A biomimetic study: Alpha-cyclodextrin and its derivatives as penicillinase models

Zhihong Helena Qi, Purdue University

Abstract

Understanding of the catalytic mechanism of penicillinases at the atomic level is crucial in order to design and formulate more effective antibiotic agents. Due to the structural complexity of these enzymes, however, details in their mechanisms are still not completely clear. Therefore, a relevant biomimetic study has been undertaken. To serve as a basic penicillinase model, $\alpha$-cyclodextrin is chosen with its cavity mimicking the hydrophobic binding site of the enzyme, and the hydroxyl group simulating the nucleophilic serine residue. The formation and the stability of an 1:1 inclusion complex between $\alpha$-cyclodextrin (host) and penicillin V (guest) in both weakly alkaline and mildly acidic solutions have been systematically investigated. Alpha-cyclodextrin exhibits considerable catalytic activity in alkaline solutions. The mechanism of the $\alpha$-cyclodextrin-catalyzed hydrolysis has been substantiated by the structural elucidation of a reaction intermediate, equivalent to the acyl-penicillinase in the enzymatic catalysis. The results have provided the important experimental evidence to indicate the C-2 hydroxyl group of $\alpha$-cyclodextrin mimicking the serine residue of penicillinase. In an attempt to design a more effective enzyme model, 3$\sp{\rm A}$-Amino-$3\sp{\rm A}$-deoxy-(2$\sp{\rm A}R),(3\sp{\rm A}S)$-$\alpha$-cyclodextrin has been synthesized along with two control compounds, 3$\sp{\rm A}$-dimethylamino-3$\sp{\rm A}$-deoxy-(2$\sp{\rm A}R)$,(3$\sp{\rm A}S)$-$\alpha$-cyclodextrin and 3$\sp{\rm A}$-amino-3$\sp{\rm A}$-deoxy-(2$\sp{\rm A}$S),(3$\sp{\rm A}R)$-$\alpha$-cyclodextrin. The molecular specificity and the stability of the inclusion complex between 3$\sp{\rm A}$-amino-3$\sp{\rm A}$-deoxy-(2$\sp{\rm A}R)$,(3$\sp{\rm A}S)$-$\alpha$-cyclodextrin and penicillin V have been studied through FAB-MS and $\sp1$H NMR. Computer-assisted molecular modeling and docking calculations have been carried out to explore the host-guest interactions between the model compounds and penicillin V. The computational results are in good agreement with experimental data. Detailed kinetic studies have revealed that 3$\sp{\rm A}$-amino-3$\sp{\rm A}$-deoxy-(2$\sp{\rm A}R)$,(3$\sp{\rm A}S)$-$\alpha$-cyclodextrin has a much improved catalytic activity compared with natural $\alpha$-cyclodextrin near neutral pH. Most remarkably, this stereo- and regioselectively modified $\alpha$-cyclodextrin has brought about strong rate enhancement toward the hydrolysis of penicillin V at the physiological pH. It is the best cyclodextrin mimic of penicillinase known thus far.

Degree

Ph.D.

Advisors

Chang, Purdue University.

Subject Area

Pharmaceuticals

COinS