Last year we highlighted a study in which virtual screening identified a number of functionally active fragments and crystallography confirmed their binding modes. In a recent issue of Bioorg. Med. Chem. Lett. researchers from Sanofi-aventis report a more complicated case: fragments that bind not only in a manner different than predicted, but in a completely different site.
The team used the computational docking method Glide to select 200 compounds likely to bind in the active site of the cytokine MIF (migration inhibitory factor). Of these, 23 were tested in crystallographic soaking studies, resulting in 5 co-crystal structures. Three of these bound in the active site, but the other two bound in a hydrophobic “cryptic” site on the protein surface formed by the rotation of a tyrosine residue. Protein rearrangements are not uncommon; a similar example was reported last year in which fragments were found to bind differently than predicted due to unforeseen protein movements. The cryptic site does appear to be real: the authors crystallized a compound reported in the patent literature and found that it binds across both the active and cryptic sites.
This is the third in a recent series of papers featured on this site in which fragment approaches found new binding sites on proteins. However, like the HIV-protease example, there is no functional data presented; I’ll take this to mean that the compounds are probably weak, if they show any detectable activity. The question of what to do with a fragment remains challenging, though (to be somewhat self-promoting) we are working on practical solutions.
What to do with a fragment is also a theme of the upcoming FBLD 2010, so if you have a success story you can share, consider submitting an abstract.