity of the amino-terminal region of the beta subunit. The kinetic data suggest that the amino-terminal region of the beta subunit comprises part of the docking site for some primary dehydrogenases and electron transfer flavoprotein-ubiquinone oxidoreductase.

Identification of the catalytic base in long chain acyl-CoA dehydrogenase

We have used molecular modeling and site-directed mutagenesis to identify the catalytic residues of human long chain acyl-CoA dehydrogenase. Among the acyl-CoA dehydrogenases, a family of flavoenzymes involved in beta-oxidation of fatty acids, only the three-dimensional structure of the medium chain fatty acid specific enzyme from pig liver has been determined (Kim, J.-J.P., Wang, M., & Paschke, R. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 7523-7527). Despite the overall sequence homology, the catalytic residue (E376) of medium chain acyl-CoA dehydrogenase is not conserved in isovaleryl- and long chain acyl-CoA dehydrogenases. A molecular model of human long chain acyl-CoA dehydrogenase was derived using atomic coordinates determined by X-ray diffraction studies of the pig medium chain specific enzyme, interactive graphics, and molecular mechanics calculations. The model suggests that E261 functions as the catalytic base in the long-chain dehydrogenase. An altered dehydrogenase in which E261 was replaced by a glutamine was constructed, expressed, purified, and characterized. The mutant enzyme exhibited less than 0.02% of the wild-type activity. These data strongly suggest that E261 is the base that abstracts the alpha-proton of the acyl-CoA substrate in the catalytic pathway of this dehydrogenase.

Mutations and polymorphisms of the gene encoding the beta-subunit of the electron transfer flavoprotein in three patients with glutaric acidemia type II

Electron transfer flavoprotein (ETF) is a heterodimeric enzyme composed of an alpha-subunit and a beta-subunit and contains a single equivalent of FAD per dimer. ETF deficiency can be demonstrated in individuals affected by a severe metabolic disorder, glutaric acidemia type II (GAII). In this study, we have investigated for the first time the molecular basis of beta-ETF deficiency in three GAII patients: two Japanese brothers, P411 and P412, and a third unrelated patient, P485. Molecular analysis of the beta-ETF gene in P411 and P412 demonstrated that both these patients are compound heterozygotes. One allele is carrying a G to A transition at nucleotide 518, causing a missense mutation at codon 164. This point mutation is maternally derived and is not detected in 42 unrelated controls. The other allele carries a G to C transversion at the first nucleotide of the intron donor site, downstream of an exon that is skipped during the splicing event. The sequence analysis of the beta-ETF coding sequence in P485 showed only a C to T transition at nucleotide 488 that causes a Thr154 to Met substitution and the elimination of a HgaI restriction site. HgaI restriction analysis on 63 unrelated controls' genomic DNA demonstrated that the C488T transition identifies a polymorphic site. Finally, transfection of wild-type beta-ETF cDNA into P411 fibroblasts suggests that wild-type beta-ETF cDNA complements the genetic defect and restores the beta-oxidation flux to normal levels.

Molecular cloning and expression of a cDNA encoding human electron transfer flavoprotein-ubiquinone oxidoreductase

Electron-transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO) in the inner mitochondrial membrane accepts electrons from electron-transfer flavoprotein which is located in the mitochondrial matrix and reduces ubiquinone in the mitochondrial membrane. The two redox centers in the protein, FAD and a [4Fe4S]+2,+1 cluster, are present in a 64-kDa monomer. We cloned several cDNA sequences encoding the majority of porcine ETF-QO and used these as probes to clone a full-length human ETF-QO cDNA. The deduced human ETF-QO sequence predicts a protein containing 617 amino acids (67 kDa), two domains associated with the binding of the AMP moiety of the FAD prosthetic group, two membrane helices and a motif containing four cysteine residues that is frequently associated with the liganding of ferredoxin-like iron-sulfur clusters. A cleavable 33-amino-acid sequence is also predicted at the amino terminus of the 67-kDa protein which targets the protein to mitochondria. In vitro transcription and translation yielded a 67-kDa immunoprecipitable product as predicted from the open reading frame of the cDNA. The human cDNA was expressed in Saccharomyces cerevisiae, which does not normally synthesize the protein. The ETF-QO is synthesized as a 67-kDa precursor which is targeted to mitochondria and processed in a single step to a 64-kDa mature form located in the mitochondrial membrane. The detergent-solubilized protein transfers electrons from ETF to the ubiquinone homolog, Q1, indicating that both the FAD and iron-sulfur cluster are properly inserted into the heterologously expressed protein.

