Characterization of human and pig kidney long-chain-acyl-CoA dehydrogenases and their role in beta-oxidation

Structural basis for expanded substrate specificities of human long chain acyl-CoA dehydrogenase and related acyl-CoA dehydrogenases

Crystal structures of human long-chain acyl-CoA dehydrogenase (LCAD) and the catalytically inactive Glu291Gln mutant, have been determined. These structures suggest that LCAD harbors functions beyond its historically defined role in mitochondrial β-oxidation of long and medium-chain fatty acids. LCAD is a homotetramer containing one FAD per 43 kDa subunit with Glu291 as the catalytic base. The substrate binding cavity of LCAD reveals key differences which makes it specific for longer and branched chain substrates. The presence of Pro132 near the start of the E helix leads to helix unwinding that, together with adjacent smaller residues, permits binding of bulky substrates such as 3α, 7α, l2α-trihydroxy-5β-cholestan-26-oyl-CoA. This structural element is also utilized by ACAD11, a eucaryotic ACAD of unknown function, as well as bacterial ACADs known to metabolize sterol substrates. Sequence comparison suggests that ACAD10, another ACAD of unknown function, may also share this substrate specificity. These results suggest that LCAD, ACAD10, ACAD11 constitute a distinct class of eucaryotic acyl CoA dehydrogenases.

Structural Basis for Expanded Substrate Specicities of Human Long Chain Acyl-CoA Dehydrogenase and Related Acyl- CoA Dehydrogenases

Crystal structures of human long-chain acyl-CoA dehydrogenase (LCAD) and the E291Q mutant, have been determined. These structures suggest that LCAD harbors functions beyond its historically defined role in mitochondrial β-oxidation of long and medium-chain fatty acids. LCAD is a homotetramer containing one FAD per 43kDa subunit with Glu291 as the catalytic base. The substrate binding cavity of LCAD reveals key differences which makes it specific for longer and branched chain substrates. The presence of Pro132 near the start of the E helix leads to helix unwinding that, together with adjacent smaller residues, permits binding of bulky substrates such as 3α, 7α, l2α-trihydroxy-5β-cholestan-26-oyl-CoA. This structural element is also utilized by ACAD11, a eucaryotic ACAD of unknown function, as well as bacterial ACADs known to metabolize sterol substrates. Sequence comparison suggests that ACAD10, another ACAD of unknown function, may also share this substrate specificity. These results suggest that LCAD, ACAD10, ACAD11 constitute a distinct class of eucaryotic acyl CoA dehydrogenases.

A clinical experience-based Chinese herbal formula improves ethanol-induced drunken behavior and hepatic steatohepatitis in mice models

BACKGROUND

Bao-Gan-Xing-Jiu-Wan (BGXJW) is a clinical experience-based Chinese herbal formula. Its efficacy, pharmacological safety, targeted function, process quality, and other aspects have met the evaluation standards and the latest requirements of preparations. It could prevent and alleviate the symptoms of drunkenness and alcoholic liver injury clinically. The present work aims to elucidate whether BGXJW could protect against drunkenness and alcoholic liver disease in mice and explore the associated mechanism.

MATERIAL AND METHODS

We used acute-on-chronic (NIAAA) mice model to induce alcoholic steatosis, and alcohol binge-drinking model to reappear the drunk condition. BGXJW at indicated doses were administered by oral gavage respectively to analyze its effects on alcoholic liver injury and the associated molecular mechanisms.

RESULTS

BGXJW had no cardiac, hepatic, renal, or intestinal toxicity in mice. Alcoholic liver injury and steatosis in the NIAAA mode were effectively prevented by BGXJW treatment. BGXJW increased the expression of alcohol metabolizing enzymes ADH, CYP2E1, and ALDH2 to enhance alcohol metabolism, inhibited steatosis through regulating lipid metabolism, counteracted alcohol-induced upregulation of lipid synthesis related proteins SREBP1, FASN, and SCD1, meanwhile it enhanced fatty acids β-oxidation related proteins PPAR-α and CPT1A. Alcohol taken enhanced pro-inflammatory TNF-α, IL-6 and down-regulated the anti-inflammatory IL-10 expression in the liver, which were also reversed by BGXJW administration. Moreover, BGXJW significantly decreased the blood ethanol concentration and alleviated drunkenness in the alcohol binge-drinking mice model.

CONCLUSIONS

BGXJW could effectively relieve drunkenness and prevent alcoholic liver disease by regulating lipid metabolism, inflammatory response, and alcohol metabolism.

