The purification and some properties of a stereospecific D-asparaginase from an extremely thermophilic bacterium, Thermus aquaticus

The enzymes from extreme thermophiles: bacterial sources, thermostabilities and industrial relevance

This review on enzymes from extreme thermophiles (optimum growth temperature greater than 65 degrees C) concentrates on their characteristics, especially thermostabilities, and their commercial applicability. The enzymes are considered in general terms first, with comments on denaturation, stabilization and industrial processes. Discussion of the enzymes subsequently proceeds in order of their E.C. classification: oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases. The ramifications of cloned enzymes from extreme thermophiles are also discussed.

Thermozymes

Enzymes synthesized by thermophiles (organisms with optimal growth temperatures > 60 degrees C) and hyperthermophiles (optimal growth temperatures > 80 degrees C) are typically thermostable (resistant to irreversible inactivation at high temperatures) and thermophilic (optimally active at high temperatures, i.e., > 60 degrees C). These enzymes, called thermozymes, share catalytic mechanisms with their mesophilic counterparts. When cloned and expressed in mesophilic hosts, thermozymes usually retain their thermal properties, suggesting that these properties are genetically encoded. Sequence alignments, amino acid content comparisons, and crystal structure comparisons indicate that thermozymes are, indeed, very similar to mesophilic enzymes. No obvious sequence or structural features account for enzyme thermostability and thermophilicity. Thermostability and thermophilicity molecular mechanisms are varied, differing from enzyme to enzyme. Thermostability and thermophilicity are usually caused by the accumulation of numerous subtle sequence differences. This review concentrates on the mechanisms involved in enzyme thermostability and thermophilicity. Their relationships with protein rigidity and flexibility and with protein folding and unfolding are discussed. Intrinsic stabilizing forces (e.g., salt bridges, hydrogen bonds, hydrophobic interactions) and extrinsic stabilizing factors are examined. Finally, thermozymes' potential as catalysts for industrial processes and specialty uses are discussed, and lines of development (through new applications, and protein engineering) are also proposed.

A specific L-asparaginase from Thermus aquaticus

The L-asparaginase from an extreme thermophile, Thermus aquaticus strain T351, was highly substrate- and stereospecific, with no activity against glutamine or D-asparagine. It had a high Km of 8.6 mM. In these aspects it closely resembled the corresponding enzymes from thermophilic bacteria. The enzyme had a molecular weight of 80,000, an isoelectric point of 4.6, and a pH optimum of 9.5. It showed some substrate inhibition above 20 mM asparagine and was also inhibited by L-aspartic acid, D- and L-lysine (Ki of 5.2 and 1.25 mM, respectively), and D- and L-serine. The half-life of the enzyme at 85 degrees C was 40 min. The Arrhenius plot showed a change in slope at 55 degrees C.

A correlation between protein thermostability and resistance to proteolysis

The loss of activity due to proteolysis of purified L-asparaginase and beta-galactosidase from different sources correlates with the thermal instability of the enzymes. A similar correlation is found when populations of soluble proteins from micro-organisms grown at different temperatures are compared for proteolytic susceptibility and thermal stability. It is proposed that there is a general correlation between the thermostability of proteins and their resistance to proteolysis.