Browsing by Author "Shehu, Amarda"
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Item Molecules in motion: Computing structural flexibility(2008) Shehu, Amarda; Kavraki, Lydia E.Growing databases of protein sequences in the post-genomic era call for computational methods to extract structure and function from a protein sequence. In flexible molecules like proteins, function cannot be reliably extracted from a few structures. The amino-acid chain assumes various spatial arrangements (conformations) to modulate biological function. Characterizing the flexibility of a protein under physiological (native) conditions remains an open problem in computational biology. This thesis addresses the problem of characterizing the native flexibility of a protein by computing conformations populated under native conditions. Such computation involves locating free-energy minima in a high-dimensional conformational space. The methods proposed in this thesis search for native conformations using systematically less information from experiment: first employing an experimental structure, then using only a closure constraint in cyclic cysteine-rich peptides, and finally employing only the amino-acid sequence of small- to medium-size proteins. A novel method is proposed to compute structural fluctuations of a protein around an experimental structure. The method combines a robotics-inspired exploration of the conformational space with a statistical mechanics formulation. Thermodynamic quantities measured over generated conformations reproduce experimental data of broad time scales on small (∼ 100 amino acids) proteins with non-concerted motions. Capturing concerted motions motivates the development of the next methods. A second method is proposed that employs a closure constraint to generate native conformations of cyclic cysteine-rich peptides. The method first explores the entire conformational space, then explores in present energy minima until no lower-energy minima emerge. The method captures relevant features of the native state also observed in experiment for 20–30 amino-acid long peptides. A final method is proposed that implements a similar exploration but for longer proteins and employing only amino-acid sequence. In its first stage, the method explores the entire conformational space at a coarse-grained level of detail. A second stage focuses the exploration to low-energy regions in more detail. All-atom conformational ensembles are obtained for proteins that populate various functional states through large-scale concerted motions. These ensembles capture well the populated functional states of proteins up to 214 amino-acids long.Item Sampling biomolecular conformations with spatial and energetic constraints(2005) Shehu, Amarda; Kavraki, Lydia E.This work extends cyclic coordinate descent to efficiently satisfy multiple spatial constraints, respect the secondary structure of proteins., and work with reduced backbone protein models. Reduced models allow us to treat large systems that are intractable under all-atom models. In addition, this thesis combines the satisfaction of multiple spatial constraints with conformational sampling and energy minimization techniques to generate spatially constrained biomolecular structures that are energetically stable under physiological conditions. The experiments in this thesis demonstrate the relevance and robustness of our method on three areas of applications: loop closure; backbone reconstruction, and physical trajectory recovery. Addressing the problem of loop closure, we obtain ensembles of spatially constrained conformations whose energy landscape is in agreement with laboratory experimental results on the energetic stability of the proteins at hand. Our experiments on backbone reconstruction agree with results from statistical approaches to this problem, but in addition guarantee the energetic feasibility of the completed models. (Abstract shortened by UMI.)