Figure 4. Proposed reaction mechanism for hEXOG.

(a) The active site of apo hEXOG illustrating R¹³⁸ and R³¹⁴ interactions with SO₄²⁻ ions, and Mg (turquoise sphere) interactions with water molecules (black spheres) and N171. (b) The active site of hEXOG–DNA complex. DNA replaces water molecules in coordinating the Mg²⁺ ion. R¹³⁸ and R³¹⁴ bind to the DNA backbone at the +1 and +4 nt, respectively. (c) The crystal structure of EXOG–DNA complex shows that the Wing domain forms a docking site for the 5′-P of the substrate strand. (d) Proposed mechanism of DNA cleavage by hEXOG.