Development of fragment-based approaches to build chemical tools for biology

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Enzymes catalyse chemical reactions within living cells, very quickly and very precisely. One way to cure (or kill) an organism is to slow down or stop an enzyme by using a small molecule called an enzyme inhibitor. Enzyme inhibitors have to be very selective (so they only affect the target enzyme) and they have to bind very tightly to that enzyme, so that they are not pushed off by the enzyme's normal substrate. By understanding how to inhibit an enzyme we can often learn something about how the enzyme works. The design and synthesis of enzyme inhibitors is also central to the pharmaceutical industry. Many medicines are enzyme inhibitors e.g. penicillin, pills for high blood pressure etc. So it is not surprising that ways to discover new enzyme inhibitors are very important. In the last ten years the main approach industry has used to find new enzyme inhibitors is to test millions of compounds very quickly to see if promising compounds can be found. These are then modified to be more potent and have the properties they need to be a medicine. However there is now growing interest in a completely different approach to discovering enzyme inhibitors. This involves finding a quite small molecule (called a fragment) that binds to the enzyme, and then using this as an anchor point to build up bigger more potent inhibitors. These 'fragment-based' approaches depend on methods of finding fragments. There are several ways to this, and we propose to use a number of these, some of which are well established and others which are more novel. The methods use techniques including NMR spectroscopy, mass spectrometry, and calorimetry. All rely on some way of showing the fragment binds to the enzyme, with the exception of virtual screening, where computational approaches are used to try and identify fragments that look like they should bind to the enzyme. Once we have identified fragments that bind to our target enzymes we will try and grow them into bigger molecules. One general way to do this is to mix a lot of fragments and let them react with each other in every possible combination (called dynamic combinatorial chemistry), and let the enzyme select out the one that binds to it most efficiently. It may be that the enzyme acts as a template to bring together two fragments that bind to it separately. If a lot of enzyme is used, it may lead to more of the key compound that binds to it being formed, allowing this compound to be identified as the major product. A more elegant way to identify the best binding compound, which uses less of the valuable enzyme, is to form the mixture of compounds in the presence of a crystal of the enzyme. If a compound binds to the enzyme, its identity can be deduced by solving the crystal structure of the enzyme with the molecule bound to it. We will devlop these ideas by trying to make inhibitors of two enzymes involved in making vitamin B5 (pantothenate). There is evidence to suggest that inhibitors of these enzymes may be useful against tuberculosis. Another reason to target these enzymes is that they bind molecules called NADPH and ATP, which are also used by other enzymes, so we might learn some useful general information about how to inhibit these other enzymes.