A homolog to the eubacteria inorganic pyrophosphatase (PPase, EC 3.6.1.1) was found in the genome of the hyperthermophilic archaeon Pyrococcus horikoshii. This inorganic pyrophosphatase (Pho-PPase) grows optimally at 88℃. To understand the structural basis for the thermostability of Pho-PPase, we have determined the crystal structure to 2.66 A resolution. The crystallographic asymmetric unit contains three monomers related by approximate threefold symmetry, and a hexamer is built up by twofold crystallographic symmetry. The main-chain fold of Pho-PPase is almost identical to that of the known crystal structure of the model from Sulfolobus acidocaldarius. A detailed comparison of the crystal structure of Pho-PPase with related structures from S. acidocaldarius, Thermus thermophilus, and Escherichia colishows significant differences that may account for the difference in their thermostabilities. A reduction in thermolabile residues, additional aromatic residues, and more intimate association between subunits all contribute to the larger thermophilicity of Pho-PPase. In particular, deletions in two loops surrounding the active site help to stabilize its conformation, while ion-pair networks unique to Pho-PPase are located in the active site and near the C-terminus. The identification of structural features that make PPases more adaptable to extreme temperature should prove helpful for future biotechnology applications.

Acylpeptide hydrolases (APH; also known as acylam-ino acid releasing enzyme) catalyze the removal of an N-acylated amino acid from blocked peptides. The crystal structure of an APH from the thermophilic arch-aeon Aeropyrum pernix K1 to 2.1 A resolution confirms it to be a member of the prolyl oligopeptidase family of serine proteases. The structure of apAPH is a sym-metric homodimer with each subunit comprised of two domains. The N-terminal domain is a regular seven-bladed β-propeller, while the C-terminal domain has a canonical α/β hydrolase fold and includes the active site and a conserved Ser445-Asp524-His556 catalytic triad. The complex structure of apAPH with an organo- phosphorus substrate, p-nitrophenyl phosphate, has also been determined. The complex structure unam- biguously maps out the substrate binding pocket and provides a basis for substrate recognition by apAPH. A conserved mechanism for protein degradation from archaea to mammals is suggested by the structural features of apAPH.

The gene APE1547 of the aerobic thermophilic Aeropyrum pernix K1 encoding 582 amino acid residues was cloned into Escherichia coli. BL21 (DE3) by using vector pET11a with a T7 promoter. An alignment of similarity analysis of APE1547 with protein sequences from A. pernix K1 databank revealed that it showed a lipase motif and low homology with the known thermophilic esterases. However, it had a high degree homology with several acyl amino acid-releasing enzymes. After purified by ion exchange chromatography and gel filtration chromatography, the recombinant protein showed both esterase activity and acylamino acid-releasing enzyme (AARE) activities. The optimum of temperature and pH of the esterase activity are 90℃ and 8.0, respectively. The recombinant protein showed the hydrolytic activity for a wide range of substrates, such as p-nitrophenyl alkanoate esters of varying alkyl chain lengths, pNA-labelled amino acid and peptide. The highest activity was observed for the substrate p-nitrophenyl caprylate. The recombinant enzyme was extremely stable and protein concentration-dependent. Its half-life at 90℃ was over 160h. at the concentration of 2.14 mg/ml, which renders this new esterase very attractive for biotechnological applications.

HPhA, a recombinant histone-like protein from Pyrococcus horikoshii OT3 strain, has compacting activity with DNA as previously reported. The extreme stability and DNA packaging activity of the HPhA make it a candidate as a DNA carrier. Here, the plasmid DNA-HPhA complexes were fully characterized by gel retardation assay and DNase resistance assay. It was further proved that HPhA has in vitro DNA transfection activity. HPhA-mediated transfection efficiency was dependent on the mass ratio of HPhA to DNA, the incubation time and the presence of calcium. A protocol for HPhA-mediated transfection in vitro was established to improve transfection efficiency. The optimal mass ratio of HPhA to DNAwas 6:1, and the incubation time required for the DNA-HPhA complex to be in contact with the cell was 4 h. In addition, the presence of 2 mM CaCl2 in the cell culture medium was required for efficient transfection. Serum did not show inhibition of HPhA-mediated transfection. Most importantly, the cytotoxicity of HPhA is lower than that of commonly used cationic liposome-based gene delivery systems, and HPhA-mediated transfection in NIH 3T3, HEK 293, HL-7702, HepG2 and Cos 7 cell lines in vitro has a higher efficiency and reproducibility. These results demonstrate that the HPhA is a new, potentially widely applicable and highly efficient gene carrier.

An open reading frame of the hyperthermophilic archaeon Aeropyrum pernix K1 APE2325, which composed of 474 bases, was cloned and expressed in Escherichia coli BL21 (DE3) Codon Plus-RIL. The recombinant protein was puriWed by Ni-chelation aYnity chromatography. It showed a single band with a molecular mass of 18kDa in SDS–PAGE. The puriWed enzyme exhibited both phospholipase A2 and esterase activities with the optimal catalytic temperature at 90℃. The enzyme activity was Ca2+-independent. Kinetic analysis revealed its Km, kcat, and Vm for the p-nitrophenyl propionate substrate were 103μM, 39s-1, and 249μmol/min/mg, respectively. The recombinant protein was thermostable and its half-life at 100℃ was about 1h.