Figure 1.

Catalytic cycle of type II topoisomerases. Step 1: The enzyme bends the gate-, or G-, segment of DNA in the presence of divalent metal ions (the physiological ion is Mg²⁺). Step 2: The enzyme cleaves and covalently attaches to the newly generated 5’-termini of the G-segment, generating the cleavage complex. Step 3: The enzyme passes the transfer-, or T-, segment of DNA through the cut it generated in the G-segment. Step 4: The enzyme religates the G-segment. Step 5: The T-segment is released from the enzyme. Step 6: The enzyme releases the G-segment and resets for another round of catalysis. Note that ATP hydrolysis is required for the enzyme to complete the catalytic cycle. Modified from Reference [22].