TOPO I

Related Reading

DNA topoisomerases are a class of enzymes that exist in the nucleus. They can catalyze the break and combination of DNA strands, thereby controlling the topological state of DNA. Topoisomerases participate in the regulation of supercoil structure templates. There are mainly two topoisomerases in mammals. DNA topoisomerase I catalyzes the change of the topoisomeric state of DNA replication by forming a short single-strand cleavage-binding cycle; on the contrary, topoisomerase II causes the instantaneous break of the double-stranded enzyme bridge, and then opens and recloses it. To change the topological state of DNA. Mammalian topoisomerase II can be divided into αII type and βII type. The anti-tumor activity of topoisomerase toxins is related to the stability of the enzyme-DNA cleavable complex. These drugs effectively convert enzymes into fibrotoxins by stabilizing the enzyme-DNA splittable complex.

Figure 1. DNA topoisomerases.

Introductions

There are many reactions that DNA topoisomerase can catalyze, which can only be briefly described here. DNA topoisomerase I has much higher affinity for single-stranded DNA than double-stranded DNA, which is the molecular basis for its recognition of negative supercoiled DNA, because negative supercoiled DNA often has a certain degree of single-stranded region. The higher the negative supercoil, the faster the action of DNA topoisomerase I. It is now known that the negative supercoil in the organism is stable at about 5%, and it does not work if it is low or not if it is high. The organism makes the negative supercoil reach a stable state through the opposite action of topoisomerase 1 and II. It has been found that the code E. Mutations in the topA gene of coli topoisomerase I will cause compensatory mutations in the gyrase gene; otherwise, the negative supercoil will increase and the cell viability will decrease. The base sequence specificity of topoisomerase I is not high, but the cut point must be 4 bases downstream of C (including C itself). After the DNA single strand is cut, topoisomerase I is connected to the 5 ends of the nick, and the energy for hydrolyzing the phosphodiester bond is stored to connect the nick, so the function of topoisomerase I does not require energy supply.

Functions

Topoisomerase inhibitors are important anti-tumor drugs, which are believed to work by stabilizing a covalent complex formed between topoisomerase and DNA, which in turn sets up an obstacle to the DNA replication mechanism. Scientists still don't know the origin of drug effects that target topoisomerase. The cover picture of this issue shows the accumulation of positive DNA super spirals due to the action of the drug. This super-entanglement of DNA can hinder the advancement of a DNA polymerase, and may also play a role in preventing or destroying the replication fork, leading to cell death.

DNA topoisomerase catalyzed reaction

Many, the essence of the reaction is to cut the phosphodiester bond of DNA first, change the number of DNA strands, and then connect them. It has the functions of DNA endonuclease and DNA ligase. However, they cannot connect the previously existing broken DNA, that is to say, their cleavage and ligation reactions are mutually coupled. In addition to DNA topoisomerase which can produce heterogeneous changes, many reagents that can intercalate between adjacent bases and affect base stacking, especially flake dye molecules. It can also change the topological state of DNA. The most obvious example is ethidium bromide. With the increase in the number of intercalating dye molecules, the first manifestation is the decrease and disappearance of negative supercoils, followed by the increase of positive supercoils. This is similar to the case where single-stranded DNA binding proteins promote the transformation of negative supercoils into vesicles.

Clinical significance

Many drugs work by interfering with topoisomerase. Broad-spectrum fluoroquinolone antibiotics work by disrupting the function of bacterial type II topoisomerase. These small molecule inhibitors act as effective antibacterial agents by hijacking the natural ability of topoisomerase to produce breaks in chromosomal DNA. Some chemotherapeutic drugs called topoisomerase inhibitors work by interfering with mammalian eukaryotic topoisomerase in cancer cells. This induces DNA breaks, which ultimately leads to programmed cell death (apoptosis). In addition to its potential therapeutic effects, this DNA damaging effect may also cause patients to develop secondary tumors.