Sandbox GGC15

Eukaryotic DNA topoisomerase I (topo I) is a protein that reduces the strain from the supercoils that are caused during transcription and translation^[1]. There are two types of topoisomerases. Type 1 topoisomerases are monomeric and break one strand of DNA^[2]. Type 2 topoisomerases are dimeric, meaning that they made up of two units and break both strands of the DNA helix^[2]. They are able to pass another part of the duplex through the cut, and close the cut using ATP^[1].

. ^[3].

Structure

Human topo 1 is composed of 765 amino acids ^[2]. The enzyme consist of 4 regions which are the NH2-terminal, core, linker, and COOH-terminal domains^[2]. The NH2-terminal is approximately 210 residues long, it is highly charged, disordered, and contains few hydrophobic amino acids^[2]. The domain is made up of residues 713 to 765 and contains the important amino aside Tyrosine 723^[2]. The location of the active site is at this amino acid^[2]. Residues 636 to 712 form the linker domain and they contribute to the enzyme catalytic activity but are not required^[2]. The core is divided into 3 subdomains: [215-232 & 320-433] The core and the COOH-terminal domain are very important for the catalytic activity^[2].

Active Site

Topo 1 reduces stress in DNA by causing a transient single strand nick in the the DNA helix^[1]. This nick enables the cut to rotate around its intact complement, thus eliminating proximal supercoils^[1].

The active site of Topo 1 is catalytic and it is the location where the nicking or cutting occurs^[2]. The nicking occurs from the trans-esterification of Tyr-723 at a DNA phophodiester bond forming a 3'-phosphotyrosine covalent enzyme–DNA complex ^[1]. After the DNA is relaxed, the covalent intermediate is reversed when the released 5'-OH of the broken strand reattacks the phosphotyrosine intermediate in a second transesterification reaction^[1].

^[4]

Relevance

Many anticancer drugs target topo 1 enzymes. This enzyme is the target of camptothecin (CPT) family of anticancer drugs^[2]. These drugs work by increasing the duration of the nicked intermediate in the topo I reaction ^[2]. The stabilized intermediates prevent transcription and replication to continue in the cancer cells^[2]. This eventually leads to DNA damage and cell death^[2].

Mutations

A mutation at amino acid 532 to Alanine almost abolishes enzyme activity^[5]. The location of Lys 532 to the scissile phosphate and other active site amino acids could be the reason why a mutation of this amino acid abolishes the enzyme activity^[5].