PI3Ka controls several cellular processes and its aberrant signalling is implicated in tumorigenesis. One of its hotspot mutations, E545K, increases PI3Ka lipid kinase activity, but its mode of action is only partially understood. Here, we perform biased and unbiased molecular dynamics simulations of PI3Ka and uncover, for the first time, the free energy landscape of the E545K PI3Ka mutant. We reveal the mechanism by which E545K leads to PI3Ka activation in atomic-level detail, which is considerably more complex than previously thought.