Many studies have shown that protein dynamics are important to enzyme function. For example, enzyme protein movements have been shown to optimize the active site, enable binding of substrate and cofactor, and facilitate product release. Whittier et al. now show that in two tyrosine phosphatases, the rate of cleavage is coupled to the motion of a loop. The two phosphatases have different catalytic rates; however, in both, a loop containing a catalytic residue switches between an inactive open and a catalytically competent closed state. The rates of closure are equivalent to the cleavage rates, suggesting that the leaving group tyrosine is protonated simultaneously with loop closure. Thus, tuning of the loop motion plays a regulatory role in the catalytic cycle.