A self-compartmentalizing hexamer serine protease from Pyrococcus horikoshii: substrate selection achieved through multimerization.

Oligopeptidases impose a size limitation on their substrates, the mechanism of which has long been under debate. Here we present the structure of a hexameric serine protease, an oligopeptidase from Pyrococcus horikoshii (PhAAP), revealing a complex, self-compartmentalized inner space, where substrates may access the monomer active sites passing through a double-gated "check-in" system, first passing through a pore on the hexamer surface and then turning to enter through an even smaller opening at the monomers' domain interface. This substrate screening strategy is unique within the family. We found that among oligopeptidases, a residue of the catalytic apparatus is positioned near an amylogenic ß-edge, which needs to be protected to prevent aggregation, and we found that different oligopeptidases use different strategies to achieve such an end. We propose that self-assembly within the family results in characteristically different substrate selection mechanisms coupled to different multimerization states.

Structure and catalysis of acylaminoacyl peptidase: closed and open subunits of a dimer oligopeptidase.

Acylaminoacyl peptidase from Aeropyrum pernix is a homodimer that belongs to the prolyl oligopeptidase family. The monomer subunit is composed of one hydrolase and one propeller domain. Previous crystal structure determinations revealed that the propeller domain obstructed the access of substrate to the active site of both subunits. Here we investigated the structure and the kinetics of two mutant enzymes in which the aspartic acid of the catalytic triad was changed to alanine or asparagine. Using different substrates, we have determined the pH dependence of specificity rate constants, the rate-limiting step of catalysis, and the binding of substrates and inhibitors. The catalysis considerably depended both on the kind of mutation and on the nature of the substrate. The results were interpreted in terms of alterations in the position of the catalytic histidine side chain as demonstrated with crystal structure determination of the native and two mutant structures (D524N and D524A). Unexpectedly, in the homodimeric structures, only one subunit displayed the closed form of the enzyme. The other subunit exhibited an open gate to the catalytic site, thus revealing the structural basis that controls the oligopeptidase activity. The open form of the native enzyme displayed the catalytic triad in a distorted, inactive state. The mutations affected the closed, active form of the enzyme, disrupting its catalytic triad. We concluded that the two forms are at equilibrium and the substrates bind by the conformational selection mechanism.

Characterization of a novel acylaminoacyl peptidase with hexameric structure and endopeptidase activity.

We have overexpressed in E. coli, purified and investigated the kinetic, thermodynamic and biophysical properties of an acylaminoacyl peptidase (AAP), from the thermophile Pyrococcus horikoshii (PhAAP). It was shown that the electrostatic environment of the catalytic site of PhAAP substantially influenced the pH dependence of the specificity rate constant (k(cat)/K(m)). However, 0.3 M NaCl, which depressed the electrostatic effects, simplified the complex pH-rate profile. The rate of formation of the enzyme-substrate complex (k(1)) was obtained from a non-linear Arrhenius plot. The lack of substrate leaving group effects indicated that k(1) is the rate determining step in the catalysis. DSC and CD measurements demonstrated that PhAAP displayed a stable structure in the catalytically competent pH range. It was shown that PhAAP is not just an acylaminoacyl peptidase, but it also has an endopeptidase activity and so differs from the mammalian AAPs. Size exclusion chromatography with PhAAP revealed a hexameric structure, which is unique among the known members of the prolyl oligopeptidase family that includes AAPs and suggests that its cellular function may be different from that of the dimeric AAP also found in the same organism.

Detection and characterisation of Giardia and Cryptosporidium in Hungarian raw, surface and sewage water samples by IFT, PCR and sequence analysis of the SSUrRNA and GDH genes.

We investigated the prevalence of Giardia and Cryptosporidium species and analysed the genotypes in 36 samples collected from different water sources and various geographic areas in Hungary. Samples were collected from drinking water and sewage treatment plants and from the recreation area of Lake Balaton. The (oo)cysts were purified according to the US EPA 1623 method and they were detected by immunofluorescence test (IFT). Genomic DNA was extracted from all samples and then the GDH target gene for Giardia and the SSUrDNA for both Giardia and for Cryptosporidium species were amplified by PCR. 24 out of 36 samples (67%) were Giardia positive and 15 (42%) were Cryptosporidium positive by IFT. PCR confirmed that 13 out of 36 samples (36%) were Giardia positive and 10 (28%) contained Cryptosporidium. Twelve Giardia and two Cryptosporidium PCR products were successfully sequenced. In seven samples G. lamblia Assemblage A and in one sample Assemblage B and in four cases Assemblages A and B have been found. In one sample C. parvum and in the other separate sample C. meleagridis were detected. Sequence analysis revealed a new subtype of G. duodenalis complex, clustered close to the Assemblage A group. This study provides the first report on simultaneous detection and genotyping of G. duodenalis and Cryptosporidium species from water supplies in Hungary.