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. 2004 Oct;87(4):2838–2854. doi: 10.1529/biophysj.104.045211

TABLE 3.

Definition of parameters and variables

Symbol Definition
d Length of one protofilament subunit (5.4 nm for actin, 8 nm for tubulin)
F Force on filament end, opposing elongation
ΔG(+)add Free energy change upon monomer·NTP binding to filament (+)-end
ΔGexchange Free energy change upon NTP exchange with monomer-bound NDP
ΔGhydrolysis Free energy change of NTP hydrolysis to form NDP and Pi
ΔG(−)loss Free energy change upon monomer·NDP dissociation from filament (−)-end
ΔInline graphic Free energy change upon hydrolysis of filament-bound NTP
ΔInline graphic Standard free energy change of NTP hydrolysis to form NDP and Pi
ΔGPi-release Free energy change upon reversible release of filament-bound Pi
k1, k−1, K1 Forward and reverse rate constants and equilibrium dissociation constant (K1 = k−1/k1) for monomer binding to filament (+)-end (Mechanism-A)
k2, k−2, K2 Forward and reverse rate constants and equilibrium dissociation constant (K2 = k−2/k2) for tracking unit translocation to terminal subunit (Mechanism-A)
Inline graphic Forward and reverse rate constants and equilibrium dissociation constant (Inline graphic) for monomer binding to tracking protein (Mechanism-B)
Inline graphic, Inline graphic Forward and reverse rate constants and equilibrium dissociation constant (Inline graphic) for monomer-tracking protein complex binding to filament end (Mechanism-B)
Inline graphic Forward and reverse rate constants and equilibrium dissociation constant (Inline graphic) for tracking protein binding to terminal subunit (Mechanism-B)
kT Thermal energy (Boltzmann constant, k, × absolute temperature, T)
KP Equilibrium dissociation constant for Pi binding to filament subunits
Kx Equilibrium constant for nucleotide exchange (Kx = [MT]eq[NDP] /[MD]eq[NTP])
MT NTP-bound monomer
MD NDP-bound monomer
[MT](+)-crit Critical concentration of NTP-bound monomers for filament (+)-ends
[MD](−)-crit Critical concentration of NDP-bound monomers for filament (−)-ends
u Probability of the end-tracking protein bound only to monomer (Mechanism-B)
α Ratio of rate constants (k−2/k−1) in Mechanism-A
β Ratio of rate constants (Inline graphic) in Mechanism-B
γ Ratio of rate constants (Inline graphic) in Mechanism-B
ɛ Energy captured from hydrolysis that is used for affinity modulation
ρ Probability of the end-tracking protein being bound to the terminal subunit (Mechanism-A or Mechanism-B)