Cloning, sequencing, and expression of the genes encoding subunits of Paracoccus denitrificans electron transfer flavoprotein

The genes encoding the two subunits of Paracoccus denitrificans electron transfer flavoprotein (ETF) were identified by screening a genomic library constructed in pBluescript II SK+ with probes generated by amplification of genomic sequences by the polymerase chain reaction. Primers for the polymerase chain reaction were designed based on peptide sequences from purified Paracoccus ETF subunits. The genes are arranged in tandem in the genomic DNA with the deoxyadenylic acid residue in the TGA termination codon of the small subunit providing the deoxyadenylic acid residue for the ATG initiating codon of the large subunit. The deduced amino acid sequences of the ETF subunits exhibits extensive sequence identity with the human ETF subunits. The Paracoccus ETF is expressed from the pBluescript vector in Escherichia coli, yielding 30 mg of purified, catalytically active protein per liter of culture.

Redox properties of electron-transfer flavoprotein ubiquinone oxidoreductase as determined by EPR-spectroelectrochemistry

We have determined the formal potential values for each electron transfer to electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO), in order to further characterize the thermodynamics of electron transport from various acyl-CoA thioesters to the mitochondrial ubiquinone pool. ETF-QO contains one [4Fe-4S]2+,1+ cluster and one FAD prosthetic group. A preliminary visible-spectroelectrochemical titration showed that the two redox centers were reduced almost simultaneously. Since the visible spectra of the chromophores overlap, it was not possible to resolve the formal potential value for each electron transfer to the protein using this method. Accordingly, an EPR-spectroelectrochemical cell was designed so that each formal potential value could be resolved by EPR quantitation of the flavin semiquinone and the reduced iron-sulfur cluster during the titration. The formal potential values for electron transfer to ETF-ubiquinone oxidoreductase at pH 7.5 and 4 degrees C were E1 degrees' = +0.028 V and E2 degrees' = -0.006 V for the first and second electron transfers, respectively, to the FAD and E degrees' = +0.047 V for the iron-sulfur cluster. The thermodynamics of electron transport from the acyl-CoA substrates of beta-oxidation to the mitochondrial electron transport chain have been fully resolved with completion of this work. The results are discussed in terms of their significance to the overall electron transport process from beta-oxidation.

Structural and redox relationships between Paracoccus denitrificans, porcine and human electron-transferring flavoproteins

Electron-transferring flavoprotein (ETF) was purified from the bacterium Paracoccus denitrificans and the structural and redox relationships to the porcine and human ETFs were investigated. The three proteins have essentially identical subunit masses and the alpha-helix content of the bacterial and porcine ETFs are very similar, indicating global structural similarity. An anti-(porcine ETF) polyclonal antibody that crossreacts with the human large and small subunits also crossreacts strongly with the large subunit of Paracoccus ETF. However, crossreactivity with the small subunit is very weak. Nonetheless, an amino-terminal peptide and four internal peptides of the small bacterial subunit show extensive sequence identity with the human small subunit. Local similarities in environment are also indicated by the intrinsic tryptophan fluorescence emission spectra of porcine and Paracoccus ETFs. Although the visible spectra of porcine and Paracoccus ETFs are virtually identical, flavin fluorescence in the bacterial protein is only 15% that of the mammalian protein. Further, the circular dichroic spectrum of the flavin in the bacterial protein is significantly more intense, suggesting that the microenvironment of the isoalloxazine ring is different in the two proteins. Enzymatic or photochemical reduction of Paracoccus ETF rapidly yields an anionic semiquinone; formation of the fully reduced flavin in the bacterial ETF is very slow. The spacing of the oxidation-reduction potentials of the flavin couples in the bacterial ETF is essentially identical to that in procine ETF as judged from the disproportionation equilibrium of the bacterial ETF flavin semiquinone. Together, the enzymatic reduction and disproportionation equilibria suggest that the flavin potentials of the two ETFs must be very close. The data indicate that the structural properties of the bacterial and mammalian proteins and the thermodynamic properties of the flavin prosthetic group of the proteins are very similar.

