Capture of a third Mg 2+ is essential for catalyzing DNA synthesis

A hit-and-run metal ion DNA polymerase is an enzyme that uses existing DNA as a template to build new DNA by adding new nucleotides to the end of the newly forming daughter strand. The enzyme mechanism that catalyzes formation of a phosphodiester bond is known to require two Mg 2+ ions, and recent crystal structures have shown that a third metal ion is present after bond formation. Gao et al. used time-resolved crystallography to visualize bond formation. The enzyme-substrate complex captures a third cation before bond formation occurs, and DNA synthesis cannot occur without the third metal ion. Binding of this metal ion requires thermal motion of the enzyme-substrate complex, so that catalysis is achieved by acquiring a transient cofactor. Science , this issue p. 1334