but the modelling strategy also made use of an active form CDK2 structure for modelling the Tloop and to impose an active conformation on the C-helix of CDK4. ProSa-Web Z-scores for the hybrid model and the CDK4 and CDK2 templates respectively, indicating that the modelling strategy has not introduced any significant packing problems. The rmsd between the active form ��hybrid model and the experimentally determined CDK4 structure, this is close to the found for comparing the active and inactive form of CDK2. FAS and CRB were docked into both, CDK2 and CDK4, using the GOLD package. GOLD treats ligands as fully flexible and allows the user to assign flexibility to a limited set of receptor residues. The best-scoring docking poses for all four systems investigated show key features known from experimentally determined CDK2/inhibitor X-ray structures such as hydrogen bonding in the hinge region. FAS and CRB docking poses are characterised by two H-bonds involving the backbone NH and carbonyl of the hinge residues Leu83CDK2 and LY335979 Val96CDK4. No significant 1242156-23-5 difference was found in the docking scores of both compounds with both CDK2 and CDK4. So while the ligand docking study generates typical kinase inhibitor binding poses, it can not explain why fascaplysin preferably binds to CDK4 rather than CDK2. A key difference between the CDK2 and CDK4 poses involves the equivalent residues His95CDK4 and Phe82CDK2. In principle three different ��species of His95CDK4 have to be considered: His95CDK4 could have a positively charged imidazole side chain and there are two uncharged species with either Nd or Ne of the imidazole ring bearing a hydrogen. We did not consider a positively charged imidazole side chain as this would unfavourably interact with the positively charged fascaplysin. However, alternative positioning of hydrogens in His95CDK4 was considered in the ligand docking process. ChemScores were by a small margin higher for the His95CDK4Nd-H/fascaplysin complex indicating a slight preference for the side-chain conformation in which the Nd-hydrogen of the imidazol ring forms an additional H-bond to the carbonyl of FAS and CRB, respectively. This conformation is different from the His95 conformation found in the experimentally determined CDK4 structures, but such a conf
