Kinesin-1 is a two-headed nanoscale motor protein that walks toward the microtubule's plus end by converting the chemical energy of ATP hydrolysis into mechanical work, one 8 nm tubulin-dimer step at a time — hand-over-hand, like walking. This 2D version adds a live force-velocity curve and a per-step detachment (processivity) model on top of the same stepping mechanics as the 3D version.
Chemical cycle (per head, per step):
1. ADP-head binds ATP (rate ∝ [ATP], Michaelis-Menten)
2. ATP hydrolysis: ATP → ADP + Pi
3. Pi release → neck-linker "power stroke"
4. Trailing head swings 16 nm forward, binds next site (net +8 nm)
Stepping is a biased random walk. Each attempt succeeds with
probability set by the load-dependent free energy (Boltzmann/
Arrhenius force-velocity relation):
P(forward) = 1 / (1 + exp[(F·d − ΔG_ATP) / (k_B T)])
F = opposing load force (pN)
d = 8 nm step size
ΔG_ATP≈ 20 pN·nm available per hydrolysis event
k_B T ≈ 4.1 pN·nm at 310 K (thermal energy scale)
Expected velocity: v(F) = d · P(forward) / (t_wait(ATP) + t_stroke)
— this is the curve plotted live in the "Force ↔ Velocity" graph,
and it falls to ~0 near the real measured stall force, F ≈ 7 pN.
Processivity: after each completed step the motor may detach from
the microtubule with a small, load-increasing probability — real
kinesin-1 averages ~100 steps (≈0.8 µm) per run before releasing.
- ATP concentration — sets the ATP-binding rate that gates each step; near-zero ATP starves the motor and it stalls.
- Load force — an opposing force (as from a cargo vesicle or optical trap) that raises the energy barrier per step; kinesin stalls near F ≈ 7 pN, matching single-molecule optical-trap measurements.
- Thermal noise — the Brownian jitter (k_BT) that both powers the diffusive search for the next binding site and occasionally knocks a head backward.
- Detachment — after each step, a small probability the trailing head fails to rebind at all, ending the run (processivity); the motor then diffuses away until reset.
Real-world relevance: kinesin motors haul vesicles, mitochondria and mRNA along microtubule "highways" inside every eukaryotic cell — the same mechanochemical coupling principle used to engineer synthetic DNA-origami walkers and nanoscale cargo-transport devices in nanobiotechnology.