拓扑异构酶- Definition, Types, Structure, Functions, Mechanism

Isomerase enzymes form or assist in forming isomers of any biological components. They assist in the rearrangement process of different biomolecules during the formation or breakage of bonds. Topoisomerase is a type of isomerase enzyme.

什么是拓扑异构酶？

拓扑异构酶is an essential enzyme that aids in the DNA replication process, segregation of chromosomes, transcription, and also in recombination.

• 它是1970年代首次由J.C. Wang发现的大肠杆菌。It was the type I topoisomerase.
• 顾名思义，它有助于更​​改DNA拓扑。It can increase or decrease the extent of the unwinding of DNA.
• It can also be called DNA topoisomerase as it only acts on DNA strands.
• It doesn’t work on RNA.
• It breaks the phosphodiester bond that is present in the backbone of DNA strands. The bonds are formed again as the enzyme leaves.

Some Important Terms

扭曲（TW）：这是DNA链的螺旋转弯的总数。

writhe（WR）：这是DNA交叉本身的双螺旋螺旋的总计数，指示DNA的超副管。

链接号码：It is the total number or addition of twists and writhes in DNA.

Linking No.= Wr+Tw

拓扑异构酶类型

There are two types of topoisomerases:

• Type I Topoisomerase
• II型拓扑异构酶

Type I Topoisomerase

Type I TopoisomeraseDefinition

I型拓扑异构酶是一种在单个DNA上切割的拓扑异构酶。它不是ATP依赖性酶（异常：反向回旋酶）。

It mainly changes the linking number by plus one.

注意：topoisomerases的奇数类型属于I型，甚至在II型下。

Type I TopoisomeraseStructure

There is the presence of multiple varying domains in the type IA. It can be from I to IV. Toprim domain is contained in domain I. HTH (Helix-Turn-Helix) is present in domains III and IV. The tyrosine residues are present in the HTH of domain III. It appears like a lock with all three domains present at bottom of the topoisomerase structure.

Type IB contains active site (tyrosine) bind with C-terminal domain, N-terminal domain, capping, and catalytic lobe.

Type I Topoisomerase类型

It is of three basic types:

IA型拓扑异构酶

它与DNA的5'碳端结合。

This type of topoisomerase show homology to topoisomerase I of大肠杆菌。

It is of further three types:

• TOPO IA：在Eubacteria中发现。
• TOPO III：在Eubacteria和Euckaryotes中发现它。
• Reverse Gyrase: It is found in archaebacteria and eubacteria as well. It is the only type of type I topoisomerase that is ATP-dependent.

（这里的TOPO指示topoisomerase）

Type IB topoisomerases

它与DNA的3'碳端结合。它形成一条尼克。这种类型的拓扑异构酶与人类的拓扑异构酶I显示同源。

Type IC topoisomerases

It contains one type of topoisomerase i.e. topoisomerase V. It binds to the 3′ Carbon end of the DNA. It is found in archaebacterial. It shows the controlled mechanism of rotation.

Type I TopoisomeraseMechanism of action

It generally occurs in the following events occurring together at the same time.

• Cutting a single strand of DNA：Active site of the topoisomerase contains an amino acid tyrosine. The disruption of phosphodiester bond and formation of intermediate with phospho-tyrinosyl linkage favors the breaking of a DNA strand. The bond formation and cleavage mechanism in detail are the same as in the case of type II topoisomerase. Tyrosine may attack 3’or 5’carbon end.

• Passing of strand:After the cleavage, the uncut DNA strand passes through the break. In this step, the enzyme changes from closed conformation to open conformation favoring the passing of strand. No ATP is utilized in this conformational change in the case of type I.

• 宗教：与酪氨酸相关的磷酸盐再次受到以前分离的链核核群的OH攻击，并导致去除酪氨酸的中间连接并重新加入裂解链。酶返回其初始阶段（闭合构型），并在下一个周期中回收。

Type I Topoisomerase职能

• They are involved in the removal of supercoils of DNA in biological processes such as replication and transcription.
• 帮助放松DNA。
• They help in breaking strands during recombination.
• They are also involved in the condensation of the chromosome.
• During mitosis, the DNA strands need to be free from interwinding which is done by topoisomerase I.

II型拓扑异构酶

II型拓扑异构酶Definition

II型拓扑异构酶是一种拓扑异构酶，一次在两个DNA上切割。它是ATP依赖性酶。它将链接号码更改两个。

II型拓扑异构酶Structure

真核生物中的拓扑异构酶IIA由两个相同的单体（A-A）组成，而在原核生物中它们形成异杀量（A2B2）。

拓扑异构酶IIB is formed of heterotetramers only.

