Reaction scheme for an enzymatic reaction with two non-essential activators. The microtubule end is viewed as an enzyme that catalyses the reaction: tubulin in solution (substrate)→tubulin in the polymer (product) by providing a much more efficient pathway than direct incorporation into the lattice. XMAP215 and EB1 are viewed as two non-essential enzyme activators that accelerate the net rate of addition at the end. Filled arrows represent the flux of polymerization through a particular vertex. M, microtubule end; T, free tubulin; X, XMAP215; E, EB1.