Enzymes are catalysts.
ATP and water do not react, however, because there is a large energy barrier separating reactants and products. A catalyst functions by lowering this energy barrier and providing a kinetic pathway for the reaction to proceed and for equilibrium to be attained. A free energy diagram can be used to illustrate the effect of a catalyst. Catalysts are neither irreversibly consumed or altered during the reaction cycle. S is converted to P but is still bound to the active site.