From lipoic acid to multi-enzyme complexes

Dietary lipoic acid-dependent changes in the activity and mRNA levels of hepatic lipogenic enzymes in rats

Effects of dietary α-lipoic acid on hepatic and serum lipid concentrations and the activity and mRNA levels of lipogenic enzymes were examined in rats. Rats were fed experimental diets containing varying amounts of lipoic acid (0, 1, 2·5, 5 g/kg) for 21 d. Lipoic acid profoundly decreased serum and liver concentrations of TAG, and also lowered serum concentrations of phospholipid and NEFA, and the concentration of cholesterol in the liver. A hypoglycaemic effect of this compound was also observed. Lipoic acid dose-dependently decreased the activity and mRNA levels of fatty acid synthase, ATP-citrate lyase, glucose 6-phosphate dehydrogenase, malic enzyme and pyruvate kinase in the liver despite that reductions were considerably attenuated in the NADPH-producing enzymes. This compound also dose-dependently lowered the mRNA levels of spot 14, adiponutrin, stearoyl-CoA desaturase 1, and Δ5- and Δ6-desaturases. In addition, lipoic acid dose-dependently lowered serum concentrations of insulin and leptin, but increased those of adiponectin. Lipoic acid appeared to reduce hepatic lipogenesis and hence decreases serum and liver lipid levels. Alterations in serum concentrations of insulin and (or) adiponectin may trigger this consequence.

The antioxidant alpha-lipoic acid improves endothelial dysfunction induced by acute hyperglycaemia during OGTT in impaired glucose tolerance

OBJECTIVE

Impaired glucose tolerance (IGT) is considered a transitional phase in the development of type 2 diabetes, and is also independently associated with the occurrence of cardiovascular disease. Endothelial dysfunction (ED) represents a very early step in the development of atherosclerosis. The aim of the present study was to examine ED in the fasting state and after a glucose challenge as well as after administration of an antioxidant agent.

PATIENTS AND METHODS

The study subjects included 42 IGT patients and 26 healthy individuals (control group). The IGT patients were randomly divided into two groups, 21 in each group (the alpha-lipoic acid group and the placebo group). In the alpha-lipoic acid group, 300 mg of alpha-lipoic acid was administrated before an oral glucose tolerance test (OGTT); in the placebo group, 250 ml of 0.9% sodium chloride was administrated before the OGTT. In addition, 250 ml of 0.9% sodium chloride was also administrated to the control subjects before the OGTT (control group), and then vascular function was examined in the fasting state and repeated 1 and 2 h after the glucose load. High-resolution ultrasound was used to measure flow-mediated endothelium-dependent arterial dilation (FMD) and glyceryltrinitrate (GTN)-induced endothelium-independent arterial dilation.

RESULTS

In the fasting state, and at 60 and 120 min, FMD in both the placebo and alpha-lipoic acid groups was significantly lower than in the controls (P < 0.01). In the control group, FMD tended to decrease at 60 min after glucose loading and returned to the baseline levels at 120 min (P > 0.05). In the placebo group, FMD decreased significantly at 60 min after glucose loading (P < 0.01) and increased markedly from 60 to 120 min (P < 0.01). The alpha-lipoic acid-treated patients showed FMD values intermediate between the control subjects and the IGT patients treated with placebo, at both 60 and 120 min, and the differences were significant (P < 0.01). In multiple regression analysis, FMD was significantly correlated to fasting blood glucose (FBG), low density lipoprotein cholesterol (LDL-C), lipoprotein (a) [Lp(a)], C-reactive protein (CRP), thiobarbituric acid reactive substances (TBARS) and age in IGT patients at baseline (P < 0.01). Spearman's analysis showed a significant negative correlation between FMD and plasma glucose levels, and between FMD and TBARS during the OGTT in IGT patients (placebo group) (P < 0.01). There was also a significant correlation between FMD and plasma glucose levels, and between FMD and TBARS during the OGTT in IGT patients treated with alpha-lipoic acid (P < 0.05), although the power of association decreased.

CONCLUSION

In subjects with IGT, FMD was impaired both in the fasting state and after a glucose challenge, probably through increased production of oxygen-derived free radicals. The ED observed after a glucose challenge is related to the extent of hyperglycaemia and TBARS, and an antioxidant agent can improve the impairment of endothelial function induced by acute hyperglycaemia.

