Hepatitis C virus NS3 NTPase/Helicase: different stereoselectivity in nucleoside triphosphate utilisation suggests that NTPase and helicase activities are coupled by a nucleotide-dependent rate limiting step11Edited by A. R. Fersht

Hepatitis C virus (HCV) NS3 protein is a multifunctional enzyme, possessing protease, NTPase and helicase activities within a single polypeptide of 625 amino acid residues. These activities are essential for the virus life cycle and are considered attractive targets for anti-HCV chemotherapy. Beside ATP, the NS3 protein has the ability to utilise deoxynucleoside triphosphates (dNTPs) as the energy source for nucleic acid unwinding. We have performed an extensive analysis of the substrate specificities of both NS3 NTPase and helicase activities with respect to all four dNTPs as well as with dideoxynucleoside triphoshate (ddNTP) analogs, including both d-(β) and l-(β)-deoxy and dideoxy-nucleoside triphosphates. Our results show that almost all dNTPs and ddNTPs tested were able to inhibit hydrolysis of ATP by the NTPase activity, albeit with different efficiencies. Moreover, this activity showed almost no stereoselectivity, being able to recognise both d-(β), l-(β)-deoxy and ddNTPs. On the contrary, the helicase activity had more strict substrate selectivity, since, among d-(β)-nucleotides, only ddTTP and its analog 2′,3′-didehydro-thymidine triphosphate could be used as substrates with an efficiency comparable to ATP, whereas among l-(β)-nucleotides, only l-(β)-dATP was utilised. Comparison of the steady-state kinetic parameters for both reactions, suggested that dATP, l-(β)-dCTP and l-(β)-dTTP, specifically reduced a rate limiting step present in the helicase, but not in the NTPase, reaction pathway. These results suggest that NS3-associated NTPase and helicase activities have different sensitivities towards different classes of deoxy and dideoxy-nucleoside analogs, depending on a specific step in the reaction, as well as show different enantioselectivity for the d-(β) and l-(β)-conformations of the sugar ring. These observations provide an essential mechanistic background for the development of specific nucleotide analogs targeting either activity as potential anti-HCV agents.