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Figure 7.
Figure 7. Helicase mechanism schematic. The binding of
polynucleotide by NS3 helicase in the absence of ATP leaves a
large cleft between domains 1 and 2. Binding of ATP occurs with
the b-phosphate binding to residues in motif I (GSGKT) and the
g-phosphate with Mg2+ binding to the conserved acidic residues
in motif II (DECH). This results in the closing of the
interdomain cleft and the binding of conserved arginines in
motif VI (QRRGRTGR) to the ATP phosphates. Val432 and Trp501
disrupt base stacking at either end of the single-stranded
region. Closure of the interdomain cleft leads to translocation
of the single strand in the 5' to 3' direction and forces
several bases to slip past Trp501. Hydrolysis of ATP facilitates
opening of the cleft and release of ADP. The orientation of
Trp501 favors movement of the polynucleotide in only one
direction such that opening of the gap results in net movement
of the helicase in a 3' ->5' direction.
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