Purification and properties of an extreme thermostable glutamate dehydrogenase from the archaebacterium Sulfolobus solfataricus

Oxidative Stickland reactions in an obligate aerobic organism - amino acid catabolism in the Crenarchaeon Sulfolobus solfataricus

The thermoacidophilic Crenarchaeon Sulfolobus solfataricus is a model organism for archaeal adaptation to extreme environments and renowned for its ability to degrade a broad variety of substrates. It has been well characterised concerning the utilisation of numerous carbohydrates as carbon source. However, its amino acid metabolism, especially the degradation of single amino acids, is not as well understood. In this work, we performed metabolic modelling as well as metabolome, transcriptome and proteome analysis on cells grown on caseinhydrolysate as carbon source in order to draw a comprehensive picture of amino acid metabolism in S. solfataricus P2. We found that 10 out of 16 detectable amino acids are imported from the growth medium. Overall, uptake of glutamate, methionine, leucine, phenylalanine and isoleucine was the highest of all observed amino acids. Our simulations predict an incomplete degradation of leucine and tyrosine to organic acids, and in accordance with this, we detected the export of branched-chain and aromatic organic acids as well as amino acids, ammonium and trehalose into the culture supernatants. The branched-chain amino acids as well as phenylalanine and tyrosine are degraded to organic acids via oxidative Stickland reactions. Such reactions are known for prokaryotes capable of anaerobic growth, but so far have never been observed in an obligate aerobe. Also, 3-methyl-2-butenoate and 2-methyl-2-butenoate are for the first time found as products of modified Stickland reactions for the degradation of branched-chain amino acids. This work presents the first detailed description of branched-chain and aromatic amino acid catabolism in S. solfataricus.

Heat-stable enzymes from extremely thermophilic and hyperthermophilic microorganisms

Only in the last decade have microorganisms been discovered which grow near or above 100°C. The enzymes that are formed by these extremely thermophilic (growth temperature 65 to 85°C) and hyperthermophilic (growth temperature 85 to 110°C) microorganisms are of great interest. This review covers the extracellular and intracellular enzymes of these exotic microorganisms that have recently been described. Polymer-hydrolysing enzymes, such as amylolytic, cellulolytic, hemicellulolytic and proteolytic enzymes, will be discussed. In addition, the properties of the intracellular enzymes involved in carbohydrate and amino-acid metabolism and DNA-binding and chaperones and chaperone-like proteins from hyperthermophiles are described. Due to the unusual properties of these heat-stable enzymes, they are expected to fill the gap between biological and chemical processes.

Carbon source-dependent modulation of NADP-glutamate dehydrogenases in isophthalate-degrading Pseudomonas aeruginosa strain PP4, Pseudomonas strain PPD and Acinetobacter lwoffii strain ISP4

Acinetobacter lwoffii strain ISP4 metabolizes isophthalate rapidly compared with Pseudomonas aeruginosa strain PP4 and Pseudomonas strain PPD. Isophthalate has been reported to be a potent competitive inhibitor of glutamate dehydrogenase (GDH). Exogenous supplementation of isophthalate with glutamate or alpha-ketoglutarate at 1 mM concentration caused strains PP4 and PPD to grow faster than in the presence of isophthalate alone; however, no such effect was observed in strain ISP4. When grown on isophthalate, all strains showed activity of NADP-dependent GDH (NADP-GDH), while cells grown on glucose, 2x yeast extract-tryptone broth (2YT) or glutamate showed activities of both NAD-dependent GDH (NAD-GDH) and NADP-GDH. Activity staining, inhibition and thermal stability studies indicated the carbon source-dependent presence of two (GDH(I) and GDH(II)), three (GDH(A), GDH(B) and GDH(C)) and one (GDH(P)) forms of NADP-GDH in strains PP4, PPD and ISP4, respectively. The results demonstrate the carbon source-dependent modulation of different forms of NADP-GDH in these bacterial strains. This modulation may help the efficient utilization of isophthalate as a carbon source by overcoming the inhibitory effect on GDH.

Purification and characterization of NADP-specific alcohol dehydrogenase and glutamate dehydrogenase from the hyperthermophilic archaeon Thermococcus litoralis

Thermococcus litoralis is a strictly anaerobic archaeon that grows at temperatures up to 98 degrees C by fermenting peptides. Little is known about the primary metabolic pathways of this organism and, in particular, the role of enzymes that are dependent on thermolabile nicotinamide nucleotides. In this paper we show that the cytoplasmic fraction of cell extracts contained NADP-specific glutamate dehydrogenase (GDH) and NADP-specific alcohol dehydrogenase (ADH) activities, neither of which utilized NAD as a cofactor. The GDH is composed of identical subunits having an M(r) of 45,000 and had an optimal pH and optimal temperature for glutamate oxidation of 8.0 and > 95 degrees C, respectively. Potassium phosphate (60 mM), KCl (300 mM), and NaCl (300 mM) each stimulated the rate of glutamate oxidation activity between two- and threefold. For glutamate oxidation the apparent Km values at 80 degrees C for glutamate and NADP were 0.22 and 0.029 mM, respectively, and for 2-ketoglutarate reduction the apparent Km values for 2-ketoglutarate, NADPH, and NH4+ were 0.16, 0.14, and 0.63 mM, respectively. This enzyme is the first NADP-specific GDH purified form a hyperthermophilic organism. T. litoralis ADH is a tetrameric protein composed of identical subunits having an M(r) of 48,000; the optimal pH and optimal temperature for ethanol oxidation were 8.8 and 80 degrees C, respectively. In contrast to GDH activity, potassium phosphate (60 mM), KCl (0.1 M), and NaCl (0.3 M) inhibited ADH activity, whereas (NH4)2SO4 (0.1 M) had a slight stimulating effect. This enzyme exhibited broad substrate specificity for primary alcohols, but secondary alcohols were not oxidized.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural features responsible for kinetic thermal stability of a carboxypeptidase from the archaebacterium Sulfolobus solfataricus

