Figure 8.

The flexible regions of hFEN1 respond to the appropriate structural features of the substrate, resulting in a shift to the catalytically viable ensemble. (A) In the absence of substrate, hFEN1 contains flexible regions in the ‘arch’ (red) and the α2–α3 loop (blue). The tops of these regions are disordered and display very fast motions (∼10⁹ s⁻¹), whereas the flanking regions are absent presumably due to intermediate exchange broadening. The observable regions of the arch and loop are likely in fast exchange because they experience little to no change in chemical environment from the millisecond timescale motion. (B) Upon binding DNA, the disordered regions experience exchange broadening, likely due to a change in chemical environment and/or change in rate of motions. However, the slow millisecond timescale movements persist in both the arch and the α2–α3 loop despite the change in the conformational ensemble. (C) Eventually, the catalytically viable ensemble forms possibly coupling motions within the arch, the α2–α3 loop and the DNA.