Answer
If only GDP, but no GTP, were present in the solution surrounding a microtubule in its shrinking phase, the microtubule would likely continue to depolymerize rapidly and eventually completely disassemble.
Work Step by Step
This is because GTP hydrolysis is necessary for the microtubule to transition from a growing to a shrinking phase, but GTP-bound tubulin subunits are required to switch back to the growing phase.
During the polymerization of a microtubule, GTP-bound tubulin subunits are added to the growing end of the microtubule, where they form stable lateral bonds and contribute to the overall strength and stability of the microtubule structure. Once the GTP-bound tubulin subunits reach the end of the microtubule, they undergo hydrolysis, which converts the GTP molecule into GDP and releases a phosphate group. This GTP hydrolysis destabilizes the lateral bonds between adjacent protofilaments, making the microtubule more susceptible to depolymerization.
In the absence of GTP, the tubulin subunits cannot switch back to the GTP-bound state, which is necessary for the formation of stable lateral bonds and the growth of the microtubule. As a result, the microtubule would continue to lose its structural integrity and depolymerize rapidly until it completely disassembles. Therefore, the presence of GTP-bound tubulin subunits is essential for the microtubule to switch back to its growing phase and maintain its structural stability.
