Enzyme des Zuckerstoffwechsels aus hyperthermophilen und halophilen Archaea: Charakterisierung von Pyruvat Kinasen, Phosphoglycerat Mutasen und einer Xylose Dehydrogenase
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Comparative analyses of sugar catabolism revealed that degradation of glucose and hexose polymers in archaea proceeds via modified glycolytic pathways, mainly via modified Embden-Meyerhof pathways. So far regulation of the modified pathways by allosteric enzymes has not been analyzed, e.g. pyruvate kinases as potential allosteric enzymes have not been characterized. Also, little is known about phosphoglycerate mutases being involved in both glycolysis and gluconeogenesis in archaea. Finally, in contrast to the catabolism of hexoses the pathways of pentose catabolism and the enzymes involved have not been analyzed in archaea. Thus, in this thesis the following three enzymes of archaeal sugar metabolism were studied: Pyruvate kinases and phosphoglycerate mutases from hyperthermophilic archaea and xylose dehydrogenase, the first enzyme of xylose degradation, from the halophilic archaeon Haloarcula marismortui. Pyruvate kinases from archaea und from the bacterium Thermotoga maritima Pyruvate kinases (PK) in bacteria and eukarya are well studied enzymes being allosterically regulated by AMP and fructose-1,6-bisphosphate as positive allosteric effectors and by ATP as negative allosteric effector. Most PKs are dependent on potassium. Here, three hyperthermophilic archaeal PKs from Archaeoglobus fulgidus strain 7324, Pyrobaculum aerophilum and from Aeropyrum pernix were purified and characterized. In particular their potential regulatoric properties were analyzed. For comparison the PK from the hyperthermophilic bacterium Thermotoga maritima was characterized. Archaeal PKs were 200 kDa homotetrameric enzymes. They showed high temperature optima and were extremely thermostable up to 100°C. All archaeal PKs exhibited sigmoidal saturation kinetics with phosphoenolpyruvate and ADP indicating positive homotropic cooperative response to both substrates. Classical heterotropic allosteric regulators of PKs from eukarya and bacteria, e.g. fructose-1,6-bisphosphate or AMP, did not affect PK activity of hyperthermophilic archaea, suggesting the absence of heterotropic allosteric regulation. The PK from Thermotoga maritima was also a homotetramer. The enzyme showed a high temperature optimum and high thermostability, and exhibited cooperative response to phosphoenolpyruvate and ADP. In contrast to its archaeal counterparts, the PK from Thermotoga maritima exhibited the classical allosteric response to the activator AMP and the inhibitor ATP. Phylogenetic analysis of PK sequences of all three domains, including the characterized hyperthermophilic PKs, indicate a distinct archaeal cluster which includes the PK from the hyperthermophilic bacterium Thermotoga maritima. In contrast to most of the bacterial and eukaryal PKs the activity of the hyperthermophilic PKs was independent on monovalent cations such as potassium and ammonium. Potassium dependent PKs contain a characteristic sequence motif which includes a conserved glutamate which is considered to be essential for the binding of potassium. This glutamate is absent in potassium independent PKs, e.g. PK of T. maritima, which contains a lysine at the equivalent position. This lysine of T. maritima PK was substituted by glutamate via site directed mutagenesis. The activity of the mutated enzyme showed dependency on monovalent cations, supporting the postulated role of glutamate for the binding of monovalent cations. Phosphoglycerate mutases from archaea Two types of PGMs are known in bacteria and eukarya, one type is independent on the cofactor 2,3-bisphosphoglycerate (dPGM) and the other type is dependent on 2,3-bisphosphoglycerate (dPGM). dPGMs show reversible inhibition of activity by vanadate. Little is known about phosphoglycerate mutases in archaea. In this thesis an iPGM and the first archaeal dPGM were characterized. In the genome of Archaeoglobus fulgidus VC16 a hypothetical iPGM gene was identified and expressed in E. coli. The recombinant protein was characterized as a monomer (46 kDa) and showed iPGM activity. Using RT-PCR experiments the in vivo transcription of the gene was de monstrated. Phylogenetic analysis of iPGM sequences from all three domains indicate a distinct archaeal cluster. In the genome of Thermoplasma acidophilum a hypothetical dPGM gene was identified and expressed in E. coli. The recombinant protein was characterized as a homodimer composed of 24 kDa subunits. The dPGM activity required 2,3-bisphosphoglycerate and could be reversibly inhibited by vanadate. Xylose dehydrogenase from the halophilic archaeon Haloarcula marismortui In archaea nothing is known about the catabolism of pentoses and the enzymes involved. In bacteria the first steps of xylose degradation are catalyzed by xylose isomerase and xylulose kinase. Both enzymes could not be detected in xylose grown cells of Ha. marismortui. Instead, during growth on xylose a xylose dehydrogenase was induced. The xylose dehydrogenase was purified and characterized. The enzyme was characterized as a homotetramer (180 kDa) and catalyzes the oxidation of xylose to xylonate with NADP+ as physiological electron acceptor. The xylose dehydrogenase also catalyzed the oxidation of glucose but in contrast to xylose the catalytic efficiency was 70fold lower. Thus the dehydrogenase differs from glucose dehydrogenases. Using the N-terminal amino acid sequence of xylose dehydrogenase, the coding gene xyldh was identified in the partially sequenced genome of Ha. marismortui. The data indicate that the initial reaction of xylose degradation in the halophilic archaeon Ha. marismortui involves the oxidation of xylose rather than an isomerization which is the mechanism in bacteria.
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