Browsing by Subject "Ligand binding (Biochemistry)"

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Computational prediction of allosteric nucleic acids
(2008-05) Hall, Bradley, 1977-; Ellington, Andrew D.
Selected nucleic acid binding species (aptamers) have been shown to undergo conformational changes in the presence of ligands, and have been adapted to function as biosensors. We were interested in whether the secondary structures of aptamers could be rationally engineered to undergo ligand dependent conformational changes. To this end, we used rational and computational design methods to generate a number of aptamer biosensors. First, we built upon previous work that showed that antisense oligonucleotides bearing reporter moieties could be used to denature aptamers. Upon addition of ligands, the conformational equilibrium is shifted towards release of the antisense oligonucleotide and a concomitant increase in fluorescence. We attempted to adapt this format to the potential detection of ricin, but were unsuccessful. In order to better evaluate rational designs, we attempted to use computational modeling methods. Again, aptamer biosensors have previously been engineered based on ligand-induced reorganization of secondary structure (as opposed to oligonucleotide displacement), a so-called 'slip-structure' model. We developed an algorithm to evaluate different lisp structures, predicted both aptamers and aptazymes that should have undergone ligand-dependent changes in conformation, and experimentally evaluated the computationally predicted sequences. A number of robust biosensors that could respond to the cytokine VegF and the small molecule flavin were discovered. The computational model was further adapted to an aptamer biosensor that underwent a larger conformational change upon ligand-binding, an antiswitch. In this model, binding of the ligand stabilizes one hairpin structure at the expense of a competing structure (as opposed to merely changing the register of the hairpin as in the previously described slip structure model). Again, we were able to computationally identify a number of antiswitches that upon synthesis were responsive to the ligand theophylline. Finally we again attempted to use rational design methods to optimize not just the degree of signal but also the kinetic performance of aptamer biosensors. To this end, we developed biosensors that signaled within seconds the presence of the coagulation protein thrombin.
Design, synthesis & thermodynamic evaluation of conformationally-constrained pseudopeptides and synthetic approaches to sieboldine A
(2008-08) Teresk, Martin Gerald, 1981-; Martin, Stephen F.
A series of conformationally constrained and flexible pseudopeptides were prepared and their thermodynamic parameters on binding to the Grb2 SH2 domain were determined by isothermal titration calorimetry (ITC). Cyclopropane constrained analogs having hydrophobic amino acid residues at the pTyr+1 position exhibited, on average, a 2-5 fold improvement in binding affinities with the enhancement in affinities due to a more favorable enthalpy, not entropy, of binding. This serves as the first set of examples which demonstrate that favorable entropies of binding are not an inherent characteristic of ligand preorganization. Incorporation of polar amino acid residues at the pTyr+1 position eventuated in a slightly different thermodynamic effect than what was observed with the hydrophobic analogs. The constrained molecules exhibited greater binding affinities for the Grb2 SH2 domain and that increase in affinity was a consequence of a more favorable enthalpy, not entropy, of binding. However, the binding entropies for the polar set of constrained and flexible molecules were all negative. Structural information obtained from the co-crystallization of selected constrained and flexible ligand pairs with the Grb2 SH2 domain revealed an increase in conformational mobility of the BC loop in the complexes of the constrained derivatives and the presence of a greater number of direct polar contacts, but fewer water-mediated interactions between the phosphate group of the constrained molecules and the pTyr binding pocket of the domain. The construction of the cis-hydrindanone ring system in sieboldine A was accomplished utilizing either a Lewis acid-mediated silyl-directed Nazarov cyclization of a functionalized divinyl ketone or a sequential Ni(0)-catalyzed 1,4-addition/5-endo-dig cyclization. Propargylzinc bromide was shown to undergo conjugate addition to the [alpha]-aminopropyl substituted enone using Ni(acac)₂, thus providing a new, mild protocol for the conjugate propargylation reaction. Further efforts toward the formation of the α-epoxy ketone are described.
SELEX: a tool to study the sequence specific molecular recognition of single stranded nucleic acids
(2004) Manimala, Joseph Chacko; Anslyn, Eric V.; Ellington, Andrew D.