Flavins are derivatives of riboflavin with a tricyclic structure called isoaloxazine. Known enzyme-active flavins are flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). They are redox active coenzymes that catalyze various chemical reactions and together with apoproteins form complexes called flavoenzymes. These enzymes include monoamine oxidase (MAO) which, bound to the outer membrane of mitochondria in a reductive half-reaction, reduce flavin to fully reduced flavin (FADH2), and in the oxidative half-reaction, oxidizing FADH2 back to FAD. The fact that flavin can be completely reduced to FADH2 is an important prerequisite for MAO regeneration with molecular oxygen. Regeneration produces hydrogen peroxide which may continue to react and cause the formation of reactive oxygen species that are responsible for oxidative stress and related neurodegeneration.
The purpose of the master's thesis was to determine the activation free energy of the reaction of flavin with oxygen using molecular modelling and quantum chemistry. The Molden programme was used to construct three-dimensional models, and the Gaussian programme to perform quantum chemical calculations. The calculations were first performed in the gas phase and then, for a more realistic simulation, the influence of the solvent on the reaction was taken into account with the help of implicit models using the self-attuned reaction field method and the Langevin dipole method.
The calculated activation energy for our gas phase reaction was 49.18 kcal/mol. After integrating the reaction field with implicit solvent models, we noticed that the energy barriers were lowered, namely for the IEFPCM method (integral equation formalism polarizable continuum model), the barrier value was 40.91 kcal/mol, for CPCM (conductor-like polarizable continuum model) 40.85 kcal/mol, for SMD (solvation model based on density) 33.65 kcal/mol and the value with the Langevin dipole method was 40.82 kcal/mol. Based on the results, we concluded that water reduces the activation free energy and accelerates the chemical reaction compared to gas phase, most notably in the case of the SMD solvation model.
We were able to calculate the free activation energies and confirm that the reactivity of flavin regeneration with oxygen can be evaluated by quantum chemical methods. We found out that the reaction in aqueous solution, without taking the enzymatic environment into account, does not take place. In the future, research could be carried out to prove whether it is possible to impact the formation of reactive oxygen species in flavoenzymes with inhibitors and other drugs.
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