A Steady-state and Pre-steady-state Kinetic Analysis of the NTPase Activity Associated with the Hepatitis C Virus NS3 Helicase Domain

Abstract

The helicase domain of hepatitis C virus NS3 (genotype 1b) was expressed in Escherichia coli and purified to homogeneity. The purified protein catalyzed the hydrolysis of nucleoside triphosphates (NTP) and the unwinding of duplex RNA in the presence of divalent metal ion. The enzyme was not selective for the NTP substrate. For example, UTP and acyclovir triphosphate were hydrolyzed efficiently by the enzyme. The rate of NTP hydrolysis was stimulated up to 27-fold by oligomeric nucleic acids (NA). Furthermore, NA bound to the enzyme with concomitant quenching of the intrinsic protein fluorescence. The dissociation constants of the enzyme for selected NA in the absence of NTP were between 10 and 35 microM at pH 7.0 and 25 degrees C. The enzyme had maximal affinity for NA with 12 or more nucleotides. A detailed steady-state and pre-steady-state kinetic analysis of ATP hydrolysis was made with (dU)18 as the effector. The kcat values for ATP hydrolysis in the presence and absence of (dU)18 were 80 s-1 and 2.7 s-1, respectively. The association (dissociation) rate constants for the enzyme and (dU)18 in the presence and absence of ATP were 5.7 microM-1 s-1 (3.9 s-1) and 290 microM-1 s-1 (2.27 s-1), respectively. The association (dissociation) rate constants for the enzyme and ATP in the presence and absence of (dU)18 were 0.4 microM-1 s-1 (<0.5 s-1) and 0.9 microM-1 s-1 (<10(-1) s-1), respectively. These data were consistent with a random kinetic mechanism.