Structural characterization of metalloprotease vibriolysin of cholera pathogen Vibrio cholerae.

Vibriolysin is among several zinc metalloproteases produced by Vibrio cholerae. It is involved in the molecular pathogenicity of cholera. Here, we cloned and expressed full-length vibriolysin gene from V. cholerae. Electrophoretic and mass spectrometric data showed that the N-terminal pro-peptide was removed from pro-vibriolysin generating a 45-kDa segment containing the metalloprotease plus the C-terminal domains, and the 35 kDa metalloprotease. The 35 kDa metalloprotease segment of vibriolysin was purified to homogeneity using ion-exchange and gel filtration chromatography. Circular dichroism (CD) analysis of vibriolysin indicated α+ß secondary structure, similar to other closely related metalloproteases of known structure. Positive dichroic absorption maxima in near-UV CD spectrum provided evidence for bound metal atom(s). Dynamic Light Scattering (DLS) measurements at different pHs were also performed to establish the aggregational properties of purified vibriolysin in solution. The results of DLS studies revealed that vibriolysin exists as a homomer with a hydrodynamic radius of 56.7 nm ± 2% under physiological conditions and remains catalytic when BSA was used as a protein substrate. While, extreme acidic (pH 3.5-5.4; R(H) = 65-239 nm) and alkaline (pH 8.5-9.4; R(H) = 57-72 nm) buffering conditions induced further aggregation of vibriolysin, without any trace of the monomeric state in solution.

Bioinformatics of granzymes: sequence comparison and structural studies on granzyme family by homology modeling.

Cytotoxic lymphocytes (CTLs), the key players of cell mediated immunity, induce apoptosis by engaging death receptors or through exocytosis of cytolytic granules containing granzyme (proteases) and pore-forming protein (perforin). The crystal structure of granzyme B from human (B(h)) and rat (B(r)), as well as that of pro-granzyme K (K(h)) has been reported recently. In the present communication, we describe the homology modeling of granzyme family (in particular Gzm A(h), M(h), B(m), and C(m) from human and mouse) based on the crystal structural coordinates of trypsin, granzyme K (K(h)), and granzyme B (B(h)). These models have been used for establishing phylogenetic relationship as well as identifying characteristic features for designing specific inhibitors. The paper also highlights key residues at the S1, S2, and S2(') binding subsites in all granzyme, which may be involved in the structure-function relationship of this enzyme family. The predicted 3D homology models show a conserved two similar domain structure, i.e., an N-terminal domain and a C-terminal domain comprising predominantly of beta-sheet structure with a little alpha-helical content. Micro-heterogeneities have been observed in the vicinity of the active site in all granzymes as compared to granzyme B(h). For example, in granzyme M(h), valine is present at the S1 subsite instead of arginine. Similarly differences at S2 (Leu-->Phe), S3 (Ser-->Gly), and S4 (Arg-->Asn) subsites are quite apparent and appear to hold the potential for selective designing of inhibitors for possible therapeutic applications. Furthermore, analysis of the electrostatic surface potential on the shape of granzyme-inhibitor binding groove reveals clear differences at the reactive site. Additionally the different posttranslational modification sites such as phosphorylation (e.g., in granzyme M Thr101, Ser109), myristoylation (Gly22, 117, and 131), and glycosylation (Ser160) have been identified, as very little is known about the functional significance of these modifications in the granzyme family. Thus, glycosylation at Ser160 in granzyme M may influence the net charge of the enzyme, resulting in altered substrate binding as compared to granzyme B. Also this modification may influence the rate of complexation and binding affinity with proteoglycans. These studies are expected to contribute towards the basic understanding of functional associations of the granzymes with other molecules and their possible role in apoptosis.

Molecular mechanism of apoptosis: prediction of three-dimensional structure of caspase-6 and its interactions by homology modeling.

Caspases, the intracellular cysteine proteinases, play a central role in the process of programmed cell death. Caspases induce apoptosis through a highly integrated and regulated biological, biochemical, and genetic mechanism. Although proper execution of apoptosis is fundamental for cell growth artificial caspase inhibition can be considered in certain degenerative diseases. This realization has attracted attention towards caspases as likely targets for pharmaceutical intervention. Here we analyze the structure of caspase-6 and also predict the possible glycosylation, phosphorylation, and myristoylation sites as very little is known about the functional role of these post translational modifications in the caspase family. These studies are expected to improve our understanding of associations of caspases with other molecules and the possible role played in apoptosis. The predicted tertiary structure of caspase-6 as well as the enzyme complexed with its inhibitor (tetra-peptide aldehyde Ac-IETD-CHO) shows similar binding feature as seen in other caspases. Cys/His catalytic dyad for caspase-6 and -8 show possible involvement of a third component, i.e., Pro29 and Arg258 in caspase-6 and caspase-8, respectively. Changes in the length and nature of loop between alpha5 and beta9, involved in defining the S4 subsite, result in modification of P4 (Ile) site. These interactions provide detail of inhibitor binding on structural level and also help in designing mutants for structure-function studies of these enzymes.