Topoisomerase

Topoisomerase

• DNA ek straight seedhi line nahi hoti — cell ke andar yeh supercoiled, twisted, aur tightly packed hoti hai.
• Jab bhi DNA open hota hai (replication, transcription ya repair ke time), double helix ke aage ka hissa overwound ho jata hai — jaise phone cord twist ho jaye.
• Isse torsional stress hota hai jo process ko rok sakta hai ya DNA ko damage kar sakta hai.

Topoisomerases ek type ke DNA untangling enzymes hote hain jo:

• DNA ko temporarily cut karte hain (1 ya 2 strands).

• DNA ko move hone dete hain taaki twist, knots, ya tangles remove ho jaye.

• Cut hone ke baad DNA ko wapas reseal kar dete hain without mistake.

Agar yeh enzymes na ho:

• DNA itna twist ho jayega ki replication aur transcription ruk jayenge.

• Replication ke baad chromosomes linked rahenge aur cell division possible nahi hoga.

Short yaad rakhne ka tarika:

• Type I = 1 strand cut karta hai (ATP ka use usually nahi).

• Type II = 2 strands cut karta hai (ATP lagta hai).

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Topoisomerase Types

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Type I Topoisomerase

Ek aisa enzyme jo ek strand cut karta hai, DNA ko rotate/passage allow karta hai aur phir reseal karta hai.

Structure

• Mostly single protein chain.

• Active site mein tyrosine hota hai jo DNA backbone se bond banata hai.

• DNA ko grip karne wala groove hota hai.

Types

1. Type IA – sirf negative supercoils relax karta hai; 5′-phosphotyrosyl linkage banata hai; ssDNA region chahiye.

2. Type IB – positive & negative dono supercoils relax karta hai; 3′-phosphotyrosyl linkage banata hai; DNA freely rotate hota hai.

3. Type IC – mostly archaea mein; IB jaise kaam karta hai lekin structure alag hota hai.

Example – Type IA (E. coli Topo I)

• Negative supercoils control karta hai.

• Gyrase ke saath balance maintain karta hai.

• Topo III recombination mein special role leta hai.

Mechanism (Type IB – Free Rotation)

1. DNA bind hota hai.

2. Tyrosine DNA backbone cut karta hai → 3′-phosphotyrosyl bond.

3. Cut end rotate hota hai stress relieve karne ke liye.

4. DNA reseal hota hai.

 

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Type II Topoisomerase

Yeh 2 strands ek saath cut karta hai, ek dusra double-stranded segment us gap se pass karta hai, aur phir DNA reseal karta hai. ATP lagta hai.

Structure

• Usually dimer ya tetramer.

• 3 regions:

  1. ATPase domain – ATP bind & hydrolyze.

  2. DNA-cleavage core – active site tyrosines.

  3. C-terminal domain – DNA recognition.

Types

• Type IIA: DNA gyrase, Topo IV, eukaryotic Topo IIα/β.

• Type IIB: Topo VI (archaea/plants).

Mechanism (Simple Steps)

1. G-segment bind hota hai.

2. ATP bind karke T-segment capture hota hai.

3. G-segment ke dono strands cut hote hain.

4. T-segment us cut gap se pass hota hai.

5. G-segment reseal hota hai.

6. ATP hydrolyze hota hai → enzyme reset.

 

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Topoisomerase Inhibition

Type I Inhibitors (Cancer Drugs)

• Camptothecin, Topotecan, Irinotecan.

• Mechanism: Topo I–DNA cut complex ko stabilize karte hain → replication se collision → double-strand breaks → cell death.

Type II Inhibitors (Cancer Drugs)

• Etoposide, Teniposide, Doxorubicin.

• Mechanism: Resealing block kar dete hain → DNA breaks.

Bacterial Topo II Inhibitors

• Fluoroquinolones (ciprofloxacin, levofloxacin) – gyrase & Topo IV target.

• DNA breaks cause karke bacteria mar jata hai.

 

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Clinical Significance

• Cancer therapy: Rapidly dividing cells kill karte hain.

• Antibiotics: Bacterial enzymes ko target karte hain, human enzymes safe rehte hain.

• Resistance: Binding site mutation, drug efflux pumps, protective proteins.

 

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Topoisomerase vs Helicase

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Topoisomerase vs Gyrase