Although the Zn(2+) cation in Zn·Cys(4), Zn·Cys(3)His, Zn·Cys(2)His(2), and Zn(2)Cys(6) cores of zinc finger (Zf) proteins typically plays a structural role, the Zn-bound thiolates in some Zf cores are reactive. Such labile Zf cores can serve as drug targets for retroviral or cancer therapies. Previous studies showed that the reactivity of a Zn-bound thiolate toward electrophiles is significantly reduced if it forms S---NH hydrogen bonds with the backbone amide. However, we found several well-known inactive Zf cores containing Cys ligands with no H-bonding interactions. Here, we show that H bonds from the peptide backbone or bonds from a second Zn cation to Zn-bound S atoms suppress the reactivity not only of these S atoms, but also of Zn-bound S* atoms with no interactions. Indeed, two or more indirect NH---S hydrogen bonds raise the free energy barrier for methylation of a Zn-bound S* in a Cys(4) core more than a direct NH---S* hydrogen bond. These findings help to elucidate why several well-known Zf cores have Cys ligands with no H bonds, but are unreactive. They also help to provide guidelines for distinguishing labile Cys-rich Zn sites from structural ones, which in turn help to identify novel potential Zf drug targets.
In viral proteins, labile Zn-sites, where Zn(2+) is crucial for maintaining the native protein structure but the Zn-bound cysteines are reactive, are promising drug targets. Here, we aim to (i) identify labile Zn-sites in viral proteins using guidelines established from our previous work and (ii) assess if clinically safe Zn-ejecting agents could eject Zn(2+) from the predicted target site and thus inhibit viral replication. As proof-of-concept, we identified a labile Zn-site in the hepatitis C virus (HCV) NS5A protein and showed that the antialcoholism drug, disulfiram, could inhibit HCV replication to a similar extent as the clinically used antiviral agent, ribavirin. The discovery of a novel viral target and a new role for disulfiram in inhibiting HCV replication will enhance the therapeutic armamentarium against HCV. The strategy presented can also be applied to identify labile sites in other bacterial or viral proteins that can be targeted by disulfiram or other clinically safe Zn-ejectors.
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