Sharma Kamlesh, Vázquez-Ramírez Ricardo, Kubli-Garfias Carlos
Laboratorio de Química Hormonal, Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México, Apartado Postal 70-228, México D.F. 04510, México.
Steroids. 2006 Jul;71(7):549-57. doi: 10.1016/j.steroids.2005.12.001. Epub 2006 Apr 18.
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.
本文描述了一种关于5-雄甾烯-3,17-二酮转化为4-雄甾烯-3,17-二酮所涉及的催化反应机理的理论方法。该模型纳入了Δ⁵-3-酮类固醇异构酶（KSI；EC 5.3.3.1）的酪氨酸（Tyr(14)）、天冬氨酸（Asp(38)）和天冬氨酸（Asp(99)）残基的侧链。该反应包括两个步骤：首先，Asp(38)作为碱，从类固醇的C-4位提取4β-H原子（质子），形成双烯醇盐中间体；接着，中间体通过质子转移至6β位而重新酮化。每个步骤都经历其自身的过渡态。Tyr(14)和Asp(99)侧链的官能团作为类固醇O1原子的氢键供体，在反应坐标上提供稳定性。计算采用高水平的Hartree-Fock理论进行评估，使用6-31G(*)基组以及反应各步骤中涉及的最重要的物理化学性质，如总能量、硬度和偶极矩。同样，为了解释反应机理，使用了最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）、原子轨道对前沿轨道形成的贡献、编码的静电势和原子电荷。通过振动频率分析确认了能量最小值和过渡态几何结构。本文所述的机理解释了所有性质以及原子电荷的流动，阐释了KSI的催化机理和效能。
