Kinetic and magnetic resonance studies of active-site mutants of staphylococcal nuclease: factors contributing to catalysis

Abstract

To determine the origin of the overall approximately 10(16)-fold rate enhancement of DNA hydrolysis catalyzed by staphylococcal nuclease, the effects of single mutations that alter the amino acid residue at each of the essential positions Asp-21, Asp-40, Thr-41, Arg-35, and Arg-87 have been examined. Metal ion and substrate analogue binding were quantitated by EPR, by the paramagnetic effects of Mn2+ on 1/T1 of water protons, and by fluorescence titrations, yielding the six dissociation constants of the ternary enzyme-Mn2+-3',5'-pdTp and enzyme-Ca2+-3',5'-pdTp complexes. The kinetic parameters kcat, KACa, KMCa, KSDNA, KMDNA, and KIMn were determined by monitoring the rate of DNA hydrolysis. By thermodynamic and kinetic criteria, Mn2+ binds tightly to the Ca2+ binding site of the enzyme but is at least 36,000-fold less effective than Ca2+ in activating the enzyme. Alterations of the liganding residues in the D40G, D40E, T41P, D21E, and D21Y mutants generally weaken the binding of Ca2+ less than or equal to 12.7-fold and of Mn2+ less than or equal to 5.4-fold, exert little effect on the KSDNA or KMDNA (less than or equal to 3.2-fold), and raise the affinity of the enzyme and its metal complexes for 3',5'-pdTp by factors less than or equal to 13.5-fold. Small changes in the ligand geometry are also reflected in the Mn2+ complexes of the liganding mutants (i.e., those in which the metal-liganding amino acids have been altered) by decreases in the electron-spin relaxation time of Mn2+. Inhibitory effects on kcat are noted in all of the liganding mutants with D40E, D40G, T41P, D21E, and D21Y showing 12-, 30-, 37-, 1500-, and greater than or equal to 29,000-fold reductions, respectively. The greater than or equal 10(3)-fold larger inhibitory effects on kcat of enlarging Asp-21 as compared to enlarging Asp-40 are ascribed to the displacement of the adjacent water molecule which attacks the phosphodiester. Mutations of each of the essential Arg residues to Gly (R35G and R87G) reduce kcat by factors greater than or equal to 35,000 but weaken metal binding less than or equal to 9-fold.(ABSTRACT TRUNCATED AT 400 WORDS)