拓扑异构酶II consists of four domains which include:

• ATPase domain at N-terminal
• A variable C-terminal domain
• Domain for binding of DNA located centrally
• A conserved domain of about a hundred amino acids i.e. toprim domain.

II型拓扑异构酶类型

它是两种基本类型：

Type IIA topoisomerases

它在病毒和所有细胞生物中发现。这是三种类型：

• Topo II:It is found in eukaryotes.
• TOPO IV：It is found in bacteria. It differs from Gyrase. It is not involved in DNA wrapping while Gyrase is involved in DNA wrapping and promoting negative supercoils.
• 回酶：它在细菌和一些真核生物中发现。它引入了负超螺旋，将链接数减少了两个。

Type IIB topoisomerases

It includes Topo VI which can be found in archaea and some plants.

II型拓扑异构酶Mechanism of action

It occurs as follows with ATP hydrolysis.

• DNA链的切割：The enzyme contains tyrosine residues. They form covalent bonds with the DNA strands and break the DNA chain. The lone pair of electrons of O-atom present in the tyrosine acts as a nucleophile and attacks on the Phosphorus in phosphate of DNA. It causes the shifting of a bond from phosphate to one of the O-atom attached to the ribose sugar forming a hydroxyl group. Hence the covalently bonded tyrosine attached with phosphorus breaks the phosphate-sugar backbone which cleaves the chain. This linking is termed 5′-phospho-tyrinosyl protein-DNA linkage.

酶对两条链的作用都破坏了双链体。

• 完整链穿过缝隙：在这种情况下,另一个全双工链通过through the gap over the broken duplex. In this the conformational change in enzyme requires ATP.

• 宗教：It is done by the attack of 3′-OH of the sugar of separated strand on phosphate group which has formed an intermediate linkage with tyrosine. It repels the bond with tyrosine and reforms the broken bond to join again. It occurs on both strands of duplex together ligating them. The enzymes regain their conformation and continue the cycle.

II型拓扑异构酶职能

• 它增加了染色体的分离。
• It does not aid in the supercoiling of DNA but is involved in their relaxation.
• DNA促旋酶促进负超螺旋的DNA。
• One of the most important functions is that it brings the change of two units in the linking number of loops in DNA.

拓扑异构酶Inhibition

一些化学成分可以抑制拓扑异构酶的作用，并被称为拓扑异构酶抑制剂。

它们可以干扰DNA的连接步骤，DNA的结扎步骤在细胞中导致细胞中骨折，导致细胞凋亡导致细胞死亡。

The topoisomerase inhibition principle is used for the development of drugs for bacterial infection. It includes antibiotics such as novobiocin, coumermycin of the class coumarins which interfere in ATP binding in type II topoisomerases in bacteria leading to its death. It also includes the quinolone class of antibiotics which prevent the religation of nicked DNA strands in the last step of the topoisomerase working mechanism.

Chemotherapeutic agents applied for the treatment of cancer can lead to inhibition of topoisomerase in humans. They can stabilize the intermediate formed by the linkage of tyrosine of topoisomerase and phosphate of DNA.

Clinical Significance of Topoisomerase

Many medications work by interfering with type II topoisomerases in bacteria. These medications include broad-spectrum antibiotics such as fluoroquinolones. They can make the topoisomerase damage the DNA.

癌细胞拓扑异构酶是某些化学疗法药物（例如I型I型和Teniposide）和II型依托泊剂等化疗药物的靶向，也称为拓扑异构酶抑制剂。

在自身免疫性疾病硬皮病中，可以看到抗昆虫异构酶抗体（也称为抗SCL-70抗体），可见针对拓扑异构酶I抗原。

拓扑异构酶与解旋酶

拓扑异构酶	Helicase
It is involved in the prevention of supercoiling of DNA i.e. decreases tension on the unwound strands.	It is involved in the unwinding of DNA strands.
It works on DNA only.	它作用于DNA和RNA。
It attacks the phosphodiester bond in the backbone of DNA.	它攻击双链之间的氢键。
Its two types are: Type I Topoisomerase Type II Topoisomerase	它的两种类型是：RNA解旋酶DNA解旋酶

拓扑异构酶与回旋酶

拓扑异构酶	回酶
It includes different types of enzymes including Gyrase.	回酶is a type of topoisomerase.
这是一大类酶。	它是II型拓扑异构酶子类中的一种类型。
它存在于原核生物和真核生物中。	它主要存在于原核生物中，仅在某些真核生物中。
It maintains the topology of DNA by the combined functions of different types of enzymes. It includes both negative and positive supercoiling of DNA.	它的特定功能是在DNA链中引入负超涂层，而不是将其删除。
拓扑异构酶可能与ATP有关。	Gyrase是拓扑异构酶的ATP依赖性类型。

References

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