3leo
From Proteopedia
Structure of human Leukotriene C4 synthase mutant R31Q in complex with glutathione
Structural highlights
Disease[LTC4S_HUMAN] Defects in LTC4S are the cause of leukotriene C4 synthase deficiency (LTC4 synthase deficiency) [MIM:246530]. LTC4 synthase deficiency is a fatal neurometabolic developmental disorder. It is associated with muscular hypotonia, psychomotor retardation, failure to thrive, and microcephaly. Function[LTC4S_HUMAN] Catalyzes the conjugation of leukotriene A4 with reduced glutathione to form leukotriene C4. Publication Abstract from PubMedHuman leukotriene C(4) synthase (hLTC(4)S) is an integral membrane enzyme that conjugates leukotriene (LT) A(4) with glutathione to form LTC(4), a precursor to the cysteinyl leukotrienes (LTC(4), LTD(4), and LTE(4)) that are involved in the pathogenesis of human bronchial asthma. From the crystal structure of hLTC(4)S, Arg-104 and Arg-31 have been implicated in the conjugation reaction. Here, we used site-directed mutagenesis, UV spectroscopy, and x-ray crystallography to examine the catalytic role of Arg-104 and Arg-31. Exchange of Arg-104 with Ala, Ser, Thr, or Lys abolished 94.3-99.9% of the specific activity against LTA(4). Steady-state kinetics of R104A and R104S revealed that the K(m) for GSH was not significantly affected. UV difference spectra of the binary enzyme-GSH complex indicated that GSH ionization depends on the presence of Arg-104 because no thiolate signal, with lambda(max) at 239 nm, could be detected using R104A or R104S hLTC(4)S. Apparently, the interaction of Arg-104 with the thiol group of GSH reduces its pK(a) to allow formation of a thiolate anion and subsequent nucleophilic attack at C6 of LTA(4). On the other hand, exchange of Arg-31 with Ala or Glu reduced the catalytic activity of hLTC(4)S by 88 and 70%, respectively, without significantly affecting the k(cat)/K(m) values for GSH, and a crystal structure of R31Q hLTC(4)S (2.1 A) revealed a Gln-31 side chain pointing away from the active site. We conclude that Arg-104 plays a critical role in the catalytic mechanism of hLTC(4)S, whereas a functional role of Arg-31 seems more elusive. Because Arg-104 is a conserved residue, our results pertain to other homologous membrane proteins and represent a structure-function paradigm probably common to all microsomal GSH transferases. Arginine 104 is a key catalytic residue in leukotriene C4 synthase.,Rinaldo-Matthis A, Wetterholm A, Martinez Molina D, Holm J, Niegowski D, Ohlson E, Nordlund P, Morgenstern R, Haeggstrom JZ J Biol Chem. 2010 Dec 24;285(52):40771-6. Epub 2010 Oct 27. PMID:20980252[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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