Structural bioinformatics: current status and future directions Swanand Gore∗, Sameer Velankar and Gerard J. Kleywegt Protein Data Bank in Europe (PDBe) EMBL-EBI Wellcome Trust Genome Campus, Hinxton, Cambridge 1 Introduction Structural biology is a key discipline in basic and applied biological research. It reveals atomic and mechanistic details of biological macromolecules in normal and diseased states and enables researchers to modulate the molecular machinery in a rational manner, through the design of important molecules of practical relevance such as drugs, inhibitors, enzymes, antibodies, pesticides etc. Structural bioinformatics - an umbrella term encompassing many techniques in biocomputing and informatics of macromolecular structures - is an essential component of structural biology due to the quantity and complexity of structural data.

2 An ensemble of sub-disciplines Structure determination - the core technique in structural biology - is made possible by computational refinement of structure models using experimental data and augmenting it with prior knowledge e.g. of basic covalent geometry. While X-ray crystallography and NMR spectroscopy remain the most popular methods, cryo-electron microscopy (Chiu et al.(2005)) and hybrid experimental techniques (Wanga et al.(2011)) are rapidly becoming more important as the field moves towards studying the structures of large assemblies and molecular machines. This has necessitated the development of more powerful structure refinement procedures (Tang et al.(2007); Adams et al.(2010)). High-throughput crystallography protocols have been developed for structural genomics (Terwilliger et al.(2009)) and fragment-based drug-discovery initiatives (Hajduk and Greer(2007)). Structure prediction is crucial for proteins whose structures are experimentally intractable and is carried out using techniques such as comparative (homology) modelling, threading and ab-initio methods. Homology models are now available on a genomic scale (Pieper et al.(2004)) produced using new powerful algorithms for mainchain and sidechain modelling (Krivov et al. (2009); Eswar et al.(2006)). New approaches are being developed to model remote homology more reliably (Zhou and Skolnick(2010)) and to include experimental restraints wherever available (Möglich et al.(2005)). Cutting-edge technologies like Rosetta and [email protected] have been very successful (Raman et al.(2009); Beberg and Pande(2009); Cooper et al.(2010)) for folding structures in silico where no suitable homologous structures exist. In the related field of molecular dynamics simulations, new methods are being developed towards speed and robustness (Shaw et al.(2010)), to include quantum-mechanical effects (Kamerlin et al.(2009)), and coarse-graining (Hall and Sansom(2009)). Possible errors in experimental or predicted structures may be detected by validation criteria (Kleywegt(2009); Kleywegt(2000); Chen et al.(2010)) such as the Ramachandran plot, realspace-R value etc. This is becoming more relevant due to a variety of non-expert users taking to structural studies (Kleywegt(2009)). The classification of structures is an important area of structural bioinformatics, and uses structure-based alignments (Sierk and Kleywegt(2004)). Among the many structure-superposition methods (Novotny et al.(2004); Kolodny et al.(2005)), some differentiate themselves by the 65

ability to handle challenging cases like partial matches or structural flexibility. Aligned structures generally have evolutionarily related sequences which often form globular domains. Domain is a fundamental concept in structure classification (Ponting and Russell(2002)). Domain classifications like SCOP (Andreeva et al.(2007)) and CATH (Greene et al.(2007)) are popular. Similar domains often exhibit conservation of functionally important residues, e.g. in surfaceaccessible pockets and active sites. These can be predicted by unusual conservation of residues (Chelliah et al.(2004)) whereas larger conserved surface patches often indicate domain-domain interfaces. In silico docking (Sousa et al.(2006)) is used to design small molecules that optimally fit known or predicted binding sites. Domain-independent structure analysis can sometimes reveal structural propensities, e.g. those of α and β secondary structures (Kihara(2005)), or a tendency to remain unstructured (Dosztányi et al.(2005)) as in domain linkers, or recurring patterns of sidechain orientations, structure templates (Tendulkar et al.(2004)) etc. Effective data organisation is key to analyses in structural bioinformatics. Experimentally determined structural models are available in PDB and mmCIF1 file formats which define data types and the relationships between them. Efforts to produce molecule- and residue-level mappings between structure and sequence, sequence families, structural domains and source organisms (Velankar et al.(2005)) enable the integration of structure and other biological databases. Various ontologies (Reeves et al.(2008)) have been defined to capture structure-sequence features in a controlled vocabulary.

3 A PDBe perspective The Worldwide Protein Data Bank (wwPDB; wwpdb.org) is the international consortium, consisting of RCSB PDB and BMRB (USA), PDBe (Europe) and PDBj (Japan). wwPDB is responsible for collecting, annotating, archiving and disseminating 3D biomacromolecular structure data. The EMBL-EBI Protein Data Bank in Europe (Cambridge, UK; pdbe.org) aims to become an integrated structure resource for all of bioscience, focussing on advanced services, ligands, integration, validation and experimental data (Velankar et al.(2010); Velankar and Kleywegt(2011); Velankar et al.(2011)). This effort reflects the realisation that 3D structure data is now used by a wide variety of non-expert scientists who need reliable information delivered in a biological or chemical context that is familiar to them (Velankar and Kleywegt(2011)). Current advanced services at PDBe include: (a) PDBeMotif (pdbe.org/motif), a service for analysing detailed molecular interactions and correlate them with sequence or structure patterns (Golovin and Henrick(2008)); (b) PDBeFold (pdbe.org/fold), a powerful interactive structurealignment tool (Krissinel and Henrick(2004)); (c) PDBePISA (pdbe.org/pisa), a quaternarystructure prediction service (Krissinel and Henrick(2007)); (d) PDBeXplore (pdbe.org/browse), a tool that allows browsing and analysis of the structural archive based on familiar chemical and biological classification systems (such EC, CATH and Pfam). Small-molecule ligands and their interactions with biomacromolecules are important, but they are often poorly determined and annotated. Therefore, PDBe plans to develop relevant services for both structure producers and users, e.g. for analysis, validation (Bruno et al.(2004)) and visualisation. Integration is a keyword in bioinformatics and the joint UniProt/PDBe mapping resource SIFTS (pdbe.org/sifts) will be further enriched with cross-references to other biological data resources. Validation is crucial for identifying reliable structures and regions in PDB entries. The wwPDB partners have convened Validation Task Forces for X-ray crystallography, NMR spectroscopy and Electron cryo-Microscopy and their recommendations will be implemented as part of the new joint structure deposition and annotation system that is currently being developed. Finally, PDBe intends to provide services where the experimental data 1

see http://www.ebi.ac.uk/pdbe/docs/mmcif/mmcif.html

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can be visualised and used to assess the reliability of structures or details of structures (such as a binding site).

4 Conclusion Structural biology and bioinformatics encompass many different techniques and areas, and often these have to be combined to solve a problem. Structure determination is no longer an end in itself, but a key tool for gaining insight into a biological problem. This has created challenges in computation and informatics of macromolecular structure that are currently being addressed by the community working together.

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Structural bioinformatics: current status and future ...

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