COENZYMES

Author: DOELKER ANDREA NICOLE.
Year: 2003.
University: AUTÓNOMA DE BARCELONA [www.uab.es].
Place of defense: FACULTAD DE CIENCIAS.
Place of preparation: ESCUELA DE DOCTORADO Y DE FORMACIÓN CONTINUADA.

Summary: The cobalaminas act as cofactors in various enzymatic processes. Two cobalaminas different biological activity: metilcobalamina and adenosilcobalamina. Both cofactors contain a covalent bond cobalto-carbono, which makes them two of the few known organometallic compounds with biological activity. In all the enzymes dependent cobalaminas as cofactors the first step of the reaction is always breaking this link. The modes of breakage, however, are different: in metilcobalamina the link is broken so heterolítica and form an anion methyl while the adenosilcobalamina rupture homolítica and form a radical adenosilo. In this thesis has been studied several factors that may influence the break both homolítica as heterolítica liaison Co-C of cobalaminas using methods of computational chemistry. The thesis is divided into three parts. The first part has analyzed the influence of the axial nitrogenous base, which occupies the site of coordination enposición "cross" regarding the link Co-C on the break enalce Co-C. It has been found that the energy of dissociation for breaking homolítica not on the basis axial while energy react breakage hetrolítica shows a clear dependence with regard to the distance between cobalt and nitrogen-based Axial. It is concluded that the coordination of axial based in cobalaminas not important for the reaction mechanism of breakage homolítica liaison Co-C in the active site, but that could be decisive to differentiate between homólisis and heterólisis in different enzymes. The second part has studied the breakage homolítica liaison Co-C in adenosilcobalamina. It has been discussed several mechanisms by which enzymes can facilitate the formation of a radical adenosilo. It has been found that both in gas phase and in solution, there is an electrostatic interaction between the cobalamina and the group adenosilo. This interaction is maintained after the break link Co-Cy stabilizes fragments radicalarios. It has been conclulido that the difference between metilcobalamina and adenosilcobalamina is mainly due to the more complex structure of the group adenosilo that allows greater interaction with the cabalamina which stabilizes the radical adenosilo. It finds that itneracción electrostatic between radical adenosilo and cobalamina is also important at the active center of the enzyme. An additional stabilization of radcial is achieved through hydrogen bond between the radical adenosilo and residues of the protein. The third part of the thesis deals with the issue of homólisis liaison Co-C in cofactors free in solution. Experimentally, it has been found that the energy of the bond is about 5 kcal / mol lowest in the adenosilcobalamina. Until then it had not been able to reproduce this gap with theoretical calculations. All previous theoretical studies have in common the use of a simplified model of cobalamina. In this thesis has been made calculations of energy liaison Co-C for the complete system using a hybrid between quantum mechanics and molecular mechanics. It has been found that the repulsion between the voluminous group adenosilo and ligands side of the cobalamina destabilizes slightly liaison Co-C. However, the difference between the energies of bond is mainly due to effects of the solvent rather than intrinsic properties of the cofactors. Therefore 8 has been co 33d ncluido the difference in the energy of dissociation observed in the free cofactors is not directly related to the mechanisms of enzymatic catalysis.