Molecular Biology
Helicase and Topoisomerase
Unwinding the double helix and relieving torsional supercoiling at the replication fork
Helicases are ATP-dependent motor proteins that unwind double-stranded DNA, separating the two strands at ~1000 bp/s in E. coli (DnaB) or ~50 bp/s in eukaryotes (the CMG helicase made of Cdc45, MCM2-7, and GINS). Each base pair separation costs one ATP and releases one helical turn (10.5 bp = 1 turn) of accumulated supercoil ahead of the fork. Topoisomerases relieve this torsional stress: type I enzymes cut a single strand and let it swivel, releasing one supercoil per cycle without ATP; type II enzymes cut both strands and pass another duplex through the gap, releasing two supercoils per ATP. Together, helicase plus topoisomerase keep replication forks moving against the otherwise insurmountable torque that would build to ~50 supercoils per kilobase.
- DnaB unwinding rate~1000 bp/s
- CMG (eukaryotic)~50 bp/s
- ATP per bp~1
- Topo I per cycleΔLk = 1 (no ATP)
- Topo II per cycleΔLk = 2 (2 ATP)
- Helix turn10.5 bp
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Why these two enzymes matter
- Without them, no fork moves. The double helix is overwound by 50 turns per kb. Helicase opens it; topoisomerase keeps the rest of the chromosome from binding up against the fork. James Wang's 1971 discovery of E. coli omega protein (now Topo I) was driven by the realization that something had to absorb torsional stress in vivo.
- The replisome is a coupled machine. CMG helicase, Pol ε, and Pol δ share the same scaffold and signal each other through Mrc1, Tof1, and Csm3. CMG slowdown stalls polymerases; polymerase stalling slows CMG. The mechanical-chemical coupling is so tight that the system tolerates only ~1-3% kinetic asymmetry before the strands desync.
- Topoisomerase II decatenates daughter chromosomes. When two replication forks converge, the parental strands remain interlocked as catenanes. Topo II resolves these in late S/G2 — failure prevents anaphase chromosome segregation. The ICRF-187/dexrazoxane class of drugs targets exactly this step.
- Quinolones killed bacteria by attacking gyrase. Ciprofloxacin (Bayer, approved 1987) traps the gyrase-DNA cleavage complex on a chromosome-wide scale, generating thousands of double-strand breaks in dividing E. coli. The class became a $2 billion/year market; gyrase mutations in gyrA codon Ser83 are the principal resistance mechanism today.
- Etoposide and doxorubicin built modern oncology. Both trap human Topo II in its cleavage complex, generating selective lethality in dividing tumor cells. Etoposide remains a backbone of testicular cancer cure (5-year survival >95%) and small-cell lung cancer regimens; doxorubicin is in nearly every aggressive lymphoma protocol despite its dose-cumulative cardiomyopathy.
- RecQ-family helicases protect against premature aging. BLM (Bloom syndrome), WRN (Werner syndrome), and RECQL4 (Rothmund-Thomson) all unwind aberrant replication intermediates. Loss of any one causes elevated cancer risk, hyper-recombination, and shortened lifespan.
- Topoisomerase relaxation underpins gene expression. RNA Pol II generates positive supercoils ahead and negative behind during transcription (the Liu-Wang twin-domain model). Topo I in front, Topo II behind. Without continuous relaxation, transcription bubbles cannot translocate more than ~100 bp.
Common misconceptions
- Helicases break covalent bonds. No — only hydrogen bonds between bases and stacking interactions between adjacent base pairs. The phosphodiester backbone is untouched. Topoisomerases break covalent bonds; helicases do not.
- Topoisomerases just cut DNA. They cut and rejoin in a single concerted reaction. The active-site tyrosine attacks the phosphodiester backbone, forming a covalent enzyme-DNA intermediate (5'-phosphotyrosyl for type IB and II; 3'-phosphotyrosyl for type IA). The other strand swivels (type I) or another duplex passes through (type II) before the same tyrosine releases.
- Helicases work alone. The replicative helicase is just one of dozens of helicases in a cell. Recombination uses RecA/RAD51-stimulated unwinding; transcription has Mfd/CSB; mismatch repair uses UvrD/PIF1. Each helicase has a specialized substrate preference, polarity, and partner protein set.
- Topoisomerase I and II are interchangeable. Eukaryotic Topo I cannot decatenate intertwined duplexes — only Topo II can. Conversely, Topo I is unmatched for relaxing transcription-induced supercoils inside a small loop because it does not need ATP and works close to the polymerase. Cells need both.
- Gyrase and Topo II are the same. Both are type II topoisomerases, but only gyrase introduces negative supercoils into relaxed DNA — using ATP to drive an otherwise unfavorable reaction. Eukaryotic Topo II only relaxes existing supercoils; it cannot supercoil naked DNA. The ability to actively introduce negative supercoils is bacteria-specific.