A role for taurine in mitochondrial function

The mitochondrial pH gradient across the inner-membrane is stabilised by buffering of the matrix. A low-molecular mass buffer compound has to be localised in the matrix to maintain its alkaline pH value. Taurine is found ubiquitously in animal cells with concentrations in the millimolar range and its pKa value is determined to 9.0 (25 degrees C) and 8.6 (37 degrees C), respectively. Localisation of such a low-molecular buffer in the mitochondrial matrix, transforms the matrix into a biochemical reaction chamber for the important matrix-localised enzyme systems. Three acyl-CoA dehydrogenase enzymes, which are pivotal for beta-oxidation of fatty acids, are demonstrated to have optimal activity in a taurine buffer. By application of the model presented, taurine depletion caused by hyperglycemia could provide a link between mitochondrial dysfunction and diabetes.

Identification of palmitoylated mitochondrial proteins using a bio-orthogonal azido-palmitate analogue

Increased levels of circulating saturated free fatty acids, such as palmitate, have been implicated in the etiology of type II diabetes and cancer. In addition to being a constituent of glycerolipids and a source of energy, palmitate also covalently attaches to numerous cellular proteins via a process named palmitoylation. Recognized for its roles in membrane tethering, cellular signaling, and protein trafficking, palmitoylation is also emerging as a potential regulator of metabolism. Indeed, we showed previously that the acylation of two mitochondrial proteins at their active site cysteine residues result in their inhibition. Herein, we sought to identify other palmitoylated proteins in mitochondria using a nonradioactive bio-orthogonal azido-palmitate analog that can be selectively derivatized with various tagged triarylphosphines. Our results show that, like palmitate, incorporation of azido-palmitate occurred on mitochondrial proteins via thioester bonds at sites that could be competed out by palmitoyl-CoA. Using this method, we identified 21 putative palmitoylated proteins in the rat liver mitochondrial matrix, a compartment not recognized for its content in palmitoylated proteins, and confirmed the palmitoylation of newly identified mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase. We postulate that covalent modification and perhaps inhibition of various mitochondrial enzymes by palmitoyl-CoA could lead to the metabolic impairments found in obesity-related diseases.

The AidB component of the Escherichia coli adaptive response to alkylating agents is a flavin-containing, DNA-binding protein

Upon exposure to alkylating agents, Escherichia coli increases expression of aidB along with three genes (ada, alkA, and alkB) that encode DNA repair proteins. In order to begin to identify the role of AidB in the cell, the protein was purified to homogeneity, shown to possess stoichiometric amounts of flavin adenine dinucleotide (FAD), and confirmed to have low levels of isovaleryl-coenzyme A (CoA) dehydrogenase activity. A homology model of an AidB homodimer was constructed based on the structure of a four-domain acyl-CoA oxidase. The predicted structure revealed a positively charged groove connecting the two active sites and a second canyon of positive charges in the C-terminal domain, both of which could potentially bind DNA. Three approaches were used to confirm that AidB binds to double-stranded DNA. On the basis of its ability to bind DNA and its possession of a redox-active flavin, AidB is predicted to catalyze the direct repair of alkylated DNA.

A single arginine residue is required for the interaction of the electron transferring flavoprotein (ETF) with three of its dehydrogenase partners

The interaction of several dehydrogenases with the electron transferring flavoprotein (ETF) is a crucial step required for the successful transfer of electrons into the electron transport chain. The exact determinants regarding the interaction of ETF with its dehydrogenase partners are still unknown. Chemical modification of ETF with arginine-specific reagents resulted in the loss, to varying degrees, of activity with medium chain acyl-coenzyme A dehydrogenase (MCAD). The kinetic profiles showed the inactivations followed pseudo-first-order kinetics for all reagents used. For activity with MCAD, maximum inactivation of ETF was accomplished by 2,3-butanedione (4% residual activity after 120 min) and it was shown that modification of one arginine residue was responsible for the inactivation. Almost 100% restoration of this ETF activity was achieved upon incubation with free arginine. However, the same 2,3-butanedione modified ETF only possessed decreased activity with dimethylglycine-(DMGDH, 44%) and sarcosine- (SDH, 27%) dehydrogenases unlike the abolition with MCAD. Full protection of ETF from arginine modification by 2,3-butanedione was achieved using substrate-protected DMGDH, MCAD and SDH respectively. Cross-protection studies of ETF with the three dehydrogenases implied use of the same single arginine residue in the binding of all three dehydrogenases. These results lead us to conclude that this single arginine residue is essential in the binding of the ETF to MCAD, but only contributes partially to the binding of ETF to SDH and DMGDH and thus, the determinants of the dehydrogenase binding sites overlap but are not identical.

