Browsing by Author "Futterer, O."

The euryarchaea Picrophilus torridus and Picrophilus oshimae are able to grow around pH 0 at up to 65degreesC, thus they represent the most thermoacidophilic organisms known. Several features that may contribute to the thermoaciclophilic survival strategy of A torridus were deduced from analysis of its 1.55-megabase genome. A torridus has the smallest genome among nonparasitic aerobic microorganisms growing on organic substrates and simultaneously the highest coding density among thermoacidophiles. An exceptionally high ratio of secondary over ATP-consuming primary transport systems demonstrates that the high proton concentration in the surrounding medium is extensively used for transport processes. Certain genes that may be particularly supportive for the extreme lifestyle of P. torridus appear to have been internalized into the genome of the Picrophilus lineage by horizontal gene transfer from crenarchaea and bacteria. Finally, it is noteworthy that the thermoaciclophiles from phylogenetically distant branches of the Archaea apparently share an unexpectedly large pool of genes.

The gene for a novel alpha-amylase, designated AmyC, from the hyperthermophilic bacterium Thermotoga maritima was cloned and heterologously overexpressed in Escherichia coli. The putative intracellular enzyme had no amino acid sequence similarity to glycoside hydrolase family (GHF) 13 alpha-amylases, yet the range of substrate hydrolysis and the product profile clearly define the protein as an alpha-amylase. Based on sequence similarity AmyC belongs to a subgroup within GHF 57. On the basis of amino acid sequence similarity, Glu185 and Asp349 could be identified as the catalytic residues of AmyC. Using a 60-min assay, the maximum hydrolytic activity of the purified enzyme, which was dithiothreitol dependent, was found to be at 90 degrees C. AmyC displayed a remarkably high pH optimum of pH 8.5 and an unusual sensitivity towards both ATP and EDTA.

In Picrophilus torridus, a euryarchaeon that grows optimally at 60 degreesC and pH 0.7 and thus represents the most acidophilic thermophile known, glucose oxidation is the first proposed step of glucose catabolism via a nonphosphorylated variant of the Entner-Doudoroff pathway, as deduced from the recently completed genome sequence of this organism. The P. torridus gene for a glucose dehydrogenase was cloned and expressed in Escherichia coli, and the recombinant enzyme, GdhA, was purified and characterized. Based on its substrate and coenzyme specificity, physicochemical characteristics, and mobility during native PAGE, GdhA apparently resembles the main glucose dehydrogenase activity present in the crude extract of P. torridus DSM 9790 cells. The glucose dehydrogenase was partially purified from P. torridus cells and identified by MS to be identical with the recombinant GdhA. P. torridus GdhA preferred NADP(+) over NAD(+) as the coenzyme, but was nonspecific for the configuration at C-4 of the sugar substrate, oxidizing both glucose and its epimer galactose (K-m values 10.0 and 4.5 mm, respectively). Detection of a dual-specific glucose/galactose dehydrogenase points to the possibility that a 'promiscuous' Entner-Doudoroff pathway may operate in P. torridus, similar to the one recently postulated for the crenarchaeon Sulfolobus solfataricus. Based on Zn2+ supplementation and chelation experiments, the P. torridus GdhA appears to contain structurally important zinc, and conserved metal-binding residues suggest that the enzyme also contains a zinc ion near the catalytic site, similar to the glucose dehydrogenase enzymes from yeast and Thermoplasma acidophilum. Strikingly, NADPH, one of the products of the GdhA reaction, is unstable under the conditions thought to prevail in Picrophilus cells, which have been reported to maintain the lowest cytoplasmic pH known (pH 4.6). At the optimum growth temperature for P. torridus, 60 degreesC, the half-life of NADPH at pH 4.6 was merely 2.4 min, and only 1.7 min at 65 degreesC (maximum growth temperature). This finding suggests a rapid turnover of NADPH in Picrophilus.