title: Temperature effects on the catalytic activity of the D38E mutant of 3-oxo-Delta5-steroid isomerase: favorable enthalpies and entropies of activation relative to the nonenzymatic reaction catalyzed by acetate ion.
abstract: 3-oxo-Delta5-steroid isomerase (ketosteroid isomerase, KSI) catalyzes the isomerization of 5-androstene-3,17-dione (1) to 4-androstene-3,17-dione (3) via a dienolate intermediate (2-). KSI catalyzes this conversion about 13 orders of magnitude faster than the corresponding reaction catalyzed by acetate ion, a difference in activation energy (DeltaG) of approximately 18 kcal/mol. To evaluate whether the decrease in DeltaG by KSI is due to enthalpic or entropic effects, the activation parameters for the isomerization of 1 catalyzed by the D38E mutant of KSI were determined. A linear Arrhenius plot of kcat/KM versus 1/T gives the activation enthalpy (DeltaH = 5.9 kcal/mol) and activation entropy (TDeltaS = -2.6 kcal/mol). Relative to catalysis by acetate, D38E reduces DeltaH by approximately 10 kcal/mol and increases TDeltaS by approximately 5 kcal/mol. The activation parameters for the microscopic rate constants for D38E catalysis were also determined and compared to those for the acetate ion-catalyzed reaction. Enthalpic stabilization of 2- and favorable entropic effects in both chemical transition states by D38E result in an overall energetically more favorable enzymatic reaction relative to that catalyzed by acetate ion.
authors.affiliation: Department of Chemistry and Biochemistry, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA.
authors.forename: Wendy J
authors.initials: WJ
authors.lastname: Houck
authors.forename: Ralph M
authors.initials: RM
authors.lastname: Pollack
mesh_terms: D002384:Catalysis; D004789:Enzyme Activation; D007700:Kinetics; D016297:Mutagenesis, Site-Directed; D013251:Steroid Isomerases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D013696:Temperature; D013816:Thermodynamics
chemical_list: D013251:Steroid Isomerases; C022660:steroid delta-isomerase