Practice question

Microtubules do not require GTP hydrolysis to polymerize hydrolysis follows incorporation and modulates stability rather than providing assembly energy which is entropically driven. After alpha beta dimer carrying GTP on beta adds to plus end longitudinal contacts induce GTPase activity converting beta bound GTP to GDP plus inorganic phosphate retained initially as GDP Pi lattice. GDP tubulin prefers bent conformation by about 12 degrees but constrained straight within wall creating mechanical strain stored as elastic energy. As long as terminal layers retain GTP cap maintains stabilizing influence over underlying strained lattice. When addition slows or stochastic fluctuations expose GDP layers at tip strain released protofilaments splay outward lateral bonds break rapid depolymerization ensues known as catastrophe releasing stored energy. Thus hydrolysis acts as timer converting stable polymer into unstable intermediate powering dynamic instability required for exploratory search and generating polymerization force harnessed at kinetochores and cortical force generation for spindle positioning.