Alpha-lipoic acid modulates gut inflammation induced by trinitrobenzene sulfonic acid in rats

BACKGROUND AND AIM

Alpha-lipoic acid (ALA) has been shown to combat oxidative stress by quenching a variety of reactive oxygen species. It is involved in the regeneration of exogenous and endogenous antioxidants, chelation of metal ions, and repair of oxidized proteins. This study aimed to evaluate the potential beneficial effect of ALA on trinitrobenzenesulfonic acid (TNBS)-induced gut ileitis and colitis in rats.

METHOD

After 48 h of fasting, Sprague-Dawley rats underwent a laparotomy under ether anesthesia. TNBS solution 30 mg/mL in 40% ethanol (1 mL) was injected into the lumen, 10 cm proximal to the ileocolonic junction to induce ileitis or intrarectally 8 cm proximal to the anal sphincter to induce colitis. ALA (25 mg/kg intraperitoneally, twice a day) was given after induction of inflammation and continued for 3 days. All animals were decapitated 3 days after induction of the inflammation. The mucosal lesions of the ileum and colon were scored macroscopically and microscopically. Samples were taken for the measurement of malondialdehyde (MDA) and glutathione (GSH) levels, tissue-associated myeloperoxidase (MPO) activity and luminol- or lucigenin-enhanced chemiluminescence (CL).

RESULTS

Macroscopic scores, morphological changes and increased tissue lipid peroxidation with a concomitant reduction in GSH of the ileitis or colitis groups were all reversed by treatment with ALA. ALA treatment was also effective in improving tissue MPO activity and CL values, which were elevated in untreated ileitis or colitis groups.

CONCLUSION

ALA is beneficial in TNBS-induced gut inflammation in rats via suppression of neutrophil accumulation, preservation of endogenous glutathione and inhibition of reactive oxidant generation.

α-lipoic acid: An inhibitor of secretory phospholipase A2 with anti-inflammatory activity

Alpha-lipoic acid (ALA) and its reduced form dihydrolipoic acid (DHLA) are powerful antioxidants both in hydrophilic and lipophylic environments with diverse pharmacological properties including anti-inflammatory activity. The mechanism of anti-inflammatory activity of ALA and DHALA is not known. The present study describes the interaction of ALA and DHALA with pro-inflammatory secretory PLA(2) enzymes from inflammatory fluids and snake venoms. In vitro enzymatic inhibition of sPLA(2) from Vipera russellii, Naja naja and partially purified sPLA(2) enzymes from human ascitic fluid (HAF), human pleural fluid (HPF) and normal human serum (HS) by ALA and DHLA was studied using (14)C-oleate labeled Escherichia coli as the substrate. Biophysical interaction of ALA with sPLA(2) was studied by fluorescent spectral analysis and circular dichroism studies. In vivo anti-inflammatory activity was checked using sPLA(2) induced mouse paw edema model. ALA but not DHLA inhibited purified sPLA(2) enzymes from V. russellii, N. naja and partially purified HAF, HPF and HS in a dose dependent manner. This data indicated that ALA is critical for inhibition. IC(50) value calculated for these enzymes ranges from 0.75 to 3.0 microM. The inhibition is independent of calcium and substrate concentration. Inflammatory sPLA(2) enzymes are more sensitive to inhibition by ALA than snake venom sPLA(2) enzymes. ALA quenched the fluorescence intensity of sPLA(2) enzyme in a dose dependent manner. Apparent shift in the far UV-CD spectra of sPLA(2) with ALA indicated change in its alpha-helical confirmation and these results suggest its direct interaction with the enzyme. ALA inhibits the sPLA(2) induced mouse paw edema in a dose dependent manner and confirms the sPLA(2) inhibitory activity in vivo also. These data suggest that ALA may act as an endogenous regulator of sPLA(2) enzyme activity and suppress inflammatory reactions.

