Any enzyme that brings about the hydrolysis of ATP. This results in the cleavage of either one phosphate group, with the formation of ADP and inorganic phosphate (P_i), or of two phosphate groups, with the formation of AMP and pyrophosphate (PP_i); the second reaction yields twice as much energy as the first. ATPase activity is associated with many energy-consuming processes; for example, in muscle contraction it is associated with myosin when activated by actin. One form of ATPase, ATP synthetase, can catalyse the synthesis of ATP, for example in the mitochondrial electron transport chain (see also chemiosmotic mechanism).