title: The structural basis of differential inhibition of human calpain by indole and phenyl α-mercaptoacrylic acids.
abstract: Excessive activity of neutrophils has been linked to many pathological conditions, including rheumatoid arthritis, cancer and Alzheimer's disease. Calpain-I is a Ca(2+)-dependent protease that plays a key role in the extravasation of neutrophils from the blood stream prior to causing damage within affected tissues. Inhibition of calpain-I with small molecule mercaptoacrylic acid derivatives slows the cell spreading process of live neutrophils and so these compounds represent promising drug leads. Here we present the 2.05 and 2.03 Å co-crystal X-ray structures of the pentaEF hand region, PEF(S), from human calpain with (Z)-3-(4-chlorophenyl)-2-mercaptoacrylic acid and (Z)-3-(5-bromoindol-3-yl)-2-mercaptoacrylic acid. In both structures, the α-mercaptoacrylic acid derivatives bind between two α-helices in a hydrophobic pocket that is also exploited by a leucine residue of the endogenous regulatory calpain inhibitor calpastatin. Hydrophobic interactions between the aromatic rings of both inhibitors and the aliphatic residues of the pocket are integral for tight binding. In the case of (Z)-3-(5-bromoindol-3-yl)-2-mercaptoacrylic acid, hydrogen bonds form between the mercaptoacrylic acid substituent lying outside the pocket and the protein and the carboxylate group is coplanar with the aromatic ring system. Multiple conformations of (Z)-3-(5-bromoindol-3-yl)-2-mercaptoacrylic acid were found within the pocket. The increased potency of (Z)-3-(5-bromoindol-3-yl)-2-mercaptoacrylic acid relative to (Z)-3-(4-chlorophenyl)-2-mercaptoacrylic acid may be a consequence of the indole group binding more deeply in the hydrophobic pocket of PEF(S) than the phenyl ring.
authors.affiliation: School of Chemistry, Main Building, Cardiff University, Park Place, Cardiff CF10 3AT, UK.
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Adams
authors.affiliation: School of Medicine, Institute of Molecular and Experimental Medicine, Heath Park, Cardiff University, Cardiff CF14 4XN, UK.
authors.forename: Pierre J
authors.initials: PJ
authors.lastname: Rizkallah
authors.affiliation: School of Chemistry, Main Building, Cardiff University, Park Place, Cardiff CF10 3AT, UK.
authors.forename: David J
authors.initials: DJ
authors.lastname: Miller
authors.affiliation: School of Medicine, Institute of Molecular and Experimental Medicine, Heath Park, Cardiff University, Cardiff CF14 4XN, UK.
authors.forename: Emma J
authors.initials: EJ
authors.lastname: Robinson
authors.affiliation: School of Medicine, Institute of Molecular and Experimental Medicine, Heath Park, Cardiff University, Cardiff CF14 4XN, UK.
authors.forename: Maurice B
authors.initials: MB
authors.lastname: Hallett
authors.affiliation: School of Chemistry, Main Building, Cardiff University, Park Place, Cardiff CF10 3AT, UK. Electronic address: allemannrk@cardiff.ac.uk.
authors.forename: Rudolf K
authors.initials: RK
authors.lastname: Allemann
mesh_terms: D000179:Acrylates / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology; D000595:Amino Acid Sequence; D001483:Base Sequence; D002118:Calcium / Q000737:chemistry / Q000378:metabolism; D002154:Calpain / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D006801:Humans; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D007211:Indoles / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D011485:Protein Binding; D055503:Protein Multimerization; D017434:Protein Structure, Tertiary; D013438:Sulfhydryl Compounds / Q000737:chemistry* / Q000494:pharmacology
chemical_list: D000179:Acrylates; D007211:Indoles; D013438:Sulfhydryl Compounds; C030374:indole; D002154:Calpain; C036658:acrylic acid; D002118:Calcium
keywords: Calpain-I; Mercaptoacrylic acids; PEF(S); Rheumatoid arthritis