Solvation energy calculation is absolutely crucial for a successful binding free energy (IC50) determination. Quantum Pharmaceuticals develops aqueous solvation models and tests them against available experimental data to validate the theoretical approaches.

The graph on the left represents two types of solvation energy calculations compared with experiments. The first series (small circles) are the energy differences on solvation for a set of molecules without conformational changes taken into account. The second set (large squares) is obtained after a single optimization run.

The correlation with the experiment clearly improves after conformational changes calculations. Apparently this does not only mean that the model is good, it also means that the molecules do change structure when inserted into water from the gas phase.

Following the classic thrombine study, we catch up with a more important target: PDK1 kinase.

Pyruvate dehydrogenase kinase, isozyme 1, also known as PDK1, is a human gene.It codes for an isozyme of pyruvate dehydrogenase kinase (PDK).Pyruvate dehydrogenase (PDH) is a part of a mitochondrial multienzyme complex that catalyzes the oxidative decarboxylation of pyruvate and is one of the major enzymes responsible for the regulation of homeostasis of carbohydrate fuels in mammals. The enzymatic activity is regulated by a phosphorylation/dephosphorylation cycle. Phosphorylation of PDH by a specific pyruvate dehydrogenase kinase (PDK) results in inactivation.

There are no as much known inhibitors as for thrombine. BindingDB gives a few more than 70 compounds with measured binding affinities, all relatively strong binders, many of them similar to each other. We run our QUANTUM software to perform docking and the affinity calculations. The results are represented on the graph and demonstrate a solid correlation. In fact the correlation shows QUANTUM's ability to identify strong binders and distinguish between similar compounds (selectivity).

The Figure on the left represents a docking study of more than 200 molecules with known activity on thrombin. The protein is a well known target ....

We have extracted the binding data from the BindingDB database and docked all the molecules onto a single (of a few available) 3D structure (2cn0 from the pdb databank).

The figure represents graphically the results of the research. The calculated and the measured activities are well correlated. Strong binders are indeed identified as strong binders (left bottom part of the graph). The accuracy of the predictions is quite good (see our discussion on the quality of the biological data here and here).

The results of the calculations can be conveniently summarized in terms of confidentiality matrix. Normally a first screen of novel compounds is performed at a certain concentration to distinguish between the active and non-active compounds. Let's take a standard, 1muM (~-35kJ/M) activity, as a separation cut-off. Then the confidence matrix has the following elements:

• Experimentally active, Predicted active: 29 molecules
• Experimentally n-active, Predicted active: 15 molecules (false positives)
• Experimentally active, Predicted n-active: 8 molecules (false negatives)
• Experimentally n-active, Predicted n-active: 156 molecules

Standard hit identification procedure with QUANTUM software implies screening of a large compound library against a given protein target. An example of such procedure for a small set of compounds with known activities is discussed in another blog entry.

Let us show first that a calculation with flexible protein gives a reasonable prediction of the binding free energy. To do that we selected a set of ~200 protein - ligand complexes from the BindingDB database. The protein-ligand pairs were selected mainly so that the complex is small and therefore the whole calculation is fast. The results are represented on the Figure. The horizontal and the vertical axis represent the calculated and the experimental value of the binding free energy calculated from the complexed positions of the ligand within the protein.

Molecular modelling software of Quantum Pharmaceuticals is used to dock small molecule to active site of target protein. The molecular docking on flexible protein is explored. The Quantum docking software is available for free use at LeadFinding.com, the online hit-to-lead optimization service to filter and profile chemical compounds in chemical database of ChemDiv - organic chemistry supplier.