Binding of the human "electron transferring flavoprotein" (ETF) to the medium chain acyl-CoA dehydrogenase (MCAD) involves an arginine and histidine residue

The interaction between the "electron transferring flavoprotein" (ETF) and medium chain acyl-CoA dehydrogenase (MCAD) enables successful flavin to flavin electron transfer, crucial for the beta-oxidation of fatty acids. The exact biochemical determinants for ETF binding to MCAD are unknown. Here we show that binding of human ETF, to MCAD, was inhibited by 2,3-butanedione and diethylpyrocarbonate (DEPC) and reversed by incubation with free arginine and hydroxylamine respectively. Spectral analyses of native ETF vs modified ETF suggested that flavin binding was not affected and that the loss of ETF activity with MCAD involved modification of one ETF arginine residue and one ETF histidine residue respectively. MCAD and octanoyl-CoA protected ETF against inactivation by both 2,3-butanedione and DEPC indicating that the arginine and histidine residues are present in or around the MCAD binding site. Comparison of exposed arginine and histidine residues among different ETF species, however, indicates that arginine residues are highly conserved but that histidine residues are not. These results lead us to conclude that this single arginine residue is essential for the binding of ETF to MCAD, but that the single histidine residue, although involved, is not.

Long-chain acyl-CoA dehydrogenase is a key enzyme in the mitochondrial beta-oxidation of unsaturated fatty acids

The first reaction of mitochondrial beta-oxidation, which is catalyzed by acyl-CoA dehydrogenases, was studied with unsaturated fatty acids that have a double bond either at the 4,5 or 5,6 position. The CoA thioesters of docosahexaenoic acid, arachidonic acid, 4,7,10-cis-hexadecatrienoic acid, 5-cis-tetradecenoic acid, and 4-cis-decenoic acid were effectively dehydrogenated by both rat and human long-chain acyl-CoA dehydrogenases (LCAD), whereas they were poor substrates of very long-chain acyl-CoA dehydrogenases (VLCAD). VLCAD, however, was active with CoA derivatives of long-chain saturated fatty acids or unsaturated fatty acids that have double bonds further removed from the thioester function. Although bovine LCAD effectively dehydrogenated 5-cis-tetradecenoyl-CoA (14:1) and 4,7,10-cis-hexadecatrienoyl-CoA, it was nearly inactive toward the other unsaturated substrates. The catalytic efficiency of rat VLCAD with 14:1 as substrate was only 4% of the efficiency determined with tetradecanoyl-CoA, whereas LCAD acted equally well on both substrates. The conclusion of this study is that LCAD serves an important, if not essential function in the beta-oxidation of unsaturated fatty acids.

Inborn errors of mitochondrial fatty acid oxidation

Inborn errors of the mitochondrial beta-oxidation of long-chain fatty acids represent an evolving field of inherited metabolic disease. Fatty acid oxidation defects demonstrate an abnormal response to the process of fasting adaptation and affect those tissues that utilize fatty acids as an energy source. These tissues include cardiac and skeletal muscle and liver. Muscle directly uses fatty acids as an energy source whilst hepatic metabolism of fatty acids is mostly directed toward the synthesis of ketone bodies for energy utilization by tissues such as brain. The clinical phenotypes of fatty acid oxidation disorders include disease of one or more of these fatty acid-metabolizing tissues. In this review, we provide an overview of the pathway, discuss the disorders that are well established, and describe recent advances in the field. Currently available diagnostic procedures are critically evaluated.

Targeted disruption of mouse long-chain acyl-CoA dehydrogenase gene reveals crucial roles for fatty acid oxidation

Abnormalities of fatty acid metabolism are recognized to play a significant role in human disease, but the mechanisms remain poorly understood. Long-chain acyl-CoA dehydrogenase (LCAD) catalyzes the initial step in mitochondrial fatty acid oxidation (FAO). We produced a mouse model of LCAD deficiency with severely impaired FAO. Matings between LCAD +/- mice yielded an abnormally low number of LCAD +/- and -/- offspring, indicating frequent gestational loss. LCAD -/- mice that reached birth appeared normal, but had severely reduced fasting tolerance with hepatic and cardiac lipidosis, hypoglycemia, elevated serum free fatty acids, and nonketotic dicarboxylic aciduria. Approximately 10% of adult LCAD -/- males developed cardiomyopathy, and sudden death was observed in 4 of 75 LCAD -/- mice. These results demonstrate the crucial roles of mitochondrial FAO and LCAD in vivo.

2,6-Dimethylheptanoyl-CoA is a specific substrate for long-chain acyl-CoA dehydrogenase (LCAD): evidence for a major role of LCAD in branched-chain fatty acid oxidation

Oxidation of straight-chain fatty acids in mitochondria involves the complicated interaction between a large variety of different enzymes. So far four different mitochondrial straight-chain acyl-CoA dehydrogenases have been identified. The physiological function of three of the four acyl-CoA dehydrogenases has been resolved in recent years especially from studies on patients suffering from certain inborn errors of mitochondrial fatty acid beta-oxidation. The physiological role of long-chain acyl-CoA dehydrogenase (LCAD) has remained obscure, however. The results described in this paper provide strong evidence suggesting that LCAD plays a central role in branched-chain fatty acid metabolism since it turns out to be the major acyl-CoA dehydrogenase reacting with 2,6-dimethylheptanoyl-CoA, a metabolite of pristanic acid, which itself is the alpha-oxidation product of phytanic acid.