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PubMed2003

The active site and mechanism of action of recombinant acetohydroxy acid synthase from tobacco.

Acetohydroxy acid synthase (AHAS) is one of several enzymes that require thiamine diphosphate and a divalent cation as essential cofactors. Recently, the three-dimensional structure of the enzyme from yeast has been determined [Pang et al., J. Mol. Biol. 317 (2002) 249-262]. While this structure sheds light on the binding of the cofactors and the reaction mechanism, the interactions between the substrates and the enzyme remain unclear. We have studied the pH dependence of kinetic parameters in order to obtain information about the chemical mechanism in the active site. Data are consistent with a mechanism in which substrate selectively catalyzed to the enzyme with an unprotonated base having a pK of 6.48, and a protonated group having a pK of 8.25 for catalysis. The temperature dependence of kinetic parameters was pH-dependent, and the enthalpies of ionization, DeltaH(ion), calculated from the slope of pK(1) and pK(2) are both pH-independent. The solvent perturbation of kinetic parameters was pH-dependent, and the pK(1) from the acidic side and the pK(2) from the basic side were shifted down 0.4 pH units and shifted up 0.6 units as water was replaced by 15% ethanol, respectively. The data are discussed in terms of the acid-base chemical mechanism.

Acetolactate Synthase↗
PubMed2004

Folate synthesis in plants: the p-aminobenzoate branch is initiated by a bifunctional PabA-PabB protein that is targeted to plastids.

It is not known how plants synthesize the p-aminobenzoate (PABA) moiety of folates. In Escherichia coli, PABA is made from chorismate in two steps. First, the PabA and PabB proteins interact to catalyze transfer of the amide nitrogen of glutamine to chorismate, forming 4-amino-4-deoxychorismate (ADC). The PabC protein then mediates elimination of pyruvate and aromatization to give PABA. Fungi, actinomycetes, and Plasmodium spp. also synthesize PABA but have proteins comprising fused domains homologous to PabA and PabB. These bipartite proteins are commonly called "PABA synthases," although it is unclear whether they produce PABA or ADC. Genomic approaches identified Arabidopsis and tomato cDNAs encoding bipartite proteins containing fused PabA and PabB domains, plus a putative chloroplast targeting peptide. These cDNAs encode functional enzymes, as demonstrated by complementation of an E. coli pabA pabB double mutant and a yeast PABA-synthase deletant. The partially purified recombinant Arabidopsis protein did not produce PABA unless the E. coli PabC enzyme was added, indicating that it forms ADC, not PABA. The enzyme behaved as a monomer in size-exclusion chromatography and was not inhibited by physiological concentrations of PABA, its glucose ester, or folates. When the putative targeting peptide was fused to GFP and expressed in protoplasts, the fusion protein appeared only in chloroplasts, indicating that PABA synthesis is plastidial. In the pericarp of tomato fruit, the PabA-PabB mRNA level fell drastically as ripening advanced, but there was no fall in total PABA content, which stayed between 0.7 and 2.3 nmol.g(-1) fresh weight.

4-Aminobenzoic Acid↗
PubMed1998

13C NMR study of the effects of leptin treatment on kinetics of hepatic intermediary metabolism.

The recent discovery of leptin receptors in peripheral tissue raises questions about which of leptin's biological actions arise from direct effects of the hormone on extraneural tissues and what intracellular mechanisms are responsible for leptin's effects on carbohydrate and lipid metabolism. The present study is focused on the action of leptin on hepatic metabolism. Nondestructive 13C NMR methodology was used to follow the kinetics of intermediary metabolism by monitoring flux of 13C-labeled substrate through several multistep pathways. In perfused liver from either ob/ob or lean mice, we found that acute treatment with leptin in vitro modulates pathways controlling carbohydrate flux into 13C-labeled glycogen, thereby rapidly enhancing synthesis by an insulin-independent mechanism. Acute treatment of ob/ob liver also caused a rapid stimulation of long-chain fatty acid synthesis from 13C-labeled acetyl-CoA by the de novo synthesis route. Chronic leptin treatment in vivo induced homeostatic changes that resulted in a tripling of the rate of glycogen synthesis via the gluconeogenic pathway from [2-13C]pyruvate in ob/ob mouse liver perfused in the absence of the hormone. Consistent with the 13C NMR results, leptin treatment of the ob/ob mouse in vivo resulted in significantly increased hepatic glycogen synthase activity. Chronic treatment with leptin in vivo exerted the opposite effect of acute treatment in vitro and markedly decreased hepatic de novo synthesis of fatty acids in ob/ob mouse liver. In agreement with the 13C NMR findings, activities of hepatic acetyl-CoA carboxylase and fatty acid synthase were significantly reduced by chronic treatment of the ob/ob mouse with leptin. Our data represent a demonstration of direct effects of leptin in the regulation of metabolism in the intact functioning liver.

Animals↗
PubMed1985

Calf muscle adaptation to peripheral vascular disease.

130 muscle biopsies were taken from the gastrocnemius of 82 patients with different degrees of peripheral vascular disease (PVD) and 19 normal controls for histochemical analysis, and 30 patients and 7 controls for biochemical analysis of "aerobic" and "anaerobic" enzymes. The results showed that the gastrocnemius of patients with PVD did not adapt by increasing its "aerobic" potential as previously suggested. Histochemical studies showed that the cross-sectional area of both Type 1 and Type 2 muscle fibres became smaller than those from age-matched controls in the presence of PVD, and in some cases biopsies from both legs confirmed that this trend was greater in those limbs with increasing evidence of PVD. There is also some evidence to suggest a decrease in absolute capillary numbers per muscle fibre. The biochemical assays confirmed decreased levels of aerobic enzymes with increasing evidence of PVD as judged by ankle systolic pressure, and those patients with intermittent claudication showed some evidence of increased anaerobic enzyme levels in comparison to both the normal controls and those with rest pain. Patients with PVD do not adapt to ischaemia/anoxia by increasing their aerobic capability, but show signs of muscle atrophy probably due to reduced mobility. Training regimes may benefit patients with intermittent claudication by reversing these changes.