Quantitation of acyl-CoA and acylcarnitine esters accumulated during abnormal mitochondrial fatty acid oxidation

We have used radio-high pressure liquid chromatography to study the acyl-CoA ester intermediates and the acylcarnitines formed during mitochondrial fatty acid oxidation. During oxidation of [U-14C]hexadecanoate by normal human fibroblast mitochondria, only the saturated acyl-CoA and acylcarnitine esters can be detected, supporting the concept that the acyl-CoA dehydrogenase step is rate-limiting in mitochondrial beta-oxidation. Incubations of fibroblast mitochondria from patients with defects of beta-oxidation show an entirely different profile of intermediates. Mitochondria from patients with defects in electron transfer flavoprotein and electron transfer flavoprotein:ubiquinone oxido-reductase are associated with slow flux through beta-oxidation and accumulation of long chain acyl-CoA and acylcarnitine esters. Increased amounts of saturated medium chain acyl-CoA and acylcarnitine esters are detected in the incubations of mitochondria with medium chain acyl-CoA dehydrogenase deficiency, whereas long chain 3-hydroxyacyl-CoA dehydrogenase deficiency is associated with accumulation of long chain 3-hydroxyacyl- and 2-enoyl-CoA and carnitine esters. These studies show that the control strength at the site of the defective enzyme has increased. Radio-high pressure liquid chromatography analysis of intermediates of mitochondrial fatty acid oxidation is an important new technique to study the control, organization and defects of the enzymes of beta-oxidation.

Tryptophan fluorescence in electron-transfer flavoprotein:ubiquinone oxidoreductase: fluorescence quenching by a brominated pseudosubstrate

We have studied the intrinsic fluorescence of the 12 tryptophan residues of electron-transfer flavoprotein:ubiquinone oxidoreductase (ETF:QO). The fluorescence emission spectrum (lambda ex 295 nm) showed that the fluorescence is due to the tryptophan residues and that the contribution of the 22 tyrosine residues is minor. The emission maximum (lambda m 334 nm) and the bandwidth (delta lambda 1/2 56 nm) suggest that the tryptophans lie in hydrophobic environments in the oxidized protein. Further, these tryptophans are inaccessible to a range of ionic and nonionic collisional quenching agents, indicating that they are buried in the protein. Enzymatic or chemical reduction of ETF:QO results in a 5% increase in fluorescence with no change of lambda m or delta lambda 1/2. This change is reversible upon reoxidation and is likely to reflect a conformational change in the protein. The ubiquinone analogue Q0(CH2)10Br, a pseudosubstrate of ETF:QO (Km = 2.6 microM; kcat = 210 s-1), specifically quenches the fluorescence of one tryptophan residue (Kd = 1.6-3.2 microM) in equilibrium fluorescence titrations. The ubiquinone homologue UQ-2 (Km = 2 microM; kcat = 162 s-1) and the analogue Q0(CH2)10OH (Km = 2 microM; kcat = 132 s-1) do not quench tryptophan fluorescence; thus the brominated analogue acts as a static heavy atom quencher. We also describe a rapid purification for ETF:QO based on extraction of liver submitochondrial particles with Triton X-100 and three chromatographic steps, which results in yields 3 times higher than previously published methods.

Electron transfer flavoprotein: ubiquinone oxidoreductase (ETF:QO) deficiency in an adult

A 19-year-old woman with mild myopathic symptoms from age 6 and fasting intolerance presented with a Reye-like syndrome and a myopathy. Investigations disclosed a lipid storage myopathy, type II glutaric acidemia, and carnitine deficiency in skeletal muscle. Riboflavin and carnitine treatment corrected the metabolic abnormalities and she improved clinically. She later died from pulmonary complications secondary to aspiration. Subsequent studies established electron transfer flavoprotein: ubiquinone oxidoreductase (ETF:QO) deficiency (fibroblast ETF:QO activity was 2.9 mU/mg, normal range is 14.1 +/- 3.8 mU/mg) as the cause of her illness. This is the first documented case of ETF:QO diagnosed in an adult.

