Estudo teórico do mecanismo de ação da enzima RTA da ricina com um ligante análogo ao substrato natural rRNA 28S

Abstract

Ricinus communis, a plant known in Brazil as castor bean, has gained attention from industry, media and governments, due to the properties of its derivatives, especially ricin and ricinoleic oil. The latter corresponds to 90% of the total oil that is extracted from the seed of the fruit and presents an important saving for the chemical industry. Already ricin is a potent toxin capable of efficiently inactivating eukaryotic ribosomes. It consists of two subunits, RTA and RTB. Functionally, RTB is responsible for the recognition and internalization of ricin in the target cells and RTA for the catalysis of the depurination of adenine 4324 located in the Sarcin-Ricin Loop region of rRNA 28S. Due to its cytotoxicity, ease of extraction and purification, ricin is classified as a category B biological threat agent according to the Center for Disease Control and Prevention (CDC), worrying world authorities for its potential use as a biological weapon. Thus, the development of an antitoxin would be an important pharmaceutical and biotechnological achievement. However, the great difficulty in designing inhibitors capable of acting efficiently as RTA blockers is attributed to the incomplete understanding of its reaction mechanism. In order to obtain further clarification about the mechanism of action of RTA, a theoretical study was carried out in this dissertation using the Molecular Dynamics (DM) to monitor the interaction of the RTA catalytic subunit with a natural substrate analog. Quantum Chemistry calculations were also performed using Reactivity Descriptors (DR) based on the Functional Theory of Conceptual Density (CDFT). In short, the DM trajectory revealed the main hydrogen bonds present in the active site of the RTA between the Val81, Arg180 and Gly121 residues with the target adenine of the mimic ligand. In addition, it evidenced the hydrogen bond that activates the target adenine throughout the simulation. The attractive interactions of adenine stabilization by residues Tyr80, Tyr123, Val81, Gly121 and Glu208 and repulsive interactions by residues Glu177 and Trp211, were identified by the study of the interaction energy. Residue 332 was also recognized as catalytic water in the mechanism. Finally, the reactivity descriptors pointed to an RTA mechanism of action where there is a susceptibility to electrophilic attack on the target adenine through the Arg180 residue, a stabilization of the adenine in a π stack between the Tyr80 and Tyr123 residues and a Glu177 propensity to activate catalytic water by donating electron density and receiving a proton.