Hepatitis C virus, NS3 protease (HepC_NS3_protease)
Type: Domain

This entry represents the Hepatitis C virus NS3 protein, a serine protease which belongs to MEROPS peptidase family S29 (hepacivirin family, clan PA(S)), which has a trypsin-like fold. The non-structural (NS) protein NS3 is one of the NS proteins involved in replication of the HCV genome. The NS2 proteinase (

), a zinc-dependent enzyme, performs a single proteolytic cut to release the N terminus of NS3. The action of NS3 proteinase (NS3P), which resides in the N-terminal one-third of the NS3 protein, then yields all remaining non-structural proteins. The C-terminal two-thirds of the NS3 protein contain a helicase. The functional relationship between the proteinase and helicase domains is unknown. NS3 has a structural zinc-binding site and requires cofactor NS4. It has been suggested that the NS3 serine protease of hepatitis C is involved in cell transformation and that the ability to transform requires an active enzyme .

Although Hepatitis A virus, Hepatitis B virus, and Hepatitis C virus have similar names, because they all cause liver inflammation, these are distinctly different viruses both genetically and clinically. The Hepatitis C virus (HCV) is a small (50-80 nm in diameter), enveloped, single-stranded, positive sense RNA virus. It is member of the family Flaviviridae. There are seven genotypes and a number of subtypes with diverse geographic distributions. The genome of HCV consists of a single open reading frame. At the 5' and 3' ends of the RNA are the UTR regions that are not translated into proteins but are important to translation and replication of the viral RNA. The 5' UTR has a ribosome binding site (IRES - Internal ribosome entry site) that starts the translation of unique polyprotein that is later cut by cellular and viral proteases into 10 active structural and non-structural smaller proteins .

Proteolytic enzymes that exploit serine in their catalytic activity are ubiquitous, being found in viruses, bacteria and eukaryotes . They include a wide range of peptidase activity, including exopeptidase, endopeptidase, oligopeptidase and omega-peptidase activity. Many families of serine protease have been identified, these being grouped into clans on the basis of structural similarity and other functional evidence . Structures are known for members of the clans and the structures indicate that some appear to be totally unrelated, suggesting different evolutionary origins for the serine peptidases .

Not withstanding their different evolutionary origins, there are similarities in the reaction mechanisms of several peptidases. Chymotrypsin, subtilisin and carboxypeptidase C have a catalytic triad of serine, aspartate and histidine in common: serine acts as a nucleophile, aspartate as an electrophile, and histidine as a base . The geometric orientations of the catalytic residues are similar between families, despite different protein folds . The linear arrangements of the catalytic residues commonly reflect clan relationships. For example the catalytic triad in the chymotrypsin clan (PA) is ordered HDS, but is ordered DHS in the subtilisin clan (SB) and SDH in the carboxypeptidase clan (SC) .