Catalyzes the reversible phosphorylation of UMP to UDP, with ATP as the most efficient phosphate donor. ATP + UMP = ADP + UDP Allosterically activated by GTP. Inhibited by UTP, 5-bromo-UTP and 5-iodo-UTP. Positive cooperativity is observed with ATP as variable substrate, but it is strongly reduced in the presence of GTP. GTP enhances the affinity for ATP whereas UTP decreases it. Optimum pH is below 6.5 at low ATP concentrations and is 7.4-7.8 at high ATP concentrations. Pyrimidine metabolism; CTP biosynthesis via de novo pathway; UDP from UMP (UMPK route): step 1/1. Homohexamer. Belongs to the UMP kinase family.