Crystal structure of a human membrane protein involved in cysteinyl leukotriene biosynthesis

摘要

The cysteinyl leukotrienes, namely leukotriene (LT)C_4 and its metabolites LTD_4 and LTE_4, the components of slow-reacting substance of anaphylaxis, are lipid mediators of smooth muscle constriction and inflammation, particularly implicated in bronchial asthma. LTC_4 synthase (LTC_4S), the pivotal enzyme for the biosynthesis of LTC_4 (ref. 10), is an 18-kDa integral nuclear membrane protein that belongs to a superfamily of membrane-associated proteins in eicosanoid and glutathione metabolism that includes 5-lipoxygenase-activating protein, microsomal glutathione S-transferases (MGSTs), and microsomal prostaglandin E synthase 1 (ref. 13). LTC_4S conjugates glutathione to LTA_4, the endogenous substrate derived from arachidonic acid through the 5-lipoxygenase pathway. In contrast with MGST2 and MGST3 (refs 15, 16), LTC_4S does not conjugate glutathione to xeno-biotics. Here we show the atomic structure of human LTC_4S in a complex with glutathione at 3.3 A resolution by X-ray crystallography and provide insights into the high substrate specificity for glutathione and LTA_4 that distinguishes LTC_4S from other MGSTs. The LTC_4S monomer has four transmembrane α-helices and forms a threefold symmetric trimer as a unit with functional domains across each interface. Glutathione resides in a U-shaped conformation within an interface between adjacent monomers, and this binding is stabilized by a loop structure at the top of the interface. LTA_4 would fit into the interface so that Arg 104 of one monomer activates glutathione to provide the thiolate anion that attacks C6 of LTA_4 to form a thioether bond, and Arg 31 in the neighbouring monomer donates a proton to form a hydroxyl group at C5, resulting in 5(S)-hydroxy-6(R)-S-glutathionyl-7,9-trans-11,14- cis-eicosatetraenoic acid (LTC_4). These findings provide a structural basis for the development of LTC_4S inhibitors for a proinflammatory pathway mediated by three cysteinyl leukotriene ligands whose stability and potency are different and by multiple cysteinyl leukotriene receptors whose functions may be non-redundant.