Statistical analysis of the loop-geometry on a non-redundant database of proteins

The conformations of protein loops from a non-redundant set of 347 proteins with less than 25% sequence homology have been studied in order to clarify the topological variation of protein loops. Loops have been classified in five types (alpha-alpha, alpha-beta, beta-alpha, beta-links and beta-hairpi ns) depending on the secondary structures that they embrace. Four variables have been used to describe the loop geometry (3 angles and the end-to-end distance between the secondary structures embracing the loop). Loops with w ell defined geometry are identified by means of the internal dependency bet ween the geometrical variables by application of information-entropy theory . From this it has been deduced that loops formed by less than 10 residues show an intrinsic dependency on the geometric variables that defines the mo tif shape. In this interval the most stable loops are found for short conne ctions owing to the entropic energy analysed.