Docking of 4-oxalocrotonate tautomerase substrates: Implications for the catalytic mechanism

The enzyme 4-oxalocrotonate tautomerase catalyzes the ketonization of dieno ls, which after further processing become intermediates in the Krebs cycle. The enzyme uses a general acid-base mechanism for proton transfer: the ami no-terminal proline has been shown to function as the catalytic base and Ar g39 has been implicated as the catalytic acid. We report the results of mol ecular docking simulations of 4-oxalocrotonate tautomerase with two substra tes, 2-hydroxymuconate and 5-carboxymethyl-2-hydroxymuconate. pK(a) calcula tions are also performed for the free enzyme. The predicted binding mode of 2-hydroxymuconate is in agreement with experimental data. A model for the binding mode of 5-carboxymethyl-2-hydroxymuconate is proposed which explain s the lower catalytic efficiency of the enzyme toward this substrate. The p K(a) predictions and docking simulations support residue Arg39 as the gener al acid for the enzyme catalysis. (C) 1999 John Wiley & Sons, Inc.