Hydrogen Bonds and n → π* Interactions in the Acetylation of Propranolol Catalyzed by Candida antarctica Lipase B: A QTAIM Study

Enzyme–substrate interactions play a crucial role in enzymatic catalysis. Quantum theory of atoms in molecules (QTAIM) calculations are extremely useful in computational studies of these interactions because they provide very detailed information about the strengths and types of molecular interactions. QTAIM also provides information about the intramolecular changes that occur in the catalytic reaction. Here, we analyze the enzyme–substrate interactions and the topological properties of the electron density in the enantioselective step of the acylation of (R,S)-propranolol, an aminoalcohol with therapeutic applications, catalyzed by Candida antarctica lipase B. Eight reaction paths (four for each enantiomer) are investigated and the energies, atomic charges, hydrogen bonds, and n → π* interactions of propranolol, the catalytic triad (composed of D187, H224, and S105), and the oxyanion hole are analyzed. It is found that D187 acts as an electron density reservoir for H224, and H224 acts as an electron density reservoir for the active site of the protein. It releases electron density when the tetrahedral intermediate is formed from the Michaelis complex and receives it when the enzyme–product complex is formed. Hydrogen bonds can be grouped into noncovalent and covalent hydrogen bonds. The latter are stronger and more important for the reaction than the former. We also found weak n → π* interactions, which are characterized by QTAIM and the natural bond orbital (NBO) analysis.酶-底物相互作用在酶催化过程中发挥着至关重要的作用。分子中的原子量子理论（Quantum theory of atoms in molecules, QTAIM）计算在这类相互作用的计算研究中极具应用价值，因其能够提供关于分子相互作用强度与类型的详尽信息。QTAIM还可揭示催化反应中发生的分子内变化。本研究针对南极假丝酵母脂肪酶B（Candida antarctica lipase B）催化的（R,S）-普萘洛尔——一种具有治疗应用价值的氨基醇——的酰化反应对映选择性步骤，分析了酶-底物相互作用与电子密度的拓扑性质。本次研究共考察了8条反应路径（每种对映异构体对应4条路径），并对普萘洛尔、由D187、H224与S105构成的催化三联体以及氧阴离子空穴的能量、原子电荷、氢键与n→π*相互作用展开了分析。研究发现，D187可作为H224的电子密度储库，而H224则可作为蛋白质活性位点的电子密度储库：当从米氏复合物形成四面体中间体时，H224会释放电子密度；而当形成酶-产物复合物时，则会接收电子密度。氢键可划分为非共价氢键与共价氢键两类，其中共价氢键的强度更高，对反应的重要性也更为显著。本研究还发现了弱的n→π*相互作用，此类相互作用可通过QTAIM与自然键轨道（Natural Bond Orbital, NBO）分析进行表征。