Glutaric acidemia type II: heterogeneity of clinical and biochemical phenotypes

We have examined 23 fibroblast lines from patients with neonatal and late onset glutaric acidemia type II and fibroblasts from four parents of these patients. Fifteen of these patients are previously unreported. Results of these investigations show deficiency of electron transfer flavoprotein or electron transfer flavoprotein-ubiquinone oxidoreductase activity in all of the patients' fibroblasts. Immunoblots indicate that the steady state levels of the antigens is very low or undetectable in most of the neonatal onset patients; however, cross-reacting antigen without electron transfer activity is observed in several glutaric acidemia type II fibroblast lines. Assay of parental lines confirm the autosomal transmission of deficiencies of proteins. Of particular interest is the clinical heterogeneity among these patients. Patients may present with an extrapyramidal movement disorder as observed in glutaric aciduria type I, without the typical organic aciduria typical of glutaric acidemia type II even in the presence of severe enzyme deficiency, or with renal cystic dysplasia accompanying electron transfer flavoprotein deficiency. Renal cystic dysplasia had previously been reported only in patients with electron transfer flavoprotein-ubiquinone oxidoreductase deficiency.

Glutaric aciduria type II: autopsy study of a case with electron-transferring flavoprotein dehydrogenase deficiency

An autopsy study of glutaric aciduria type II in a 62-day-old Japanese boy is presented. The diagnosis was made by analysis of organic acids in the urine. Immunoblot analysis of liver homogenate confirmed the diagnosis, revealing absence of electron-transferring flavoprotein dehydrogenase. The major findings were fatty changes of variable degree in many organs and tissues, the most severe being found in cardiac myocytes, hepatocytes, renal tubular epithelium, and skeletal muscle fibers. Other pertinent findings included multicystic and dysplastic kidney, pulmonary alveolar proteinosis, and spongiosis and gliosis of the spinal cord. The thymus was markedly depleted, and lymphocytes in the lymph nodes were mainly B cells. Although some of these changes may have been secondary to the sepsis and immunosuppression complicating 2 months of intensive care, the abnormal organic acid metabolism with severe acidosis may have been a significant contributing factor.

Glutaric aciduria type II: review of the phenotype and report of an unusual glomerulopathy

A male infant with glutaric aciduria II secondary to electron transfer flavoprotein: ubiquinone oxidoreductase deficiency is compared to previously reported cases of glutaric aciduria II. A common pattern of anomalies in patients with malformations (8/16) includes macrocephaly, large anterior fontanelle, high forehead, flat nasal bridge, telecanthus, and malformed ears. Abnormalities such as hypotonia, cerebral gliosis, heterotopias, hepatomegaly, hepatic periportal necrosis, polycystic kidneys, and genital defects in glutaric aciduria II are reminiscent of those in Zellweger syndrome, whereas elevations of glutaric, ethylmalonic, adipic, and isovaleric acids are quite distinctive. A unique ultrastructural alteration of the glomerular basement membrane was observed in the proposita. This manifestation may represent an early stage in renal cyst formation and provide a diagnostic criterion for glutaric aciduria II when enzyme studies are unavailable.

Striatal degeneration in glutaric acidaemia type II

A girl of first cousin parents presented in the 1st year of life with a progressive neurological disease with muscle weakness and hypotonia, accompanied later by dystonia. Investigations, including gas chromatography of urine, showed no abnormality. Autopsy showed marked neuronal loss and gliosis in the putamen and globus pallidus. The activity of glutaryl-CoA dehydrogenase in cultured fibroblasts was normal, but the activity of electron transfer flavoprotein was markedly diminished. Retrospective study of urine by capillary gas chromatography/mass spectrometry showed small amounts of glutaric and other organic acids. This is the first report of striatal degeneration in association with glutaric acidaemia type II. The neuropathological changes were milder than those in glutaric acidaemia type I.

Electron paramagnetic resonance and magnetic circular dichroism studies of electron-transfer flavoprotein-ubiquinone oxidoreductase from pig liver

Pig liver electron-transfer flavoprotein-ubiquinone oxidoreductase has been investigated by room temperature UV-visible, low-temperature electron paramagnetic resonance and low-temperature magnetic circular dichroism spectroscopies. The results provide unambiguous evidence for the presence of a single [4Fe-4S] cluster that is diamagnetic in the isolated enzyme and becomes paramagnetic with an S = 1/2 ground state on reduction with dithionite or enzymatically with the physiological electron donor. The EPR data for samples at pH 7.8 indicate that FAD is reduced by one electron to the anionic semiquinone form in the enzymatically reduced enzyme, and by two electrons to the hydroquinone form by excess dithionite. The possibility of weak spin-spin interaction between the FAD semiquinone and the [4Fe-4S]1+ center is discussed in the light of the observation of a small increase in the linewidth of the Fe-S EPR in enzymatically reduced samples.

