Designed repeat proteins catalyze the 1,3-dipolar reaction between an imine and a pi-deficient dipolarophile in THE solution to form unnatural nitroproline esters, a reaction that no enzyme can catalyze. NMR studies and mutation experiments show that both acidic and basic residues can catalyze the reaction. The diastereocontrol of the reaction depends on the flexibility of the protein and on the number and location of the active lysine and glutamate residues, which can participate independently or forming dyads that promote the formation of unusual diastereomeric cycloadducts. QM/MM calculations permit one to rationalize the origins of this Huisgenase activity and of its diastereocontrol.