Long-term safety of alpha-lipoic acid (ALA) consumption: A 2-year study

Alpha-lipoic acid (ALA) (CAS RN 1077-28-7), also referred to as thioctic acid, has been demonstrated to exhibit strong anti-oxidant properties. In order to test the long-term toxicity of ALA, groups of 40-50 male and female, 5-6-week-old, Sprague-Dawley rats were subjected to oral administration of 20, 60, or 180 mg/kg body weight (bw)/day ALA for 24 months. There was no significant difference between control animals and treated animals at 20 or 60 mg/kg bw/day with respect to body weight gain, food consumption, behavioural effects, haematological and clinical chemistry parameters, and gross and histopathological findings. In all treatment groups, mortality was slightly lower as compared to the control. The absolute weights of the heart (high-dose males), thymus (high-dose males), and left adrenal (mid-dose males), liver (high-dose females), and lungs (high-dose females) were decreased in comparison to controls. These changes were of no toxicological significance. The only notable finding in rats of both sexes dosed at 180 mg/kg bw/day was a reduction in food intake relative to the controls and a concomitant decrease in body weight. This decrease in body weight led to significant differences between the control and high-dose rats with respect to the absolute weights of certain organs. However, no gross or histopathological changes were associated with these findings. The no-observed-adverse-effect level (NOAEL) is considered to be 60 mg/kg bw/day.

alpha-Lipoic acid prevents the increase in atherosclerosis induced by diabetes in apolipoprotein E-deficient mice fed high-fat/low-cholesterol diet

Considerable evidence indicates that hyperglycemia increases oxidative stress and contributes to the increased incidence of atherosclerosis and cardiovascular complications in diabetic patients. To examine the effect of alpha-lipoic acid, a potent natural antioxidant, on atherosclerosis in diabetic mice, 3-month-old apolipoprotein (apo) E-deficient (apoE(-/-)) mice were made diabetic by administering streptozotocin (STZ). At 4 weeks after starting the STZ administration, a high-fat diet with or without alpha-lipoic acid (1.65 g/kg) was given to the mice and to nondiabetic apoE(-/-) controls. At 20 weeks, markers of oxidative stress were significantly lower in both the diabetic apoE(-/-) mice and their nondiabetic apoE(-/-) controls with alpha-lipoic acid supplement than in those without it. Remarkably, alpha-lipoic acid completely prevented the increase in plasma total cholesterol, atherosclerotic lesions, and the general deterioration of health caused by diabetes. These protective effects of alpha-lipoic acid were accompanied by a reduction of plasma glucose and an accelerated recovery of insulin-producing cells in the pancreas, suggesting that part of its effects are attributable to protecting pancreatic beta-cells from damage. Our results suggest that dietary alpha-lipoic acid is a promising protective agent for reducing cardiovascular complications of diabetes.

Flexibility of thiamine diphosphate revealed by kinetic crystallographic studies of the reaction of pyruvate-ferredoxin oxidoreductase with pyruvate

Pyruvate-ferredoxin oxidoreductases (PFOR) are unique among thiamine pyrophosphate (ThDP)-containing enzymes in giving rise to a rather stable cofactor-based free-radical species upon the decarboxylation of their first substrate, pyruvate. We have obtained snapshots of unreacted and partially reacted (probably as a tetrahedral intermediate) pyruvate-PFOR complexes at different time intervals. We conclude that pyruvate decarboxylation involves very limited substrate-to-product movements but a significant displacement of the thiazolium moiety of ThDP. In this respect, PFOR seems to differ substantially from other ThDP-containing enzymes, such as transketolase and pyruvate decarboxylase. In addition, exposure of PFOR to oxygen in the presence of pyruvate results in significant inhibition of catalytic activity, both in solution and in the crystals. Examination of the crystal structure of inhibited PFOR suggests that the loss of activity results from oxime formation at the 4' amino substituent of the pyrimidine moiety of ThDP.

α-Lipoic Acid and Cardiovascular Disease

Alpha-lipoic acid (ALA) has been identified as a powerful antioxidant found naturally in our diets, but appears to have increased functional capacity when given as a supplement in the form of a natural or synthetic isolate. ALA and its active reduced counterpart, dihydrolipoic acid (DHLA), have been shown to combat oxidative stress by quenching a variety of reactive oxygen species (ROS). Because this molecule is soluble in both aqueous and lipid portions of the cell, its biological functions are not limited solely to one environment. In addition to ROS scavenging, ALA has been shown to be involved in the recycling of other antioxidants in the body including vitamins C and E and glutathione. Not only have the antioxidant qualities of this molecule been studied, but there are also several reports pertaining to its blood lipid modulating characteristics, protection against LDL oxidation and modulation of hypertension. Therefore, ALA represents a possible protective agent against risk factors of cardiovascular disease (CVD). The objective of this review is to examine the literature pertaining to ALA in relation to CVD and describe the most powerful actions and potential uses of this naturally occurring antioxidant. Despite the numerous studies on ALA, many questions remain relating to the use of ALA as a supplement. There is no consensus on dosage, dose frequency, form of administration, and/or preferred form of ALA. However, collectively the literature increases our understanding of the potential uses for supplementation with ALA and identifies key areas for future research.