Reaction of electron-transfer flavoprotein ubiquinone oxidoreductase with the mitochondrial respiratory chain

Submitochondrial particles catalyze the reduction of electron-transfer flavoprotein (ETF) by NADH and succinate under anaerobic conditions in reactions that are totally inhibited by rotenone and thenoyl trifluoroacetone, respectively. The particles also catalyze the ATP-dependent reduction of NAD+ by enzymatically reduced ETF. The latter reaction is inhibited by rotenone and carbonyl cyanide chlorophenylhydrazone and all three reactions are inhibited by antibody to electrontransfer flavoprotein-ubiquinone oxidoreductase (ETF-QO). These observations indicated that ETF-QO reacts with the pool of ubiquinone that is reduced by NADH and succinic dehydrogenases. Consistent with this hypothesis, NADH- and succinic-ETF reductase activities are inhibited 99% in ubiquinone-depleted particles, and reincorporation of exogenous ubiquinone restores at least 90% of these activities. Reduction of the bc1 complex by ETF and acyl CoA oxidase activity are also inhibited by antibody to ETF-QO. Myxothiazole and antimycin which inhibit the quinonol oxidation and quinone reduction sites, respectively, in the bc1 complex also inhibit electron transport from ETF-QO through the complex according to current models of the Q-cycle (Rich, P.R. (1986) J. Bioenerg. Biomembranes 18, 145-156). The results show that ETF-QO is an obligatory component of the electron transport pathway between ETF and the ubiquinone pool and suggest a mechanism for the steady-state turnover of ETF-QO.

Recent progress in understanding glutaric acidemias

Glutaric acidemia, which is due to inherited deficiency of glutaryl-CoA dehydrogenase, is characterized clinically by progressive dystonia and dyskinesia in childhood, and pathologically by degeneration of the caudate and putamen. Results using newer imaging techniques (computer tomography and magnetic resonance image scanning) suggest that neurological involvement in this condition begins before birth, and that gliosis of the basal ganglia is a relatively late event. Glutaric acidemia type II is usually due to inherited deficiency of electron transfer flavoprotein (ETF) or ETF:ubiquinone oxidoreductase, but some patients with typical disease may have another, to date undefined, abnormality. There may also be a clinical phenotype of glutaric acidemia type II which, like glutaryl-CoA dehydrogenase deficiency, is characterized by a movement disorder and by degeneration of the basal ganglia.

Systemic carnitine deficiency due to lack of electron transfer flavoprotein:ubiquinone oxidoreductase

A child with myopathy and systemic carnitine deficiency died at age 8 years in an acute metabolic attack. He had glutaric aciduria type II, and his cultured fibroblasts contained normal activity of four different acyl CoA dehydrogenases, but there was deficiency of electron transfer flavoprotein:ubiquinone oxidoreductase (ETF-QO). This enzyme is thought to reduce coenzyme Q in the respiratory chain, funneling reducing equivalents from seven flavoproteins in the beta-oxidation of acyl CoAs. There was massive urinary excretion of the short-chain acylcarnitines that accumulated in mitochondria as a result of the ETF-QO defect. Carnitine therefore acts as a buffer for excessive accumulation of intramitochondrial acyl CoAs, and defective beta-oxidation can cause carnitine insufficiency.

Electron-transfer flavoprotein-ubiquinone oxidoreductase from pig liver: purification and molecular, redox, and catalytic properties

