A thermophilic esterase, SsoPEst, from Sulfolobus solfataricus P2 was cloned and expressed in E. coli AD494 (DE3). Gene sequencing indicated the encoded 353 amino acids had less than 32% identity with reported esterases. The recombinant enzyme hydrolyzed p-nitrophenyl esters but not tributyrin or tricaprylin, exhibiting the highest specific activity (1.1 U/mg) with p-nitrophenyl caprylate. The enzyme was optimally active at pH 5.5 and 80A degrees C. It retained 50% activity after 1 h incubation at 80A degrees C. Activity was significantly inhibited by PMSF. Five SsoPEst mutants were generated by site-directed mutagenesis. One mutant had a higher specific activity of 2.8 U/mg at 37A degrees C and 14 U/mg at 80A degrees C than the wild-type enzyme which exhibited 0.7 U/mg at 37A degrees C and 3.8 U/mg at 80A degrees C against p-nitrophenyl butyrate.