DNA Supercoiling & Topoisomerases Explained

Type 1a topoisomerases use three structural domains: domains 1 and 3 capture DNA at the active site while domain 2 acts as a hinge for strand passage. The reaction involves DNA capture, cleavage creating a phosphotyrosine intermediate, strand separation, strand passage, and religation. Type 1b topoisomerases employ a distinctive rotatory mechanism where the enzyme wraps around DNA, creating a 3' phosphotyrosine bond that leaves the 5' end free to rotate, releasing torsional stress before religation. Type 1c topoisomerases (recently discovered in Mycobacterium tuberculosis) share mechanistic features with Type 1b but lack sequence similarity. Type 2 topoisomerases create double-strand breaks forming a transient gate through which other DNA segments pass, changing linking number by two units. Type 2a enzymes (gyrase, topo IV) produce 4-base pair overhangs and significantly alter topology equilibrium, while Type 2b enzymes (topo VI) produce 2-base pair overhangs and cannot simplify topology beyond a specific equilibrium point.

Topoisomerases are classified into Type I and Type II based on their mechanism of action. Type I topoisomerases cleave only one DNA strand and allow the other strand to pass through before resealing, changing linking number by one. Type II topoisomerases cleave both strands and allow the entire double helix to pass through, changing linking number by two. Type I topoisomerases are further classified into Type IA and Type IB, both using strand passage mechanism. Type IA includes topoisomerase III (bacteria and eukaryotes) and reverse gyrase (bacteria and archaea), which uniquely adds positive supercoils and requires ATP. Type IB is found in both bacteria and eukaryotes. Type II topoisomerases are classified into Type IIA (topoisomerase IV in bacteria, topoisomerase II in eukaryotes) and Type IIB (topoisomerase VI in plants, topoisomerase I in archaea). All Type II topoisomerases require ATP hydrolysis to power strand passage.

There are two main categories of topoisomerases with distinct mechanisms. Type I topoisomerases break one DNA strand and pass the other strand through the gap, changing the linking number by 1 unit. These reactions do not require additional energy input. Type II topoisomerases break both DNA strands and pass the entire double helix through the gap, changing the linking number by 2 units. These reactions require energy sources such as ATP or NADH. Both types work by breaking DNA strands and passing them through the break, but they differ in how many strands are broken and how much the linking number changes.

Topoisomerases are enzymes that resolve DNA topology problems by introducing transient breaks in the DNA backbone; Type 1 topoisomerases cut only one DNA strand using a tyrosine nucleophile and relieve supercoiling through strand swiveling without requiring ATP, while Type 2 topoisomerases cut both strands and use ATP hydrolysis to pass one DNA segment through another, changing the linking number by multiples of two units.

Helicase unwinding creates torsional stress that must be relieved by topoisomerases. Every 10 base pairs of unwinding creates one supercoil turn. Type I topoisomerases (1A and 1B) create transient single-strand breaks through tyrosine-mediated cleavage, allowing rotation without ATP requirement since bonds spontaneously reform. Type II topoisomerases create double-strand breaks using a tetrameric structure (two A and two B subunits), requiring ATP hydrolysis and ligase for sealing. Type I enzymes are ATP-independent, while type II enzymes are strictly ATP-dependent. This distinction in mechanism reflects different evolutionary solutions to the common problem of maintaining proper DNA topology during replication fork progression.

Fluoroquinolones are antibiotics that inhibit bacterial DNA gyrase (a type II topoisomerase) by blocking the ligase domain, preventing DNA resealing after cleavage; this causes bacterial DNA fragmentation and cell death. Similarly, etoposide and teniposide are anti-cancer drugs that work on human topoisomerases using the same mechanism to kill rapidly dividing cancer cells.

Topoisomerases relieve supercoils created when helicase unwinds DNA ahead of replication forks. Type I topoisomerases don't require ATP, while Types II and IV do. Clinically, topoisomerase inhibitors are used in cancer treatment: irinotecan/topotecan inhibit topoisomerase I in eukaryotic cells, while etoposide/teniposide inhibit topoisomerase II. Fluoroquinolone antibiotics (ciprofloxacin, levofloxacin) specifically target bacterial topoisomerases by enhancing nuclease activity while inhibiting ligase function, causing persistent DNA damage.

Fluoroquinolones are bactericidal antibiotics that inhibit bacterial topoisomerases (DNA gyrase in gram-positive bacteria and DNA topoisomerase IV in gram-negative bacteria), preventing DNA supercoiling relaxation and thereby blocking DNA replication, which ultimately leads to bacterial cell death.

