Discovery and synthesis of novel chloride transport protein inhibitors - Howery, Andrew Everett; Du Bois, Justin; Khosla, Chaitan, 1964-; Wender, Paul A; Stanford University, Department of Chemistry (2011)
CLC chloride channels and transporters play diverse physiological roles in processes ranging from regulating bone-density, muscle excitability, and blood pressure, to facilitating extreme-acid survival of pathogenic bacteria. Defects in CLC proteins cause human disorders in these processes. Small-molecule inhibitors of the CLCs would be useful as drugs for treating a variety of CLC-related human diseases and also to investigate CLC physiology. In addition, inhibitors are powerful tools for studying molecular mechanisms of Cl-- gating. Trapping channels or transporters in particular conformational states with high-affinity ligands could potentially advance our understanding of the structural basis for CLC activity. Despite their usefulness, specific small-molecule inhibitors for CLC proteins are scarce. To address this shortfall, we have exploited the 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) scaffold to develop two novel classes of CLC inhibitors. DIDS has been used as an anion-transport inhibitor for decades and was first used to inhibit CLCs over 30 years ago. However, experiments to determine the compound's mode of inhibition led us to discover that minor contaminants in the DIDS solutions inhibit CLC proteins more effectively than DIDS itself. The contaminants were found to derive from hydrolysis of the labile isothiocyanate moieties. The structures of five major hydrolysis products were determined by 1H NMR, HRMS analysis, and chemical synthesis to be DIDS-based polythioureas. These compounds bind directly to the CLC proteins, as evidenced by the fact that they inhibit purified, reconstituted ClC-ec1 and that inhibition of ClC-Ka can be prevented by the point mutation N68D. These polythioureas are the highest affinity inhibitors known for the CLCs and provide a new class of chemical probes for dissecting the molecular mechanisms of chloride transport. The second class of identified CLC inhibitors combines the DIDS core structure with alkyl chain carboxylic acids. The most potent inhibitor identified, 4,4'-octanamidostilbene-2,2'-disulfonic acid (OADS), inhibits ClC-ec1 with an apparent affinity of 29 [Mu]M. As a means to identify the inhibitor-binding site, we synthesized photo-reactive diazirine derivatives of OADS and showed that these photo-affinity reagents specifically inhibit ClC-ec1. Experiments to identify the binding site using 'top-down' mass spectrometry, in which the protein is cleaved into peptide fragments via electron-capture dissociation, have identified an intracellular binding region encompassing 76 amino acids, or 16% of the protein. Current efforts using protease digestion procedures are focused on further refinement of the binding region. Once located, protein/inhibitor interactions gleaned from the labeling of ClC-ec1 could allow us to rationally design more potent inhibitors of CLC transporters and channels.
Submitted to the Department of Chemistry.
Ph.D. Stanford University 2011
Rights: (c) Copyright 2011 by Andrew Everett Howery
Rights: This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
Keywords: theses