title: Structural basis for the insensitivity of a serine enzyme (palmitoyl-protein thioesterase) to phenylmethylsulfonyl fluoride.
abstract: Palmitoyl-protein thioesterase-1 (PPT1) is a newly described lysosomal enzyme that hydrolyzes long chain fatty acids from lipid-modified cysteine residues in proteins. Deficiency in this enzyme results in a severe neurodegenerative storage disorder, infantile neuronal ceroid lipofuscinosis. Although the primary structure of PPT1 contains a serine lipase consensus sequence, the enzyme is insensitive to commonly used serine-modifying reagents phenylmethylsulfonyl fluoride (PMSF) and diisopropylfluorophosphate. In the current paper, we show that the active site serine in PPT1 is modified by a substrate analog of PMSF, hexadecylsulfonylfluoride (HDSF) in a specific and site-directed manner. The apparent K(i) of the inhibition was 125 micrometer (in the presence of 1.5 mm Triton X-100), and the catalytic rate constant for sulfonylation (k(2)) was 3.3/min, a value similar to previously described sulfonylation reactions. PPT1 was crystallized after inactivation with HDSF, and the structure of the inactive form was determined to 2.4 A resolution. The hexadecylsulfonyl was found to modify serine 115 and to snake through a narrow hydrophobic channel that would not accommodate an aromatic sulfonyl fluoride. Therefore, the geometry of the active site accounts for the reactivity of PPT1 with HDSF but not PMSF. These observations suggest a structural explanation as to why certain serine lipases are resistant to modification by commonly used serine-modifying reagents.
authors.affiliation: Department of Internal Medicine and the Hamon Center for Therapeutic Oncology Research, University of Texas Southwestern Medical Center, Dallas, Texas 75390-8593, USA.
authors.forename: A K
authors.initials: AK
authors.lastname: Das
authors.forename: J J
authors.initials: JJ
authors.lastname: Bellizzi
authors.forename: S
authors.initials: S
authors.lastname: Tandel
authors.forename: E
authors.initials: E
authors.lastname: Biehl
authors.forename: J
authors.initials: J
authors.lastname: Clardy
authors.forename: S L
authors.initials: SL
authors.lastname: Hofmann
mesh_terms: D000215:Acylation; D000477:Alkylating Agents / Q000494:pharmacology; D000818:Animals; D020134:Catalytic Domain; D002417:Cattle; D008247:Lysosomes / Q000201:enzymology; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009472:Neuronal Ceroid-Lipofuscinoses / Q000209:etiology; D010170:Palmitoyl-CoA Hydrolase / Q000187:drug effects; D010664:Phenylmethylsulfonyl Fluoride / Q000494:pharmacology*; D011994:Recombinant Proteins / Q000187:drug effects; D013450:Sulfones / Q000494:pharmacology; D013869:Thiolester Hydrolases / Q000187:drug effects* / Q000235:genetics
chemical_list: D000477:Alkylating Agents; D011994:Recombinant Proteins; D013450:Sulfones; D010664:Phenylmethylsulfonyl Fluoride; C069088:hexadecanesulfonyl fluoride; D013869:Thiolester Hydrolases; D010170:Palmitoyl-CoA Hydrolase; C095299:palmitoyl-protein thioesterase