ENZYMES AND PROTEINS FROM ORGANISMS THAT GROW NEAR AND ABOVE 100-DEGREES-C

Microorganisms that can grow at and above 100-degrees-C were discovered a decade ago, and about 20 different genera are now known. These so-called hyperthermophiles are the most ancient of all extant life; all but two genera are classified as Archaea. All have been isolated from geothermal heated environments including deep-sea hydrothermal vents. This group includes some methanogenic and sulfate-reducing species, but the majority are strictly anaerobic heterotrophs that utilize complex peptide mixtures as sources of energy, carbon, and nitrogen. Only a few species are saccharolytic. Most of the hyperthermophiles absolutely depend on the reduction of elemental sulfur (S0) to H2S for significant growth, a property that severely limits their large-scale culture in conventional fermentation systems. Consequently, most physiological and metabolic studies have focused on those that can also grow in the absence of S0, including species of the Archaea, Pyrococcus and Thermococcus, and the bacterium Thermotoga. The fermentative pathways for the metabolism of both peptides and carbohydrates in the Archaea appear to depend upon enzymes that contain tungsten, an element seldom used in biological systems. The mechanisms of S0 reduction and energy conservation remain unclear. Enzymes purified from the S0-reducing hyperthermophiles include proteases, amylolytic-type enzymes, hydrogenases, redox proteins, various ferredoxin-linked oxidoreductases, dehydrogenases, and DNA polymerases, some of which are active up to 140-degrees-C. However, complete amino acid sequences are known for only a handful of these proteins, and the three-dimensional structure of only one hyperthermophilic protein has been determined. Potential mechanisms by which proteins and various biological cofactors and organic intermediates are stabilized at extreme temperatures are only now beginning to emerge.