RESUMEN
Hydrolyzable tannin (3,6-bis-O-digalloyl-1,2,4-tri-O-galloyl-ß-d-glucose) has a dual effect on the cell membrane: (1) it binds to a plasmalemmal protein of the Chara corallina cell (C50â¯=â¯2.7⯱â¯0.3⯵M) and (2) it forms ionic channels in the lipid membrane. Based on these facts, a molecular model for the interaction of tannins with the cell membrane is proposed. The model suggests that the molecules of hydrolyzable tannin bind electrostatically to the outer groups of the membrane protein responsible for the Ca2+-dependent chloride current and blocks it. Some tannin molecules penetrate into the hydrophobic region of the membrane, and when a particular concentration is reached, they form ion-conducting structures selective toward Cl-.
Asunto(s)
Membrana Celular/química , Taninos Hidrolizables/química , Membrana Dobles de Lípidos/química , Chara/química , Chara/citología , Proteínas de la Membrana/químicaRESUMEN
The study reports about the influence of binding of orthosteric ligands on the conformational dynamics of ß-2-adrenoreceptor. Using molecular dynamics (MD) simulation, we found that there was a little fraction of active states of the receptor in its apo (ligand-free) ensemble. Analysis of MD trajectories indicated that such spontaneous activation of the receptor is accompanied by the motion in intracellular part of its alpha-helices. Thus, receptor's constitutive activity directly results from its conformational dynamics. On the other hand, the binding of a full agonist resulted in a significant shift of the initial equilibrium towards its active state. Finally, the binding of the inverse agonist stabilized the receptor in its inactive state. It is likely that the binding of inverse agonists might be a universal way of constitutive activity inhibition in vivo. Our results indicate that ligand binding redistribute pre-existing conformational degrees of freedom (in accordance to the Monod-Wyman-Changeux Model) of the receptor rather than cause induced fit in it. Therefore, the ensemble of biologically relevant receptor conformations is encoded in its spatial structure, and individual conformations from that ensemble might be used by the cell in conformity with the physiological behavior.
Asunto(s)
Simulación de Dinámica Molecular , Conformación Proteica , Receptores Adrenérgicos beta 2/química , Rodopsina/metabolismo , Sitios de Unión , Ligandos , Modelos Moleculares , Unión Proteica , Receptores Adrenérgicos beta 2/genética , Receptores Adrenérgicos beta 2/metabolismo , Rodopsina/químicaRESUMEN
Distributions of phosphate backbone-produced electrostatic potentials around several tRNAs were calculated by solving the nonlinear Poisson-Boltzmann equation. The tRNAs were either free or bound to the proteins involved in translation: aminoacyl-tRNA and elongation factor EF-Tu. We identified several regions of strong negative potential related to typical structural patterns of tRNA and invariant throughout the tRNAs. The patterns are conserved upon binding of tRNAs to the synthetase and the EF-Tu. Variation of tRNA charge in our theoretical calculations of electrostatic potential-mediated pK shifts of pH-dependent labels attached to tRNA, compared to experimentally observed pK shifts for those labels, shows that the total charge of tRNA is large, within the interval of -40 to -70 proton charges. The electrostatic field of tRNA is sufficient to cause ionization of histidine residues of ARSase, causing additional free energy of ARSase-tRNA interaction of at least several kcal/mol. This may discriminate proteins with respect to the particular tRNA at large distances. Two types of tRNA-protein electrostatic recognition mechanisms are discussed. One, more specific, involves charges induced on protein by the large electrostatic potential of tRNA, while the other, less specific, does not involve induced charges.