Interchangeable enzyme modules. Functional replacement of the essential linker of the biotinylated subunit of acetyl-CoA carboxylase with a linker from the lipoylated subunit of pyruvate dehydrogenase

Biotin carboxyl carrier protein (BCCP) is the small biotinylated subunit of Escherichia coli acetyl-CoA carboxylase, the enzyme that catalyzes the first committed step of fatty acid synthesis. E. coli BCCP is a member of a large family of protein domains modified by covalent attachment of biotin. In most biotinylated proteins, the biotin moiety is attached to a lysine residue located about 35 residues from the carboxyl terminus of the protein, which lies in the center of a strongly conserved sequence that forms a tightly folded anti-parallel beta-barrel structure. Located upstream of the conserved biotinoyl domain sequence are proline/alanine-rich sequences of varying lengths, which have been proposed to act as flexible linkers. In E. coli BCCP, this putative linker extends for about 42 residues with over half of the residues being proline or alanine. I report that deletion of the 30 linker residues located adjacent to the biotinoyl domain resulted in a BCCP species that was defective in function in vivo, although it was efficiently biotinylated. Expression of this BCCP species failed to restore normal growth and fatty acid synthesis to a temperature-sensitive E. coli strain that lacks BCCP when grown at nonpermissive temperatures. In contrast, replacement of the deleted BCCP linker with a linker derived from E. coli pyruvate dehydrogenase gave a chimeric BCCP species that had normal in vivo function. Expression of BCCPs having deletions of various segments of the linker region of the chimeric protein showed that some deletions of up to 24 residues had significant or full biological activity, whereas others had very weak or no activity. The inactive deletion proteins all lacked an APAAAAA sequence located adjacent to the tightly folded biotinyl domain, whereas deletions that removed only upstream linker sequences remained active. Deletions within the linker of the wild type BCCP protein also showed that the residues adjacent to the tightly folded domain play an essential role in protein function, although in this case some proteins with deletions within this region retained activity. Retention of activity was due to fusion of the domain to upstream sequences. These data provide new evidence for the functional and structural similarities of biotinylated and lipoylated proteins and strongly support a common evolutionary origin of these enzyme subunits.

Crystal structure of the free radical intermediate of pyruvate:ferredoxin oxidoreductase

In anaerobic organisms, the decarboxylation of pyruvate, a crucial component of intermediary metabolism, is catalyzed by the metalloenzyme pyruvate: ferredoxin oxidoreductase (PFOR) resulting in the generation of low potential electrons and the subsequent acetylation of coenzyme A (CoA). PFOR is the only enzyme for which a stable acetyl thiamine diphosphate (ThDP)-based free radical reaction intermediate has been identified. The 1.87 A-resolution structure of the radical form of PFOR from Desulfovibrio africanus shows that, despite currently accepted ideas, the thiazole ring of the ThDP cofactor is markedly bent, indicating a drastic reduction of its aromaticity. In addition, the bond connecting the acetyl group to ThDP is unusually long, probably of the one-electron type already described for several cation radicals but not yet found in a biological system. Taken together, our data, along with evidence from the literature, suggest that acetyl-CoA synthesis by PFOR proceeds via a condensation mechanism involving acetyl (PFOR-based) and thiyl (CoA-based) radicals.

Characterization of two cDNAs encoding mitochondrial lipoamide dehydrogenase from Arabidopsis

In contrast to peas (Pisum sativum), where mitochondrial lipoamide dehydrogenase is encoded by a single gene and shared between the alpha-ketoacid dehydrogenase complexes and the Gly decarboxylase complex, Arabidopsis has two genes encoding for two mitochondrial lipoamide dehydrogenases. Northern-blot analysis revealed different levels of RNA expression for the two genes in different organs; mtLPD1 had higher RNA levels in green leaves compared with the much lower level in roots. The mRNA for mtLPD2 shows the inverse pattern. The other organs examined showed nearly equal RNA expressions for both genes. Analysis of etiolated seedlings transferred to light showed a strong induction of RNA expression for mtLPD1 but only a moderate induction of mtLPD2. Based on the organ and light-dependent expression patterns, we hypothesize that mtLPD1 encodes the protein most often associated with the Gly decarboxylase complex, and mtLPD2 encodes the protein incorporated into alpha-ketoacid dehydrogenase complexes. Due to the high level of sequence conservation between the two mtLPDs, we assume that the proteins, once in the mitochondrial matrix, are interchangeable among the different multienzyme complexes. If present at high levels, one mtLPD might substitute for the other. Supporting this hypothesis are results obtained with a T-DNA knockout mutant, mtlpd2, which shows no apparent phenotypic change under laboratory growth conditions. This indicates that mtLPD1 can substitute for mtLPD2 and associate with all these multienzyme complexes.

