Biochemical diversity among sulfur-dependent, hyperthermophilic microorganisms

Abstract

Hyperthermophiles are a recently discovered group of microorganisms that grow at and above 90 degrees C. They currently comprise over 20 different genera, and except for two novel bacteria, all are classified as Archaea. The majority of these organisms are obligately anaerobic heterotrophs that reduce elemental sulfur (S degree) to H2S. The best studied from a biochemical perspective are the archaeon, Pyrococcus furiosus, and the bacterium, Thermotoga maritima, both of which are saccharolytic. P. furiosus is thought to contain a new type of Entner-Doudoroff pathway for the conversion of carbohydrates ultimately to acetate, H2 and CO2. The pathway is independent of nicotinamide nucleotides and involves novel types of ferredoxin-linked oxidoreductases, one of which has tungsten, a rarely used element, as a prosthetic group. The only site of energy conservation is at the level of acetyl CoA, which is the presence of ADP and phosphate is converted to acetate and ATP in a single step. In contrast, T. maritima utilizes a conventional Embden-Meyerhof pathway for sugar oxidation. P. furiosus also utilizes peptides as a sole carbon and energy source. Amino acid oxidation is thought to involve glutamate dehydrogenase together with at least three types of novel ferredoxin-linked oxidoreductases which catalyze the oxidation of 2-ketoglutarate, aryl pyruvates and formaldehyde. One of these enzymes also utilizes tungsten. In P. furiosus, virtually all of the reductant that is generated during the catabolism of both carbohydrates and peptides is channeled to a cytoplasmic hydrogenase. This enzyme is now termed sulhydrogenase, as it reduces both protons to H2 and S degrees (or polysulfide) to H2S. S degrees reduction appears to lead to the conservation of energy in P. furiosus but not in T. maritima, although the mechanism by which this occurs is not known.