- The hexameric helicase clamps both strands. No — replicative helicases encircle only one strand. DnaB and MCM2-7 surround the lagging-strand template (DnaB) or the leading-strand template (MCM2-7 in eukaryotes — the polarity is opposite). The other strand is ejected through a side channel and runs out the back of the ring.
How the two work together at the fork
Replication starts when origin-binding proteins (DnaA in bacteria, ORC plus Cdc6/Cdt1 in eukaryotes) load the inactive helicase ring around the duplex. Activation requires DnaC release in bacteria, or CDK plus DDK phosphorylation that recruits Cdc45 and GINS to MCM2-7 in eukaryotes, forming the active CMG helicase. CMG begins translocating 3' to 5' on the leading-strand template at ~50 bp/s, pulling the lagging-strand template through its central channel and ejecting it laterally. Each base pair of unwinding adds 1/10.5 of a positive supercoil ahead of the fork. With nothing to relieve it, the torque would reach the helicase's stall threshold (~12 pN nm of torque) within a few hundred base pairs.
Topoisomerase I sits ahead of the fork, cutting one strand on every encounter, allowing the duplex to swivel and discharge one positive supercoil, then re-ligating. The reaction is fast (~10/s per enzyme) and ATP-free. When the fork approaches the end of a topological domain or two converging forks meet, daughter strand catenanes accumulate behind the fork — these can only be resolved by Topo II, which clamps two duplex segments, cleaves one, passes the other through, and re-seals. In E. coli, the same enzyme family includes gyrase, which uniquely introduces negative supercoils into relaxed DNA (~10 per cycle), creating the slight underwinding that primes origins for melting and transcription start sites for promoter opening. The combined choreography — helicase pulling, Topo I discharging single supercoils, Topo II resolving catenanes, gyrase setting baseline negative tone — is essentially fault-tolerant: any one missing element causes specific defects, and the cell builds the system redundantly enough that small perturbations are rescued by neighbors.
Helicase and topoisomerase variants
| Enzyme | Type | Role | ATP? | ΔLk per cycle | Drug target? |
|---|---|---|---|---|---|
| DnaB | Hexameric helicase (5' to 3') | Bacterial replication fork | Yes | n/a | No clinical drug |
| CMG (Cdc45-MCM2-7-GINS) | Hexameric helicase (3' to 5') | Eukaryotic replication fork | Yes | n/a | No |
| RecQ / BLM / WRN | Monomeric helicase (3' to 5') | Resolves stalled forks, recombination | Yes | n/a | No |
| XPD | Monomeric helicase (5' to 3') | Nucleotide excision repair, transcription | Yes | n/a | No |
| Topoisomerase IA (E. coli Topo I, III) | Type I, breaks one strand | Relax negative supercoils, decatenate ssDNA | No | 1 | No |
| Topoisomerase IB (eukaryotic Topo I) | Type I, breaks one strand | Relax + and - supercoils at forks, transcription | No | 1 | Camptothecin, irinotecan, topotecan |
| DNA Gyrase (bacterial) | Type II, breaks both strands | Introduce negative supercoils, decatenate | Yes (2 ATP) | 2 | Fluoroquinolones, novobiocin |
| Topoisomerase II (eukaryotic) | Type II, breaks both strands | Relax supercoils, decatenate sister chromatids | Yes (2 ATP) | 2 | Etoposide, doxorubicin, mitoxantrone |
| Topoisomerase IIIa/B | Type IA | Dissolves double Holliday junctions (with BLM) | No | 1 | No |
Famous experiments
- James Wang, 1971 (PNAS). Discovered E. coli omega protein, now Topo I, by demonstrating an enzyme that relaxed superhelical DNA without ATP — opened the entire field of DNA topology.
- Martin Gellert, 1976. Discovered DNA gyrase (Topo II in bacteria) and showed it actively introduces negative supercoils with ATP, explaining why bacterial chromosomes are constitutively underwound by σ ≈ -0.06.
- Bruce Alberts & Bob Lehman, late 1970s. Reconstituted phage T4 replication in vitro from purified components: gp41 helicase, gp43 polymerase, gp32 SSB, gp45 clamp. Showed helicase-polymerase coupling at ~400 bp/s, foundation for understanding all replisomes.
- Smita Patel, 2003-2007 (single-molecule magnetic tweezers). Measured T7 gp4 helicase unwinding force-velocity relations, ATP step size of ~1 nucleotide per ATP, and force-dependent unwinding rates that match in vivo speeds.
- James Berger lab, 2010-2018 (cryo-EM of CMG). Solved the active eukaryotic CMG helicase structure (Cdc45-MCM2-7-GINS), defined the central channel and the staircase mechanism by which AAA+ ATPases pull DNA one nucleotide at a time. The full eukaryotic replisome with two CMG, Pol ε, Pol δ, primase, and Ctf4 was reconstituted by John Diffley in 2017.