Characterization of two cDNAs encoding mitochondrial lipoamide dehydrogenase from Arabidopsis

In contrast to peas (Pisum sativum), where mitochondrial lipoamide dehydrogenase is encoded by a single gene and shared between the alpha-ketoacid dehydrogenase complexes and the Gly decarboxylase complex, Arabidopsis has two genes encoding for two mitochondrial lipoamide dehydrogenases. Northern-blot analysis revealed different levels of RNA expression for the two genes in different organs; mtLPD1 had higher RNA levels in green leaves compared with the much lower level in roots. The mRNA for mtLPD2 shows the inverse pattern. The other organs examined showed nearly equal RNA expressions for both genes. Analysis of etiolated seedlings transferred to light showed a strong induction of RNA expression for mtLPD1 but only a moderate induction of mtLPD2. Based on the organ and light-dependent expression patterns, we hypothesize that mtLPD1 encodes the protein most often associated with the Gly decarboxylase complex, and mtLPD2 encodes the protein incorporated into alpha-ketoacid dehydrogenase complexes. Due to the high level of sequence conservation between the two mtLPDs, we assume that the proteins, once in the mitochondrial matrix, are interchangeable among the different multienzyme complexes. If present at high levels, one mtLPD might substitute for the other. Supporting this hypothesis are results obtained with a T-DNA knockout mutant, mtlpd2, which shows no apparent phenotypic change under laboratory growth conditions. This indicates that mtLPD1 can substitute for mtLPD2 and associate with all these multienzyme complexes.

Developmental expression of the mitochondrial pyruvate dehydrogenase complex in pea (Pisum sativum) seedlings

In order to better understand control of the mitochondrial pyruvate dehydrogenase complex (PDC), total catalytic activity was determined during development of the primary leaves of pea (Pisum sativum L.) seedlings, as well as in each leaf pair of 21-day-old plants. Activity of the PDC in clarified homogenates was highest in the youngest organs and then dropped dramatically as the leaves matured and became photosynthetically competent. As leaves began to senesce, total PDC activity dropped to zero. Steady-state mRNA levels were determined using E1 and E3 cDNA probes. The overall pattern of transcript abundance matched the pattern observed for total PDC activity; transcript levels for E1alpha and E1beta approached zero during senescence. Levels of the E1alpha, E1beta, E2 and E3 subunits of the PDC were analyzed in the same samples, using specific antibodies. Quantitation of the immunoblotting results throughout this developmental series showed a pattern in parallel with that of catalytic activity and mRNA levels, although the relative changes in subunit protein levels were not as extreme as the changes in activity. The exception to the global pattern was that of the E3 subunit: lipoamide dehydrogenase. Expression of this enzyme was highest in mature, fully expanded leaves, which were active in photosynthesis and photorespiration, reflecting the additional role of E3 as a component of glycine decarboxylase.

Swinging arms and swinging domains in multifunctional enzymes: catalytic machines for multistep reactions

Multistep chemical reactions are increasingly seen as important in a growing number of complex biotransformations. Covalently attached prosthetic groups or swinging arms, and their associated protein domains, are essential to the mechanisms of active-site coupling and substrate channeling in a number of the multifunctional enzyme systems responsible. The protein domains, for which the posttranslational machinery in the cell is highly specific, are crucially important, contributing to the processes of molecular recognition that define and protect the substrates and the catalytic intermediates. The domains have novel folds and move by virtue of conformationally flexible linker regions that tether them to other components of their respective multienzyme complexes. Structural and mechanistic imperatives are becoming apparent as the assembly pathways and the coupling of multistep reactions catalyzed by these dauntingly complex molecular machines are unraveled.

