Intracellular storage of molybdenum. Binds polyoxomolybdates. Can bind at least 90 molybdenum atoms per protein molecule. Optimum pH is 6.5 to 7.0. Above pH 7.1 molybdenum is released. Octamer consisting of 4 alpha and 4 beta chains. Requires molybdenum. Belongs to the UMP kinase family. Highly divergent.