Adaptation, Physiological↗
PubMed2002

Effect of AICAR treatment on glycogen metabolism in skeletal muscle.

AMP-activated protein kinase (AMPK) is proposed to stimulate fat and carbohydrate catabolism to maintain cellular energy status. Recent studies demonstrate that pharmacologic activation of AMPK and mutations in the enzyme are associated with elevated muscle glycogen content in vivo. Our purpose was to determine the mechanism for increased muscle glycogen associated with AMPK activity in vivo. AMPK activity and glycogen metabolism were studied in red and white gastrocnemius muscles from rats treated with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) in vivo, and also in muscles incubated with AICAR in vitro. In vivo AICAR treatment reduced blood glucose and increased blood lactate compared with basal values. AICAR increased muscle alpha2 AMPK activity, glycogen, and glucose-6-phosphate concentrations. Glycogen synthase activity was increased in the red gastrocnemius but was decreased in the white gastrocnemius. Glycogen phosphorylase activity increased in both muscles, with an inhibition initially observed in the red gastrocnemius. In vitro incubation with AICAR activated alpha2 AMPK but had no effect on either glycogen synthase or glycogen phosphorylase. These results suggest that AICAR treatment does not promote glycogen accumulation in skeletal muscle in vivo by altering glycogen synthase and glycogen phosphorylase. Rather, the increased glycogen is due to the well-known effects of AICAR to increase glucose uptake.

AMP-Activated Protein Kinases↗
PubMedDate not supplied

[Effect of L-arginine and N(omega)-nitro-L-arginine on oxidative phosphorylation and lipid peroxidation in rats with various tolerance to hypoxia under stressful conditions].

The influence of L-arginine (600 mg/kg) and NO-synthase blocator N omega-nitro-L-arginine L-NNA (35 mg/kg) on processes of ADP-stimulated respiration (under using 0.35 mM succinate, 1 mM alpha-ketoglutarate, 2 mM pyruvate, 2 mM glutamate, 2 Mm malate and succinate dehydrohenase blocator--2 mM malonate as substrates of oxidation), lipid peroxidation (concentration of DK and MDA), activities of succinate dehydrohenase and aminotransferases in rats tissues with different resistance to hypoxia under stress conditions have been investigated. It have been shown that the energy metabolism indices (respiration rate and efficiency of phosphorilation ADP/O) are higher in high resistent (HR) animals in the control group. Stress causes the increase of ADP-stimulated respiration in low resistent (LR) under succinate oxidation and decrease of NADPH-dependent utilization, indicative of more effort of energy system in LR animals. Stress conditions are connected with the increase of lipid peroxidation products in blood both in LR and in HR animals, though in hepar their concentration change unimportantly. Injection of L-arginine decreases aerobic component of energy metabolism on the background decreasing aminotransferases ways of oxidation and succinate dehydrohenase activity. L-arginine causes decrease of lipid peroxidation products in LR, in HR the same effect reaches by L-NNA injection. The has been made conclusion about tight correlation between energy metabolism, processes of lipid peroxidation with resistance to hypoxia and functioning of nitric oxide cycle under stress conditions.

Adenosine Diphosphate↗
PubMed1983

Atlantic hagfish cardiac muscle: metabolic basis of tolerance to anoxia.

Oxygen tensions in the major venous inputs to the systemic and portal-vein hearts of normoxic Atlantic hagfish (12.3 +/- 1.7 and 11.0 +/- 1.6 mmHg, respectively) are low compared with typical vertebrate values. Anoxia and poisoning with cyanide and azide do not significantly affect in situ performance of the systemic heart. Idoacetate poisoning, however, results in a significant decrease in cardiac performance of the systemic heart to 12% of the initial value after 3 h. Activities of mitochondrial enzymes of hagfish ventricle suggest a small potential for aerobic metabolism compared with those in the aerobic ventricle of Atlantic cod. Activities of enzymes of carbohydrate metabolism indicate similar anaerobic capacity in hagfish and cod ventricle. The ratio of pyruvate kinase to cytochrome c oxidase, an index of anaerobic to aerobic capacity, is 5.6 times greater in hagfish than cod ventricle. Metabolite concentrations in freeze-clamped ventricles of normoxic and hypoxic hagfish indicate hypoxia-induced activation of glycogenolysis, enhanced substrate flow across 6-phosphofructokinase, and an apparent secondary constriction of glycolysis at the level of glyceraldehyde-phosphate dehydrogenase. Carbohydrate utilization via the glycolytic pathway appears essential for maintenance of cardiac performance in both normoxic and anoxic hagfish. Under conditions of severe hypoxia, ATP provision is probably met by anaerobic glycolysis.

Alcohol Oxidoreductases↗
PubMed1993

Molecular genetic characterization of an X-linked form of Leigh's syndrome.

We report a patient with necrotizing encephalomyelopathy (Leigh's syndrome) associated with a deficiency of pyruvate dehydrogenase complex activity. The underlying mutation is an A to C transversion in the pyruvate dehydrogenase complex E1 alpha subunit gene. As the E1 alpha subunit is encoded on the X chromosome, this observation confirms that some patients with Leigh's syndrome may potentially exhibit X-linked inheritance.

Amino Acid Sequence↗