Frequently asked questions
What does a helicase actually do?
It mechanically separates the two strands of duplex DNA (or RNA) by translocating along one strand and using ATP hydrolysis to break the hydrogen bonds and base-stacking that hold the duplex together. Each base pair separation costs roughly one ATP at the canonical stoichiometry, though tight coupling varies by family. The replicative helicase DnaB in E. coli moves at ~1000 bp/s, hexameric ring around the lagging-strand template, with the leading-strand template excluded. The eukaryotic CMG complex (Cdc45 + MCM2-7 + GINS) is slower at ~50 bp/s but works on every replication origin in parallel. Other helicases unwind at transcription bubbles (RecQ, BLM, WRN), repair sites (XPD, FANCJ), and during recombination (RecA, RAD51-stimulated motors).
Why is supercoiling a problem during replication?
DNA is a double helix with one full turn every 10.5 bp. As the helicase unwinds the duplex, that twist has to go somewhere — and on a closed circular chromosome (or a constrained linear domain) it cannot diffuse away into the bulk. Instead it accumulates ahead of the fork as positive supercoils. Without intervention, just 50 bp of unwinding adds nearly five extra turns of overwound DNA in the next ~50 kb. By the time the helicase has unwound ~1000 bp, the torque becomes large enough to stall the fork outright. Topoisomerases continuously bleed off this excess writhe so the fork can keep moving. In E. coli, gyrase introduces negative supercoils ahead of the fork; in eukaryotes, topoisomerase I and II share the work.
How do type I and type II topoisomerases differ?
Type I enzymes cut one strand of duplex DNA, let the broken strand swivel around the intact partner (releasing one linking-number unit per swivel), then re-ligate. They do not require ATP. Eukaryotic Topo I (the camptothecin target) and bacterial Topo III are examples. Type II enzymes cut both strands of one duplex, pass another double helix through the transient gap, then re-seal — releasing two linking-number units per cycle and consuming two ATPs. Eukaryotic Topo II (the etoposide and doxorubicin target) and bacterial DNA gyrase (the fluoroquinolone target) are examples. Type I is for relieving torsion ahead of forks and behind transcription bubbles; type II is for both torsion relief and decatenation of intertwined daughter chromosomes after replication.
Why is the replicative helicase a hexameric ring?
Topology and processivity. A ring around one strand cannot fall off, no matter how long the substrate, because there is no free end to slide off. DnaB (E. coli) and MCM2-7 (eukaryotes) both load as toroids around single-stranded DNA at the origin. The six subunits also let ATP hydrolysis couple to mechanical motion sequentially: each subunit fires in rotation, pulling one nucleotide of the strand through the central pore and pushing the displaced complementary strand out a side channel. Patel and lab (single-molecule tweezers, T7 helicase, 2007) showed the rotation step size is ~one nucleotide per ATP. The hexameric architecture also forces the helicase to encircle one strand at the start, requiring a separate loader (DnaC in bacteria; Cdc6+Cdt1 in eukaryotes) to crack the ring open at the origin.
How do quinolone and topoisomerase poisons work?
Both classes trap topoisomerase in its covalent intermediate state — the moment after the enzyme has cleaved DNA but before re-ligation. Fluoroquinolones (ciprofloxacin, levofloxacin) bind to the bacterial gyrase-DNA cleavage complex, preventing re-sealing and leaving double-strand breaks across the chromosome — a bactericidal effect. Etoposide and doxorubicin do the same to eukaryotic Topo II, killing dividing tumor cells but causing dose-dependent secondary leukemias from MLL gene rearrangements at trapped Topo II sites. Camptothecin (and irinotecan, topotecan) traps eukaryotic Topo I, leaving single-strand breaks that collapse into double-strand breaks at replication forks. The mechanism explains why all three drug classes are dose-limited by hematologic toxicity: every dividing cell needs working topoisomerase.
What happens to forks if topoisomerase fails?
The fork stalls within seconds because the accumulating positive supercoils ahead make further unwinding mechanically impossible. Two emergency mechanisms then kick in. First, the fork can rotate around its own axis, pushing the unrelieved superhelicity behind it as precatenanes — interlocked daughter strands. Topo II must later decatenate these or sister chromatids cannot separate at anaphase. Second, the stalled fork can collapse into a double-strand break, triggering ATR/ATM checkpoint signaling and homologous-recombination-mediated restart. In humans, mutations in TOP2A (decatenation defect) cause embryonic lethality; TOP3A and TOP3B mutations cause Bloom-like syndromes with hyper-recombination. Trypanosomes have a unique mitochondrial topoisomerase II essential for kinetoplast catenane decatenation — it is the target of antitrypanosomal drugs.