Electron-transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO) was purified to homogeneity from pig liver submitochondrial particles. It is comparable in molecular weight and general properties to ETF-QO from beef heart [Ruzicka, F. J., & Beinert, H. (1977) J. Biol. Chem. 252, 8440-8445], and the electron spin resonance signals of the reduced iron-sulfur cluster are essentially identical. ETF-QO catalyzes the transfer of electrons from electron-transfer flavoprotein (ETF) to nitro blue tetrazolium, with a sluggish reaction turnover number of about 10-30 min-1. In contrast, the enzyme rapidly disproportionates ETF semiquinone, with a turnover number of 200 s-1. The reverse reaction, comproportionation of oxidized and hydroquinone ETF, provides an enzymatic assay for ETF-QO with picomolar sensitivity. Equilibrium spectrophotometric titrations show that ETF-QO accepts a maximum of two electrons from ETF and accepts three electron equivalents from dithionite or by photochemical reduction. All electrons from the enzymatically or chemically reduced protein can be transferred to 2,3-dimethoxy-5-methyl-6-pentyl-1,4-benzoquinone (PB), and this reaction is readily reversible. Reduction of ETF-QO by 2,3-dimethoxy-5-methyl-6-pentyl-1,4-benzohydroquinone is pH dependent and indicates the enzyme to have a redox potential that decreases by 47 mV per pH unit. Therefore, ETF-QO binds one to two protons upon reduction. The EO' at pH 7.3 is 38 mV. The ability of ETF-QO to catalyze the equilibration of ETF redox states has been used to evaluate the equilibrium 2ETFsq + nH+ in equilibrium ETFox + ETFhq.(ABSTRACT TRUNCATED AT 250 WORDS)

Reaction of electron-transfer flavoprotein with electron-transfer flavoprotein-ubiquinone oxidoreductase

The oxidative half-reaction of electron-transfer flavoprotein (ETF), electron transfer from ETF to electron-transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO), is dependent on complementary surface charges on the two proteins. ETF is the positively charged member of the redox pair. The evidence is based on the pH and ionic strength dependencies of the comproportionation of oxidized ETF and ETF hydroquinone catalyzed by ETF-QO and on the effects of chemical modification of ETF on the comproportionation reaction. Acetylation of one and five epsilon-amino groups of lysyl residues results in 3- and 13-fold increases, respectively, in the Km of ETF-QO for ETF but no change in Vmax. Amidination, which maintains positive charge at modified loci, has no effect on steady-state kinetic constants. These chemical modifications have no effect on the equilibrium constant for equilibration of ETF redox states. The Km of ETF-QO for ETF is pH dependent above pH 8.5, suggesting titration of lysyl residues as previously observed in studies of the reductive half-reaction of ETF [Beckmann, J. D., & Frerman, F. E. (1983) J. Biol. Chem. 258, 7563-7569]. The ionic strength dependence of TN/KmETF for the reaction follows the limiting Brønsted equation ln (TN/Km) = ln k0 + 2 alpha Z1Z2I1/2, and Z1Z2, the product of charges on the reacting proteins, is similar to the value of Z1Z2 for the reductive half-reaction of ETF by the general acyl-CoA dehydrogenase. The ETF-QO-catalyzed comproportionation reaction exhibits a primary deuterium isotope effect in D2O, perhaps indicating the participation of solvent water in the electron-transfer reaction.

Long-chain acyl coenzyme A dehydrogenase deficiency: an inherited cause of nonketotic hypoglycemia

Three children from unrelated families presented in early childhood with hypoglycemia and cardiorespiratory arrests associated with fasting. Significant hepatomegaly, cardiomegaly, and hypotonia were present at the time of initial presentation. Ketones were not present in the urine at the time of hypoglycemia in any patient; however, dicarboxylic aciduria was documented in one patient at the time of the acute episode and in two patients during fasting studies. Total plasma carnitine concentration was low with an increased esterified carnitine fraction. These findings suggested a defect in mitochondrial fatty acid oxidation, and specific assays were performed for the acyl coenzyme A (CoA) dehydrogenases. These analyses showed that the activity of the long-chain acyl CoA dehydrogenase was less than 10% of control values in fibroblasts, leukocytes, and liver tissue. Activities of the medium-chain, short-chain, and isovaleryl CoA dehydrogenases were not different from control values. With cultured fibroblasts, CO2 evolution from long-chain fatty acids was significantly reduced, while CO2 evolution from medium-chain and short-chain fatty acids was comparable to control values--findings consistent with a defect early in the beta-oxidation sequence. Studies of acyl CoA dehydrogenase activities in fibroblasts and leukocytes from parents of the patients showed levels of long-chain acyl CoA dehydrogenase activity intermediate between affected and control values and indicated an autosomal recessive form of inheritance of this enzymatic defect.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial metabolism of 3-mercaptopropionic acid. Chemical synthesis of 3-mercaptopropionyl coenzyme A and some of its S-acyl derivatives

