Halogen Hydrogen Bonding...Designable or Not?

The use of brominated fragments for X-ray screening is well known; it was the basis for former company SGX (now part of Lilly).  The purported advantage of brominated fragment is that you can identify the fragment unambiguously using anomolous dispersion.  In this paper, they are focused on using fragments to identify surface binding sites on HIV protease.  Prior work has focused on creating a new crystal form (complexed with TL-3, a known active site inhibitor) that has four solvent accesible sites: the exosite, the flap, and the two previous identified sites.  They took 68 brominated fragments and soaked these crystals: 23 fragments were found.  However, most of these actives were uninteresting.  Two compounds were found to be interesting, one bound in the exosite and one in the flap site. 

So, what's interesting in this paper?  Well, they (re)discover that brominated fragments can bind all over with a variety of affinities.  However, the bromine allows you to unambiguously identify those fragments through anomolous dispersion.  This is NOT interesting.  They discover that although it is a subject of much debate lately: specific interactions of the ligand with the target dominate the "bromine interaction".  This IS interesting.  They do not discuss this in much detail, but their grand extrapolations of this method to general applicability I don't buy. 

 I think the key take away from this paper is whether the halogen hydrogen bond undesignable and just a subject of serendipity? 

HIV protease vs fragments

HIV protease (HIV PR) is a well-known and successfully exploited Achilles heel (OK, maybe more of an Alexandrian sword) of the virus HIV. Although there is no shortage of successful drugs on the market that target this enzyme, resistance is an issue, and new approaches are always welcome. To this end, researchers led by C. David Stout at Scripps Research Institute have performed a fragment screen against HIV protease, the results of which are reported in the March issue of Chemical Biology and Drug Design.

This is really a crystallography paper, and gives a thorough, nuts-and-bolts description of doing a crystallographic fragment screen. The authors screened a library of 384 commercially available fragments with an average molecular weight of only 142 Da. Some of this work was done at Active Sight, and although I believe Active Sight has closed, some of the folks have moved to Zenobia, so I suspect their fragment library incorporates some of the same features.

The researchers used five different crystal forms of HIV PR and examined a total of 808 crystals, 507 of which were co-crystallization experiments and 301 of which had the fragments soaked into crystals that had previously been grown. In all, 378 data sets were collected. Most of these were done in the presence of an active site inhibitor, thus specifically targeting the search for fragments that bind outside of the active site. Three fragments were identified binding to two different sites, and these data have been deposited in the protein data bank. The authors argue that these fragments could be binding to allosteric sites that might keep the protease in its “closed,” inhibited conformation.

Like the recent p53 example, there is still a long way to go: it is not even clear that these fragments have functional activity. Still, the discovery of these small-molecule binding sites illustrates that fragment methods can reveal something new even about an enzyme as well-characterized as HIV protease.