Molecular cloning of the structural gene for Acinetobacter citrate synthase

Promotion of mitochondrial energy metabolism during hepatocyte apoptosis in a rat model of acute liver failure

Hepatocyte apoptosis and energy metabolism in mitochondria have an important role in the mechanism of acute liver failure (ALF). However, data on the association between apoptosis and the energy metabolism of hepatocytes are lacking. The current study assessed the activity of several key enzymes in mitochondria during ALF, including citrate synthase (CS), carnitine palmitoyltransferase-1 (CPT-1) and cytochrome c oxidase (COX), which are involved in hepatocyte energy metabolism. A total of 40 male Sprague-Dawley rats were divided into five groups and administered D-galactosamine and lipopolysaccharide to induce ALF. Hepatic pathology and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling examinations indicated that hepatocyte apoptosis was observed at 4 h and increased 8 h after ALF. Hepatocyte necrosis appeared at 12 h and was significantly higher at 24 h with inflammatory cell invasion. The results measured by electron microscopy indicated that ultrastructural changes in mitochondria began at 4 h and the mitochondrial outer membrane was completely disrupted at 24 h resulting in mitochondrial collapse. The expression of CS, CPT-1 and COX was measured and analyzed using assay kits. The activity and protein expression of CS, CPT-1 and COX began to increase at 4 h, reached a peak at 8 h and decreased at 12 h during ALF. The activities of CS, CPT-1 and COX were enhanced during hepatocyte apoptosis suggesting that these enzymes are involved in the initiation and development of ALF. Therefore, these results demonstrated that energy metabolism is important in hepatocyte apoptosis during ALF and hepatocyte apoptosis is an active and energy-consuming procedure. The current study on how hepatocyte energy metabolism affects the transmission of death signals may provide a basis for the early diagnosis and development of an improved therapeutic strategy for ALF.

Citrate synthase and 2-methylcitrate synthase: structural, functional and evolutionary relationships

Following the complete sequencing of the Escherichia coli genome, it has been shown that the proposed second citrate synthase of this organism, recently described by the authors, is in fact a 2-methylcitrate synthase that possesses citrate synthase activity as a minor component. Whereas the hexameric citrate synthase is constitutively produced, the 2-methylcitrate synthase is induced during growth on propionate, and the catabolism of propionate to succinate and pyruvate via 2-methylcitrate is proposed. The citrate synthases of the psychrotolerant eubacterium DS2-3R, and of the thermophilic archaea Thermoplasma acidophilum and Pyrococcus furiosus, are approximately 40% identical in sequence to the Escherichia coli 2-methylcitrate synthase and also possess 2-methylcitrate synthase activity. The data are discussed with respect to the structure, function and evolution of citrate synthase and 2-methylcitrate synthase.

Sequencing and expression of the gene encoding a cold-active citrate synthase from an Antarctic bacterium, strain DS2-3R

The gene encoding citrate synthase from a novel bacterial isolate (DS2-3R) from Antarctica has been cloned, sequenced and over expressed in Escherichia coli. Both the recombinant enzyme and the native enzyme, purified from DS2-3R, are cold-active, with a temperature optimum of 31 degrees C. In addition the enzymes are rapidly inactivated at 45 degrees C, and show significant activity at 10 degrees C and below. Comparison of amino acid sequences indicates that DS2-3R citrate synthase is most closely related to the enzyme from gram-positive bacteria. The amino acid sequence of the DS2-3R enzyme shows several features previously recognised in other cold-active enzymes, including an extended surface loop, an increase in the occurrence of charged residues and a decrease in the number of proline residues in loops. Other changes observed in some psychrophilic enzymes, such as a decrease in isoleucine content and in arginine/(arginine+lysine) content, were not seen in this case.

Does Escherichia coli possess a second citrate synthase gene?

Escherichia coli possesses a hexameric citrate synthase that exhibits allosteric kinetics and regulatory sensitivity, and for which the gene (gltA) has previously been cloned and sequenced. A citrate-synthase-deficient strain of E. coli (K114) has been mutated to generate a revertant (K114r4) that produces a dimeric citrate synthase with altered kinetic and regulatory properties. On cloning and sequencing the gltA gene from both K114 and K114r4, a single mutation was found that caused the replacement of Asp362 with Asn. Asp362 has been previously shown to be a catalytically essential residue in E. coli citrate synthase, and we demonstrate that the hexameric enzyme produced on expression of the gltA gene from K114 and K114r4 is inactive. The dimeric citrate synthase from K114r4 has been purified and shown to be immunologically distinct from the wild-type hexameric enzyme. Determination of its N-terminal amino acid sequence demonstrates that the mutant citrate synthase is encoded by a gene distinct from the E. coli gltA gene. The N-terminal sequence is compared with those of other eukaryotic, eubacterial and archaebacterial citrate synthases.