Investigations were performed on the structural features responsible for kinetic thermal stability of a thermostable carboxypeptidase from the thermoacidophilic archaebacterium Sulfolobus solfataricus which had been purified previously and identified as a zinc metalloprotease [Colombo, D'Auria, Fusi, Zecca, Raia and Tortora (1992) Eur. J. Biochem. 206, 349-357]. Removal of Zn2+ by dialysis led to reversible activity loss, which was promptly restored by addition of 80 microM ZnCl2 to the assay mixture. For the first-order irreversible thermal inactivation the metal-depleted enzyme showed an activation energy value of 205.6 kJ.mol-1, which is considerably lower than that of the holoenzyme (494.4 kJ.mol-1). The values of activation free energies, enthalpies and entropies also dropped with metal removal. Thermal inactivation of the apoenzyme was very quick at 80 degrees C, whereas the holoenzyme was stable at the same temperature. These findings suggest a major stabilizing role for the bivalent cation. Chaotropic salts strongly destabilized the holoenzyme, showing that hydrophobic interactions are involved in maintaining the native conformation of the enzyme. However, the inactivation rate was also increased by sodium sulphate, acetate and chloride, which are not chaotropes, indicating that one or more salt bridges concur in stabilizing the active enzyme. Furthermore, at the extremes of the pH-stability curve, NaCl did not affect the inactivation rate, confirming the stabilizing role of intramolecular ionic bonds, as a pH-dependent decrease in stability is likely to occur from breaking of salt bridges involved in maintaining the native conformation of the protein.

Glutamate dehydrogenase from the extremely thermophilic archaebacterial isolate AN1

Glutamate dehydrogenase (L-glutamate:NADP+ oxidoreductase, deaminating and transaminating, EC 1.4.1.4) was purified to homogeneity from the extremely thermophilic archaebacterial isolate AN1 (a member of the Thermococcales). The enzyme comprised a large proportion of the soluble cell protein (11%) and was purified in high yield. The molecular mass of the native enzyme was 204 kDa, while the subunit molecular mass was 47 kDa, indicating a tetrameric structure. The enzyme is specific for NADP(H) rather than NAD(H) by a factor of greater than 1000, as judged by Vmax/Km. Glutamate synthase activity was about 50% of the glutamate dehydrogenase activity. Activity was markedly enhanced by calcium, magnesium and manganese ions. The enzyme was highly thermostable with t1/2 values of 12.5 h and 47 min at 90 degrees C and 103 degrees C, respectively.

A heat-stable serine proteinase from the extreme thermophilic archaebacterium Sulfolobus solfataricus

A proteinase was purified to electrophoretic homogeneity from crude extracts of the thermoacidophilic archaebacterium Sulfolobus solfataricus. Molecular mass values assessed by SDS-PAGE and gel filtration were 54 and 118 kDa, respectively, which points to a dimeric structure of the molecule. An isoelectric point of 5.6 was also determined. The enzyme behaved as a chymotrypsin-like serine proteinase, as shown by the inhibitory effects exerted by phenylmethanesulfonyl fluoride, 3,4-dichloroisocoumarin, tosylphenylalaninechloromethyl ketone and chymostatin. Consistently with the inhibition pattern, the enzyme cleaved chromogenic substrates at the carboxyl side of aromatic or bulky aliphatic amino acids; however, it effectively attacked only a small number of such substrates, thus, displaying a specificity much narrower than and clearly different from that of chymotrypsin. This was confirmed by its inability to digest a set of natural substrate proteins, as well as insulin chains A and B; only after alkylation casein was degraded to some extent. Proteinase activity was significantly stimulated by Mn2+ which acted as a mixed-type nonessential activator. The enzyme also displayed a broad pH optimum in the range 6.5-8.0. Furthermore, it was completely stable up to 90 degrees C; above this temperature it underwent first-order thermal inactivation with half-lives ranging from 342 min (92 degrees C) to 7 min (101 degrees C). At 50 degrees C it could withstand 6 M urea and, to some extent, different organic solvents; however, at 95 degrees C it was extensively inactivated by all of these compounds. None of the chemical physical properties of the enzyme, including amino-acid analysis, provided evidence of a possible relation to other well-known microbial serine proteinases.

Characterization of an extremely thermostable glutamate dehydrogenase: a key enzyme in the primary metabolism of the hyperthermophilic archaebacterium, Pyrococcus furiosus

Glutamate dehydrogenase (L-glutamate:NAD(P)+ oxidoreductase, deaminating, EC 1.4.1.3) from the hyperthermophilic Archeon Pyrococcus furiosus was purified to homogeneity by chromatography on anion-exchange, molecular-exclusion and hydrophobic-interaction media. The purified native enzyme had an M(r) of 270,000 +/- 15,000 and was shown to be a hexamer with identical subunits of M(r) 46,000. The enzyme was exceptionally thermostable, having a half-life of 3.5 to more than 10 h at 100 degrees C, depending on the concentration of enzyme. The Km of the enzyme for ammonia was high (9.5 mM), indicating that the enzyme is probably active in the deaminating, catabolic direction. The coenzyme utilization of the enzyme resembled the equivalent enzymes from eukaryotes rather than eubacteria, since both NADH and NADPH were recognized with high affinity. The enzyme displayed a preference for NADP+ over NAD+ that was more pronounced at low assay temperatures (50-70 degrees C) compared with the optimal temperature for enzyme activity, 95 degrees C.