title: A theoretical model of the catalytic mechanism of the Delta5-3-ketosteroid isomerase reaction.
abstract: The present paper describes a theoretical approach to the catalytic reaction mechanism involved in the conversion of 5-androstene-3,17-dione to 4-androstene-3,17-dione. The model incorporates the side chains of the residues tyrosine (Tyr(14)), aspartate (Asp(38)) and aspartic acid (Asp(99)) of the enzyme Delta(5)-3-ketosteroid isomerase (KSI; EC 5.3.3.1). The reaction involves two steps: first, Asp(38) acts as a base, abstracting the 4beta-H atom (proton) from C-4 of the steroid to form a dienolate as the intermediate; next, the intermediate is reketonized by proton transfer to the 6beta-position. Each step goes through its own transition state. Functional groups of the Tyr(14) and Asp(99) side chains act as hydrogen bond donors to the O1 atom of the steroid, providing stability along the reaction coordinate. Calculations were assessed at high level Hartree-Fock theory, using the 6-31G(*) basis set and the most important physicochemical properties involved in each step of the reaction, such as total energy, hardness, and dipole moment. Likewise, to explain the mechanism of reaction, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), atomic orbital contributions to frontier orbitals formation, encoded electrostatic potentials, and atomic charges were used. Energy minima and transition state geometries were confirmed by vibrational frequency analysis. The mechanism described herein accounts for all of the properties, as well as the flow of atomic charges, explaining both catalytic mechanism and proficiency of KSI.
authors.affiliation: Laboratorio de Química Hormonal, Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México, Apartado Postal 70-228, México D.F. 04510, México.
authors.forename: Kamlesh
authors.initials: K
authors.lastname: Sharma
authors.forename: Ricardo
authors.initials: R
authors.lastname: Vázquez-Ramírez
authors.forename: Carlos
authors.initials: C
authors.lastname: Kubli-Garfias
mesh_terms: D000735:Androstenedione / Q000737:chemistry / Q000378:metabolism; D001224:Aspartic Acid / Q000737:chemistry / Q000378:metabolism; D002384:Catalysis; D008956:Models, Chemical; D008958:Models, Molecular; D013237:Stereoisomerism; D013251:Steroid Isomerases / Q000737:chemistry* / Q000378:metabolism; D014443:Tyrosine / Q000737:chemistry / Q000378:metabolism
chemical_list: D001224:Aspartic Acid; D000735:Androstenedione; D014443:Tyrosine; C003528:5-androstene-3,17-dione; D013251:Steroid Isomerases; C022660:steroid delta-isomerase