Chimeric allosteric citrate synthases: construction and properties of citrate synthases containing domains from two different enzymes

The citrate synthases of the gram-negative bacteria, Escherichia coli and Acinetobacter anitratum, are allosterically inhibited by NADH. The kinetic properties, however, suggest that the equilibrium between active (R) and inactive (T) conformational states is shifted toward the T state in the E. coli enzyme. We have now manipulated the cloned genes for the two bacterial enzymes to produce two chimeric proteins, in which one folding domain of each subunit is derived from each enzyme. One chimera (the large domain from A. anitratum and the small domain from the E. coli enzyme) is designated CS ACI::eco; the other is called CS ECO::aci. Both chimeras are roughly as active as the wild type parents, but their Km values for both substrates are lower than those for the E. coli enzyme, and NADH inhibition is markedly sigmoid, while that for E. coli citrate synthases is hyperbolic. Curve-fitting to the allosteric equation suggests that these differences are the result of the destabilization of the T state in the chimeras. The ACI::eco chimera exists almost entirely as a hexamer, like the A. anitratum enzyme, while the ECO::aci chimera, like the E. coli synthase, forms three major bands on nondenaturing polyacrylamide gels, two of them hexamers of different net charge, and one a dimer. These findings indicate that subunit interactions leading to hexamer formation in allosteric citrate synthases of gram-negative bacteria involve mainly the large domains. The chimeras are also used to show that the NADH binding site of E. coli citrate synthase is located entirely in the large domain. Sensitivity of the chimeras to denaturation by urea, to which the A. anitratum enzyme is much more resistant than the E. coli enzyme, is determined by the large domains. Sensitivity to inactivation by subtilisin is intermediate between those shown by the E. coli (very sensitive) and A. anitratum (quite resistant) synthases. This result suggests that digestibility by subtilisin is determined by conformational factors as well as the amino acid sequences of the target regions.

Citrate synthase from the thermophilic archaebacterium Thermoplasma acidophilium. Cloning and sequencing of the gene

The gene encoding the citric acid cycle enzyme, citrate synthase, has been cloned from the thermoacidophilic archaebacterium, Thermoplasma acidophilum. We report the sequencing of this gene and its flanking regions, and the derived amino acid sequence of the enzyme is compared by multiple-sequence alignment analysis with those of citrate synthases from eubacterial and eukaryotic organisms. The similarity is less than 30% between the archaebacterial and non-archaebacterial sequences, although the majority of residues implicated in the catalytic action of the enzyme have been conserved across all three kingdoms. The cloned archaebacterial gene has been expressed in Escherichia coli to produce catalytically active citrate synthase. This is the first reported sequence of citrate synthase from the archaebacteria.

Cloning, sequencing, and expression of the gene for NADH-sensitive citrate synthase of Pseudomonas aeruginosa

The structural gene for the allosteric citrate synthase of Pseudomonas aeruginosa has been cloned from a genomic library by using the Escherichia coli citrate synthase gene as a hybridization probe under conditions of reduced stringency. Subcloning of portions of the original 10-kilobase-pair (kbp) clone led to isolation of the structural gene, with its promoter, within a 2,083-bp length of DNA flanked by sites for KpnI and BamHI. The nucleotide sequence of this fragment is presented; the inferred amino acid sequence was 70 and 76% identical, respectively, with the citrate synthase sequences from E. coli and Acinetobacter anitratum, two other gram-negative bacteria. DEAE-cellulose chromatography of P. aeruginosa citrate synthase from an E. coli host harboring the cloned P. aeruginosa gene gave three peaks of activity. All three enzyme peaks had subunit molecular weights of 48,000; the proteins were identical by immunological criteria and very similar in kinetics of substrate saturation and NADH inhibition. Because the cloned gene contained only one open reading frame large enough to encode a polypeptide of such a size, the three peaks must represent different forms of the same protein. A portion of the cloned P. aeruginosa gene was used as a hybridization probe under stringent conditions to identify highly homologous sequences in genomic DNA of a second strain classified as P. aeruginosa and isolates of P. putida, P. stutzeri, and P. alcaligenes. When crude extracts of each of these four isolates were mixed with antiserum raised against purified P. aeruginosa citrate synthase, however, only the P. alcaligenes extract cross-reacted.