Topoisomerase poisons stabilize the DNA-enzyme complex rather than inhibiting the enzyme directly. Bleomycin, isolated from Streptomyces verticillus in 1962, chelates iron through its amine groups, generating hydroxy radicals that abstract hydrogen from DNA, causing strand breaks between purine-pyrimidine nucleotides. Etoposide and teniposide, semi-synthetic derivatives of epipodophyllotoxins, stabilize the DNA-topoisomerase II complex and undergo free radical formation from 4'-phenolic oxidation. Camptothecin, from Camptotheca acuminata, stabilizes topoisomerase I-DNA complexes, causing single-strand breaks that lead to replication errors. Fluoroquinolones (ciprofloxacin, norfloxacin) are synthetic antibacterials that bind bacterial topoisomerase at the DNA nicking site, with multiple molecules stacking coplanar and carbonyl/carboxylate groups forming hydrogen bonds with DNA.

Quinolones and fluoroquinolones are antibiotics that inhibit bacterial DNA synthesis by targeting topoisomerases II (DNA gyrase) and IV; fluoroquinolones inhibit both enzymes while non-fluoroquinolones only inhibit topoisomerase II, giving them bactericidal properties with post-antibiotic effects. The drug class evolved through generations: first-generation (nalidixic acid) had limited use, second-generation (ciprofloxacin) added Pseudomonas coverage, third-generation (levofloxacin) covered gram-positive cocci, and fourth-generation (moxifloxacin) expanded to anaerobes and atypical pathogens. These drugs are absorbed orally with 50-100% bioavailability, distribute to intracellular compartments, and are eliminated renally. Key adverse effects include CNS disturbances, tendonitis, QT prolongation, and photosensitivity, with contraindications during pregnancy due to teratogenic risks.

The nucleosome model, proposed by Kornberg and Thomas in 1974, explains how DNA and histone proteins are organized in chromosomes. A nucleosome consists of 200 base pairs of DNA wrapped around a histone octamer (8 histone protein molecules: two each of H2A, H2B, H3, and H4), with H1 histone supporting the linker DNA between nucleosomes. The DNA wraps approximately 1.75 turns around the histone octamer at a 30° angle, forming a 'beads on a string' structure visible under electron microscopy. This first-degree coiling allows the 2-meter DNA in a human cell to be compacted into the nucleus, with further coiling forming solenoids (6 nucleosomes per solenoid, 300 Å diameter) and super-solenoids to create metaphase chromosomes.

Negative supercoiling facilitates DNA replication and transcription initiation by reducing the energy required to separate DNA strands. Bacterial chromosomes lack nucleosomes but contain DNA, negative supercoiling, and topoisomerases. The nucleosome core particle contains an octamer of histone proteins (two copies each of H2A, H2B, H3, and H4) wrapped around 146 base pairs of DNA. Histone H1 binds to linker DNA and stabilizes the nucleosome structure.

The 30nm fiber forms through two models: solenoid (stacked nucleosomes) or zigzag (alternating arrangement), with species preferring different models based on linker DNA length. Higher-order structures involve folding into loops held by scaffold proteins, then coiling to form condensed chromosomes. During replication, nucleosomes partially disassemble—H3-H4 tetramers stay bound randomly to daughter strands while H2A-H2B dimers release. Old histones distribute to daughter strands, and new histones require chaperones (ASF1, CAF1, NAP1) for proper assembly. PCNA serves as a marker for newly synthesized DNA.

Chromatin is DNA-protein complex in eukaryotic nuclei. Nucleosomes are connected by linker DNA and bound by H1 histone. Multiple nucleosomes coil into a 30-nanometer fiber (solenoid structure) with approximately 6 nucleosomes per turn. This higher-order structure further compacts DNA. Chromatin exists as euchromatin (loose, active) and heterochromatin (tight, inactive).

Eukaryotic DNA is packaged with histone proteins to form chromatin. Nucleosomes consist of histone octamers (H2A, H2B, H3, H4 - two each) with 146 base pairs of DNA wrapped in 1.65 left-handed superhelical turns. Linker DNA between nucleosomes is 60-80 base pairs. Nucleosomes coil into 30nm fibers, which form loops and higher-order structures into chromosomes. Chromosomes contain 40% DNA, 50% histones, 1.2% RNA, and 0.3% other components. This hierarchical packaging allows meters of DNA to fit within the nucleus.