title: Escherichia coli dihydrofolate reductase catalyzed proton and hydride transfers: temporal order and the roles of Asp27 and Tyr100.
abstract: The reaction catalyzed by Escherichia coli dihydrofolate reductase (ecDHFR) has become a model for understanding enzyme catalysis, and yet several details of its mechanism are still unresolved. Specifically, the mechanism of the chemical step, the hydride transfer reaction, is not fully resolved. We found, unexpectedly, the presence of two reactive ternary complexes [enzyme:NADPH:7,8-dihydrofolate (E:NADPH:DHF)] separated by one ionization event. Furthermore, multiple kinetic isotope effect (KIE) studies revealed a stepwise mechanism in which protonation of the DHF precedes the hydride transfer from the nicotinamide cofactor (NADPH) for both reactive ternary complexes of the WT enzyme. This mechanism was supported by the pH- and temperature-independent intrinsic KIEs for the C-H→C hydride transfer between NADPH and the preprotonated DHF. Moreover, we showed that active site residues D27 and Y100 play a synergistic role in facilitating both the proton transfer and subsequent hydride transfer steps. Although D27 appears to have a greater effect on the overall rate of conversion of DHF to tetrahydrofolate, Y100 plays an important electrostatic role in modulating the pKa of the N5 of DHF to enable the preprotonation of DHF by an active site water molecule.
authors.affiliation: Department of Chemistry, Pennsylvania State University, University Park, PA 16802;
authors.forename: C Tony
authors.initials: CT
authors.lastname: Liu
authors.affiliation: Department of Chemistry, The University of Iowa, Iowa City, IA 52242; and.
authors.forename: Kevin
authors.initials: K
authors.lastname: Francis
authors.affiliation: Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801-3364.
authors.forename: Joshua P
authors.initials: JP
authors.lastname: Layfield
authors.affiliation: Department of Chemistry, Pennsylvania State University, University Park, PA 16802;
authors.forename: Xinyi
authors.initials: X
authors.lastname: Huang
authors.affiliation: Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801-3364.
authors.forename: Sharon
authors.initials: S
authors.lastname: Hammes-Schiffer
authors.affiliation: Department of Chemistry, The University of Iowa, Iowa City, IA 52242; and amnon-kohen@uiowa.edu sjb1@psu.edu.
authors.forename: Amnon
authors.initials: A
authors.lastname: Kohen
authors.affiliation: Department of Chemistry, Pennsylvania State University, University Park, PA 16802; amnon-kohen@uiowa.edu sjb1@psu.edu.
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Benkovic
mesh_terms: D001224:Aspartic Acid / Q000378:metabolism*; D055162:Biocatalysis; D004926:Escherichia coli / Q000201:enzymology*; D011522:Protons; D013696:Temperature; D013762:Tetrahydrofolate Dehydrogenase / Q000737:chemistry / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism*
chemical_list: D011522:Protons; D001224:Aspartic Acid; D014443:Tyrosine; D013762:Tetrahydrofolate Dehydrogenase
keywords: dihydrofolate reductase; kinetic isotope effect; mechanism; synergism