Helicases separate nucleic acid duplexes into their component strands using energy from ATP hydrolysis. The crystal structure of this DNA helicase from bacteriophage T7, reveals an hexagonal arrangement of six identical subunits. Surprisingly, the ring is not sixfold symmetric, but is slightly squished. A model for the mechanism of how the enzyme might work explains this structural asymmetry. Of the six potential ATP binding sites, two opposing ones bind ATP tightly, two are more likely to bind ADP and phosphate, and two are empty. These three states may interconvert in a coordinate fashion as ATP is hydrolyzed, creating a ripple effect that continuously runs around the ring. Because of these conformational changes, the loops that extend into the center hole of the ring—that are proposed to bind DNA—oscillate up and down, as seen in this cross section. The oscillating loops might pull a DNA strand through the central hole, thus unwinding the double helix in the process. A frontal view shows the full dynamics of this fascinating protein machine.
