  Helicases are molecular motors which unwind double-stranded nucleic acids
(dsNA) in cells. Many helicases move with directional bias on single-stranded
(ss) nucleic acids, and couple their directional translocation to strand
separation. A model of the coupling between translocation and unwinding uses an
interaction potential to represent passive and active helicase mechanisms. A
passive helicase must wait for thermal fluctuations to open dsNA base pairs
before it can advance and inhibit NA closing. An active helicase directly
destabilizes dsNA base pairs, accelerating the opening rate. Here we extend
this model to include helicase unbinding from the nucleic-acid strand. The
helicase processivity depends on the form of the interaction potential. A
passive helicase has a mean attachment time which does not change between ss
translocation and ds unwinding, while an active helicase in general shows a
decrease in attachment time during unwinding relative to ss translocation. In
addition, we describe how helicase unwinding velocity and processivity vary if
the base-pair binding free energy is changed.
