I have been thinking a lot recently about studies in which people have determined the bound conformation of a ligand by transfer-NOESY experiments, essentially by transferring magnetization off another ligand to the protein and then back to the ligand of interest. With the known bound conformation of the first ligand, one can apparently locate the conformation of the second one. Many such unknown protein-bound conformations have been worked out. In my field of research, the ones which are relevant are of agents that bind to tubulin, especially discodermolide. In this case, the conformation of discodermolide was deduced via competition transfer-NOESY experiments with epothilone. These experiments are non-trivial to carry out and, as is the case for other biomolecular NMR studies, should be interpreted carefully. But in the end they look like nifty techniques that can shed light on unknown bioactive conformations, something that's very valuable for drug design.
Essentially it's again a problem of fitting the bound conformation NMR data to a single conformation. In solution we know for sure that this is a fallacious step. The (not so) obvious assumption in doing this for bound conformations is that there's got to be only one conformation in the active site too. But I have always wondered if a ligand in a protein active site could also have multiple conformations. MJ's comments on a past post and the discussion there makes me think that even in a protein active site, there could possibly be multiple conformations of a ligand, something that runs counter to what we conventionally think. How diverse those conformations might be is a different question; one would probably not expect large conformational changes. But even 'small' conformational changes could be significant enough to distinguish between different conformations in the active site. It's a problem worth thinking about.
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