Molecular skins: a new concept for quantitative shape matching of a protein with its small molecule mimics

A novel analytical method for comparing molecular shapes by optimizing the intersection of molecular "SKINS" has been developed. This method provides a quantitative measure of the shape similarity by maximizing the intersection volume of molecular surfaces with a finite thickness; a molecular skin. We report shape matching of a small tripeptide inhibitor (DFKi) of elastase class proteins with the 56 residue turkey ovomucoid inhibitor (TOMI). To match a large elastase inhibitor such as TOMI with a small inhibitor or drug, we found that it is necessary to use a skin match rather than molecular volume. Skin based comparisons of TOMI protein with DFKi successfully found the alignment expected from comparison of their respective crystallographic complexes with elastase (i.e. HLE/TOMI complex and PPE/tripeptide complex). In the skin comparison of the tripeptide with the TOMI protein, blind searching for skin matches involved optimization of the skin intersection from 172 starting positions randomly selected from a set of 500 points on the TOMI van der Waals surface [within 9.5 A of the Leu-18 on the TOMI binding loop (1 point/A2)]. The tripeptide center of mass was placed at these points and its orientation was randomized before optimization was initiated. The best skin intersection, 86.4 A3, was found three times and corresponds to the experimental alignment. The next best skin intersection was 78.1 A3 giving a discrimination factor in this case of 10%. Searches over the entire surface of the TOMI protein did not identify any new matches with skin intersection greater than 78.1 A3. Matching the DFKi with a TOMI structure relaxed from its crystal conformation by molecular dynamics gives similar results.