Fig. 2.
Reaction pathway models of the nucleotide addition cycle. Four variants of the Brownian ratchet model for elongation are presented. Each model differs in the placement of the force-dependent translocation step in relation to the NTP-binding, catalysis, and phosphate-release steps. Equilibrium constants (for reversible steps) and forward rate constants (for essentially irreversible steps) are shown. Groups of states corresponding to the translocation event(s) are shaded (orange).