The metabolism of 3-mercaptopropionic acid in mitochondria was studied by use of purified mitochondrial enzymes and rat heart mitochondria. Metabolites of 3-mercaptopropionic acid were separated by high performance liquid chromatography and identified by comparing them with chemically synthesized derivatives of 3-mercaptopropionic acid. The initial step in the metabolism of 3-mercaptopropionic acid is its conversion to a CoA thioester, most likely catalyzed by medium-chain acyl-CoA synthetase. The resulting 3-mercaptopropionyl-CoA is a poor substrate of acyl-CoA dehydrogenase but substitutes effectively for CoASH in reactions catalyzed by 3-ketoacyl-CoA thiolase and acetoacetyl-CoA thiolase. S-Acyl-3-mercaptopropionyl-CoA thioesters formed in the thiolase-catalyzed reactions are not at all or only poorly acted upon by acyl-CoA dehydrogenases. However, they are hydrolyzed by thioesterase(s) to CoASH and S-acyl-3-mercaptopropionic acid. The hydrolysis of S-acyl-3-mercaptopropionyl-CoA thioesters proceeds more rapidly than the hydrolysis of fatty acyl-CoA thioesters of comparable chain lengths. Free CoASH is also regenerated from S-acetyl-3-mercaptopropionyl-CoA and more rapidly from 3-mercaptopropionyl-CoA as a result of their reactions with carnitine catalyzed by carnitine acetyltransferase. These findings lead to the suggestion that the major mitochondrial CoA-containing metabolites of 3-mercaptopropionic acid are S-acyl-3-mercaptopropionyl-CoA thioesters.

Fluorometric assay of acyl-CoA dehydrogenases in normal and mutant human fibroblasts

A fluorimetric, ETF-linked procedure to determine activities of acyl-CoA dehydrogenase in cultured human fibroblasts is described. The assay readily distinguishes between cell lines deficient in medium-chain acyl-CoA dehydrogenase, long-chain acyl-CoA dehydrogenase, isovaleryl-CoA dehydrogenase, and controls, and may allow for the diagnosis of heterozygous carriers of these disorders. The method has been made feasible with the development of rapid and efficient procedures to isolate ETF, and offers several advantages over procedures that are currently employed.

Crystallization and preliminary X-ray data for the general acyl-CoA dehydrogenase

The general acyl-CoA dehydrogenase from pig liver mitochondria has been crystallized in a form suitable for detailed three-dimensional x-ray structure analysis. Crystals grown from Tris buffer and polyethylene glycol solution diffract to high resolution and have the space group C2221, a = 128.2, b = 136.1, and c = 106.3 A. A measured crystal density of 1.178 g/cm3 and a crystal volume/unit of molecular mass, Vm = 2.5 A3/dalton, suggest that the asymmetric unit contains two monomers of the tetrameric dehydrogenase molecule.

Glutaric acidaemia type II (multiple acyl-CoA dehydrogenation deficiency)

The clinical and biochemical phenotype of glutaric acidaemia type II (GAII) has led to the suggestion that the defect in the disorder affects electron transfer from primary FAD-containing dehydrogenases into the respiratory chain. Two proteins are involved in this process, i.e. electron transfer flavoprotein (ETF) and ETF dehydrogenase, an iron--sulphur flavoprotein with a distinctive EPR signal. Reliable catalytic assays for these proteins are not available, but both proteins have been purified and antisera against them prepared in rabbits. SDS-PAG electrophoresis of liver mitochondrial membranes from a GAII infant with congenital anomalies, locating ETF dehydrogenase with specific antiserum, showed no cross-reactive material. EPR of the same membranes showed a marked decrease in the ETF dehydrogenase signal. These results suggest that the defect in GAII in some patients is indeed in electron transport, and specifically in ETF dehydrogenase.

