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From Protein Structure to Function with Bioinformatics.pdf

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12 Prediction of <strong>Protein</strong> <strong>Function</strong> from Theoretical Models 299Table 12.2 The accuracy and added value of structure-derived properties in single-templatebased comparative models (Chakravarty and Sanchez 2004; Chakravarty et al. 2005)Model-derived property Accuracy Added valueAllIncreases <strong>with</strong> template-<strong>to</strong>-targetidentityIncreases when template<strong>to</strong>-targetidentity dropsResidue exposure state Decreases <strong>with</strong> the protein size;affected by alignment errorsbelow 30% sequence identityNo added valueBuried residuesneighbourhoodExposed residuesneighbourhoodAccessible surface area (ASA)Surface pockets identificationSurface pockets compositionElectrostatic potential (EP)No clear dependence on proteinsize, higher than for exposedresidues; affected by alignmenterrors below 30%sequence identityNo clear dependence on proteinsize, lower than for buriedresidues; affected by alignmenterrors below 30%sequence identityError in <strong>to</strong>tal ASA increases<strong>with</strong> protein size, influenceof misalignment is verysmallPocket artefacts; increasednumber of surface pockets incomparison <strong>to</strong> the templateand target structure; alignmenterrors have no cleareffect on the number ofpocketsAffected by alignment errorsbelow 50% sequence identityNo added valueModerate added valueModerate added valueNegative added valueHigh added valueHigh added valuewhen the sequence identity drops below 50% (Table 12.2). It has <strong>to</strong> be stressed thatthe analysis was performed on the basis of the set of comparative single-templatemodels where no loop modelling was performed. Thus the results are representativeof the models produced by large-scale fully au<strong>to</strong>mated methods. More elaboratemodelling procedures consisting of the use of multiple templates and refinementmay provide improved models and may increase accuracy of the predictions ofparticular properties for a given sequence similarity.Knowing how reliable the different model-derived properties are, it is interesting<strong>to</strong> investigate what additional information (added value) they carry <strong>with</strong> reference<strong>to</strong> the template structures used for model building. Again, systematic analysis of themodel added value was performed on the large scale only for single-template models(Chakravarty and Sanchez 2004), but it provides valuable guidelines as <strong>to</strong> whichparticular model-derived properties can be informative (Table 12.2).In general, the greater the difference between target and template sequences, themore significant the added value becomes. This results from the fact that lower-simi-

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