Pisum sativum mitochondrial pyruvate dehydrogenase can be assembled as a functional alpha(2)beta(2) heterotetramer in the cytoplasm of Pichia pastoris

Pea (Pisum sativum) mitochondrial pyruvate dehydrogenase (E1) was produced by coexpression of the mature alpha and beta subunits in the cytoplasm of the yeast Pichia pastoris. Size-exclusion chromatography of recombinant E1, using a Superose 12 column, yielded a peak at M(r) 160,000 that contained both alpha and beta subunits as well as E1 activity. This corresponds to the size of native alpha(2)beta(2) E1. Recombinant E1 alpha (His(6))-E1 beta was purified by affinity chromatography using immobilized Ni(+), with a yield of 2.8 mg L(-1). The pyruvate-decarboxylating activity of recombinant E1 was dependent upon added Mg(2+) and thiamin-pyrophosphate and was enhanced by the oxidant potassium ferricyanide. Native pea mitochondrial E1-kinase catalyzed phosphorylation of Ser residues in the alpha-subunit of recombinant E1, with concomitant loss of enzymatic activity. Thus, mitochondrial pyruvate dehydrogenase can be assembled in the cytoplasm of P. pastoris into an alpha(2)beta(2) heterotetramer that is both catalytically active and competent for regulatory phosphorylation.

The alpha-ketoglutarate dehydrogenase complex

The alpha-ketoglutarate dehydrogenase complex (KGDHC) is an important mitochondrial constituent, and deficiency of KGDHC is associated with a number of neurological disorders. KGDHC is composed of three proteins, each encoded on a different and well-characterized gene. The sequences of the human proteins are known. The organization of the proteins into a large, ordered multienzyme complex (a "metabolon") has been well studied in prokaryotic and eukaryotic species. KGDHC catalyzes a critical step in the Krebs tricarboxylic acid cycle, which is also a step in the metabolism of the potentially excitotoxic neurotransmitter glutamate. A number of metabolites modify the activity of KGDHC, including inactivation by 4-hydroxynonenal and other reactive oxygen species (ROS). In human brain, the activity of KGDHC is lower than that of any other enzyme of energy metabolism, including phosphofructokinase, aconitase, and the electron transport complexes. Deficiencies of KGDHC are likely to impair brain energy metabolism and therefore brain function, and lead to manifestations of brain disease. In general, the clinical manifestations of KGDHC deficiency relate to the severity of the deficiency. Several such disorders have been recognized: infantile lactic acidosis, psychomotor retardation in childhood, intermittent neuropsychiatric disease with ataxia and other motor manifestations, Friedreich's and other spinocerebellar ataxias, Parkinson's disease, and Alzheimer's disease (AD). A KGDHC gene has been associated with the first two and last two of these disorders. KGDHC is not uniformly distributed in human brain, and the neurons that appear selectively vulnerable in human temporal cortex in AD are enriched in KGDHC. We hypothesize that variations in KGDHC that are not deleterious during reproductive life become deleterious with aging, perhaps by predisposing this mitochondrial metabolon to oxidative damage.

Fatty acid and lipoic acid biosynthesis in higher plant mitochondria

Fatty acid and lipoic acid biosynthesis were investigated in plant mitochondria. Although the mitochondria lack acetyl-CoA carboxylase, our experiments reveal that they contain the enzymatic equipment necessary to transform malonate into the two main building units for fatty acid synthesis: malonyl- and acetyl-acyl carrier protein (ACP). We demonstrated, by a new method based on a complementary use of high performance liquid chromatography and mass spectrometry, that the soluble mitochondrial fatty-acid synthase produces mainly three predominant acyl-ACPs as follows: octanoyl(C8)-, hexadecanoyl(C16)-, and octadecanoyl(C18)-ACP. Octanoate production is of primary interest since it has been postulated long ago to be a precursor of lipoic acid. By using a recombinant H apoprotein mutant as a potential acceptor for newly synthesized lipoic acid, we were able to detect limited amounts of lipoylated H protein in the presence of malonate, several sulfur donors, and cofactors. Finally, we present a scheme outlining the new biochemical pathway of fatty acid and lipoic acid synthesis in plant mitochondria.

Structure and electron transfer mechanism of pyruvate:ferredoxin oxidoreductase

The first crystal structure of pyruvate:ferredoxin oxidoreductase to be determined has provided significant new information on its structural organization and redox chemistry. Spectroscopic analyses of a radical reaction intermediate have shed more light on its thiamin-based mechanism of catalysis. Different approaches have been used to study the interaction between the enzyme and ferredoxin, its redox partner.