Chemical and catalytic properties of the peroxisomal acyl-coenzyme A oxidase from Candida tropicalis

The peroxisomal acyl-CoA oxidase has been purified from extracts of the yeast Candida tropicalis grown with alkanes as the principal energy source. The enzyme has a molecular weight of 552,000 and a subunit molecular weight of 72,100. Using an experimentally determined molar extinction coefficient for the enzyme-bound flavin, a minimum molecular weight of 146,700 was determined. Based on these data, the oxidase contains eight perhaps identical subunits and four equivalents of FAD. No other beta-oxidation enzyme activities are detected in purified preparations of the oxidase. The oxidase flavin does not react with sulfite to form an N(5) flavin-sulfite complex. Photochemical reduction of the oxidase flavin yields a red semiquinone; however, the yield of semiquinone is strongly pH dependent. The yield of semiquinone is significantly reduced below pH 7.5. The flavin semiquinone can be further reduced to the hydroquinone. The behavior of the oxidase flavin during photoreduction and its reactivity toward sulfite are interpreted to reflect the interaction in the N(1)-C(2)O region of the flavin with a group on the protein which acts as a hydrogen-bond acceptor. Like the acyl-CoA dehydrogenases which catalyze the same transformation of acyl-CoA substrates, the oxidase is inactivated by the acetylenic substrate analog, 3-octynoyl-CoA, which acts as an active site-directed inhibitor.

The effects of pH, ionic strength, and chemical modifications on the reaction of electron transfer flavoprotein with an acyl coenzyme A dehydrogenase

The effects of pH and ionic strength on the steady state kinetic parameters for reduction of electron transfer flavoprotein (ETF) by general acyl-CoA dehydrogenase were determined. The effect of pH on the turnover number (TN) of the reaction indicates the participation of an essential base with a pK alpha of 6.9. The KmETF of the dehydrogenase is invariant between pH 5.4 and 8.5, but increases 40-fold between pH 8.5 and 9.8. The parameter TN/KmETF follows the limiting Bronsted equation (In TN/KmETF = ln ko + 2.34ZAZB I 1/2) at ionic strength values between 0.01 and 0.125 M, indicating complementary charge interactions between the two flavoproteins. Covalent modifications of amino groups of ETF with trinitrobenzene sulfonate and acetic anhydride remove positive charges and result in an increase in KmETF of the dehydrogenase with no change of TN. However, exhaustive acetimidation of ETF amino groups, which maintains cationic charge at modified loci, does not alter the steady state kinetic parameters of the reaction. These results, in conjunction with previous chemical covalent modifications of dehydrogenase carboxyl residues (Frerman, F. E., Mielke, D., and Huhta, K. (1980) J. Biol. Chem. 255, 2199-2202), indicate that general acyl-CoA dehydrogenase and ETF interact in an electrostatic manner.

Leucine catabolism during the differentiation of 3T3-L1 cells. Expression of a mitochondrial enzyme system

Leucine can be utilized efficiently as a precursor for lipid biosynthesis by adipose tissue, especially in the presence of glucose or glucose and insulin. During the differentiation of 3T3-L1 fibroblasts to adipocytes, the rate of lipid biosynthesis from L-[U-14C]leucine increases at least 30-fold and lipogenesis, with [U-14C] acetate as the precursor, increases by 10- to 15-fold. The specific activities of two mitochondrial dehydrogenases in the leucine oxidative pathway, the branched chain alpha-ketoacid dehydrogenase and isovaleryl-CoA dehydrogenase, as well as of leucine:alpha-ketoglutarate transaminase, increase at least 20-fold during the adipose conversion. Isovaleryl-CoA dehydrogenase was assayed in crude extracts using a specific fluorimetric method employing electron transfer flavoprotein as the electron acceptor for the flavoprotein dehydrogenase. The specific activity of 3-hydroxy-3-methylglutaryl-CoA lyase, the mitochondrial enzyme catalyzing the terminal reaction in the leucine degradation pathway, increases 4-fold during differentiation. The increases in the specific activities of the mitochondrial enzymes occur without a change in the specific activity of cytochrome oxidase, indicating that the increases do not simply reflect proliferation of mitochondria. The biosynthesis of at least 20 soluble mitochondrial polypeptides is enhanced during the adipose conversion of the fibroblasts as determined by polyacrylamide gel electrophoresis following incubation of the cells with [35S] methionine. The results provide a conservative estimate of the extent of changes in mitochondrial soluble proteins during the adipose conversion. They also establish that differentiated 3T3-L1 adipocytes metabolize leucine like mature adipose tissue and illustrate the roles of the branched chain alpha-ketoacid dehydrogenase and isovaleryl-CoA dehydrogenase in lipogenesis.