DNA topoisomerase I (TopoI)
Type: Family

DNA topoisomerases regulate the number of topological links between two DNA strands (i.e. change the number of superhelical turns) by catalysing transient single-or double-strand breaks, crossing the strands through one another, then resealing the breaks . These enzymes have several functions: to remove DNA supercoils during transcription and DNA replication; for strand breakage during recombination; for chromosome condensation; and to disentangle intertwined DNA during mitosis . DNA topoisomerases are divided into two classes: type I enzymes (

; topoisomerases I, III and V) break single-strand DNA, and type II enzymes (

; topoisomerases II, IV and VI) break double-strand DNA .

Type I topoisomerases are ATP-independent enzymes (except for reverse gyrase), and can be subdivided according to their structure and reaction mechanisms: type IA (Topo IA; bacterial and archaeal topoisomerase I, topoisomerase III and reverse gyrase) and type IB (Topo IB; eukaryotic topoisomerase I and topoisomerase V). These enzymes are primarily responsible for relaxing positively and/or negatively supercoiled DNA, except for reverse gyrase, which can introduce positive supercoils into DNA. This function is vital for the processes of replication, transcription, and recombination. Unlike Topo IA enzymes, Topo IB enzymes do not require a single-stranded region of DNA or metal ions for their function. The type IB family of DNA topoisomerases includes eukaryotic nuclear topoisomerase I, topoisomerases of poxviruses, and bacterial versions of Topo IB . They belong to the superfamily of DNA breaking-rejoining enzymes, which share the same fold in their C-terminal catalytic domain and the overall reaction mechanism with tyrosine recombinases . The C-terminal catalytic domain in topoisomerases is linked to a divergent N-terminal domain that shows no sequence or structure similarity to the N-terminal domains of tyrosine recombinases .

This family of DNA topoisomerase I enzymes includes both type IA enzymes from bacteria and type IB enzymes from eukaryotes and viruses.

Human topoisomerase I has been shown to be inhibited by camptothecin (CPT), a plant alkaloid with antitumour activity . The crystal structures of human topoisomerase I comprising the core and carboxyl-terminal domains in covalent and noncovalent complexes with 22-base pair DNA duplexes reveal an enzyme that "clamps" around essentially B-form DNA. The core domain and the first eight residues of the carboxyl-terminal domain of the enzyme, including the active-site nucleophile tyrosine-723, share significant structural similarity with the bacteriophage family of DNA integrases. A binding mode for the anticancer drug camptothecin has been proposed on the basis of chemical and biochemical information combined with the three-dimensional structures of topoisomerase I-DNA complexes .

Vaccinia virus, a cytoplasmically-replicating poxvirus, encodes a type I DNA topoisomerase that is biochemically similar to eukaryotic-like DNA topoisomerases I, and which has been widely studied as a model topoisomerase. It is the smallest topoisomerase known and is unusual in that it is resistant to the potent chemotherapeutic agent camptothecin. The crystal structure of an amino-terminal fragment of vaccinia virus DNA topoisomerase I shows that the fragment forms a five-stranded, antiparallel β-sheet with two short α-helices and connecting loops. Residues that are conserved between all eukaryotic-like type I topoisomerases are not clustered in particular regions of the structure .