DNA topoisomerase II–DNA complex (Top2–DNA complex (or TOP2cc for "topoisomerase II cleavage complex"))

The **Topoisomerase II–DNA complex**, also known as the "cleavage complex" or "TOP2cc," is a transient covalent intermediate formed when type II DNA topoisomerases cleave both strands of duplex DNA to resolve supercoiling, catenanes, and other topological problems during essential cellular processes such as replication, transcription, and chromosome segregation. The enzyme forms a reversible phosphotyrosine bond with each broken strand; under normal conditions this break is quickly resealed. However, anticancer agents known as "topo-II poisons" stabilize this otherwise fleeting state by preventing religation. This leads to persistent double-strand breaks that trigger cell death—an effect exploited therapeutically against cancer but also associated with significant risks including secondary cancers and cardiotoxicity. The structure-function relationship within these ternary complexes has been elucidated at high resolution; key domains involved include the winged helix domain (WHD) containing the active site tyrosine responsible for catalysis and metal ion coordination by the TOPRIM domain critical for breakage/religation chemistry. The clinical importance stems from both their central role in cell proliferation control—and their vulnerability to pharmacologic intervention—making them validated targets across multiple chemotherapeutic regimens.[1][4][5][6]

Other names

Topoisomerase II–DNA cleavage complexTOP2ccDNA topoisomerase II covalent intermediateType IIA topoisomerase–DNA complex

Drugs targeting this molecule act primarily as **topoisomerase poisons**, which: - Stabilize the transient covalent Top2–DNA cleavage complexes, preventing religation of double-stranded breaks.[5][6][3] - Lead to accumulation of double-strand breaks, blocking replication/transcription and triggering apoptosis in rapidly dividing cells.[5][6] Some drugs are **catalytic inhibitors**, which: - Inhibit the catalytic activity without stabilizing the cleavage complex or causing direct DNA damage.[5]