So, Ras is big. We keep on talking about it. And sometimes we talk about the same work repeatedly. This recent paper from AZ is a publication of work we have talked about here and here. This follows on closely to work done by Vanderbilt and Genentech. Those two papers were done using NMR and this one took a X-ray approach. The AZ folks were taking a different approach to this PPI: stabilization of the interface. They took 1160 fragments in pools of 4 and screened against HRas (homolog)-catalytic domain of SOS stable complex. There were able to identify 3 bindings sites on HRas-SOS (Figure 1):
|Figure 1. HRas-SOS Complex. HRas (Green), SOS (Blue), A: SOS binding site (gold) (same as Vanderbilt), B SOS-Hras Interface binding site (Red) (same as Genentech), and C HRas covalent binding site (black).|
Site A was the same site identified by the Vanderbilt group Site B was the same as identified as Genentech. However, the AZ compounds bound to both proteins at the interface. Their initial hope was to use this site to stabilize the Ras-SOS interface. Both of the fragments binding to these sites had their affinity determined by TROSY-HSQC NMR. However, they were not potent enough to elicit a biological effect, which was not unexpected. After several rounds of chemistry, they were not able to improve these fragments significantly, or even show that they actually stabilized the interface.
Looking at the growing covalent literature, they hypothesized that an irreversible inhibitor may be the only way to inhibit GTPase activity, especially considering the pM affinity of GTP for Ras. They identified Cys118R (conserved between HRas and KRas) as a potentially reactive sidechain proximal to the GDP binding site on Ras. To go after this site covalently, AZ assembled a 400 fragment covalent library (Figure 2) and screened it by mass spectrometry.
They chose the N-substituted maleimide was deemed "ideal"; other warheads were either insufficiently reactive or overly reactive. Covalent modification of Cys118R by a fragment partially occludes the nucleotide binding site and potentially prevents the reorganization of the Cys118R loop, thus locking it into the catalytically inactive Ras-SOS complex. Interestingly, their covalent compounds only inhibited catalytically activity when pre-incubated with Ras-GDP-SOS. This supports the hypothesis that Cys118R becomes more accessible during SOS-mediated nucleotide exchange.
|Figure 2. Chemotypes represented in AZ 400 fragment covalent library.|