Type II Topoisomerase - Classification

Classification

There are two subclasses of type II topoisomerases, type IIA and IIB.

• Type IIA topoisomerases include the enzymes DNA gyrase, eukaryotic topoisomerase II (topo II), and bacterial topoisomerase IV (topo IV). These enzymes span all domains of life and are essential for function.
• Type IIB topoisomerases are structurally and biochemically distinct, and comprise a single family member, topoisomerase VI (topo VI). Type IIB topoisomerases are found in archaea and some higher plants.

Some organisms have two isoforms of topoisomerase II: alpha and beta. In cancers, the topoisomerase II-alpha is highly expressed in highly proliferating cells. In certain cancers, such as peripheral nerve sheath tumors, high expression of its encoded protein is also associated to poor patient survival.

The two classes of topoisomerases possess a similar strand passage mechanism and domain structure (see below), however they also have several important differences. Type IIA topoisomerases form double-stranded breaks with four-base pair overhangs, while type IIB topoisomerases form double-stranded breaks with two base overhangs (Buhler, Lebbink, Bocs, Ladenstein, and Forterre, Journal of Biological Chemistry 2001). In addition, type IIA topoisomerases are able to simplify DNA topology (Rybenkov Science 1997), while type IIB topoisomerases do not (Corbett Journal of Molecular Biology, 2006).