TABLE 2.

Thermodynamic constraints on passive and active end-tracking reactions

Mechanism-A: end-tracking stepping motor
Steps:
1. Monomer binding from solution, ΔG1 = − kT ln([MT]/K1).
2. Step of tracking unit to terminal subunit, ΔG2 = kT ln(K2).
Active motor: Hydrolysis energy ɛ facilitates Step 2.
Type	Energy	Equilibrium constants	Implications
Passive	ΔG1 + ΔG2 = ΔG(+),add ∼ O(−kT)	K1K2 = [MT](+)-crit	Favorable monomer binding ⇒ unfavorable stepping
			Favorable stepping ⇒ unfavorable monomer binding
Active	ΔG1 + ΔG2 = ΔG(+),add−ɛ ≪ −kT	K1K2 = [MT](+)-crite−ɛ/kT	Monomer binding and stepping may both be favorable.
Mechanism-B: direct-transfer end-tracking motor
Steps:
1′. Monomer binding from solution,
2′. Transfer of monomer to filament end,
3′. Release of tracking unit, ).
Active motor: Hydrolysis energy ɛ facilitates Step 3
Type	Energy	Equilibrium constants	Implications
Passive		K1K2/K3 = [MT](+)-crit	Favorable monomer binding ⇒ unfavorable monomer transfer and/or tracker release
			Favorable monomer transfer ⇒ unfavorable monomer binding and/or tracker release
			Favorable tracking-unit release ⇒ unfavorable monomer binding and/or monomer transfer
Active		K1K2/K3 = [MT](+)-crite−ɛ/kT	Monomer binding, transfer, and tracking unit release may all be favorable.