Location, Location, Location: Compartmentalization of Early Events in Leukotriene Biosynthesis

Abstract

Over the last 25؉ years, substantial progress has been made in understanding how LTs exert their effects, and a broader appreciation for the numerous biological processes they mediate has emerged. LT biosynthesis is initiated by the action of 5-lipoxygenase (5-LOX), which catalyzes the transformation of AA to LTA 4 in a two-step reaction. Ca 2؉ targets 5-LOX to the nuclear membrane, where it co-localizes with the 5-LOX-activating protein FLAP and, when present, the downstream enzyme LTC 4 synthase, both transmembrane proteins. Crystal structures of the AA-metabolizing LOXs, LTC 4 synthase, and FLAP combined with biochemical data provide a framework for understanding how subcellular organizations optimize the biosynthesis of these labile hydrophobic signaling compounds, which must navigate pathways that include both membrane and soluble enzymes. The insights these structures afford and the questions they engender are discussed in this minireview.The polyunsaturated fatty acid arachidonic acid (AA) 2 serves as a precursor to prostaglandins, leukotrienes (LTs), and other eicosanoids derived from the 20-carbon substrate. The proteins that constitute the LT biosynthetic pathway are located in several cellular compartments and include extra-and intracellular as well as membrane and soluble proteins. Products of the intracellular pathway are exported by specific pumps for further elaboration in extra-or transcellular biosynthesis or for access to the extracellular receptors they target. The complex compartmentalized biosynthetic pathway for LTs offers numerous opportunities for control of pathway flux through dynamic control of the locations of key proteins. We focus in this minireview on the early events in the LT biosynthetic pathway, specifically a major branch point in LT biosynthesis that determines whether cysteinyl-LTs that result from the pathway that requires conjugation with glutathione are produced or whether the non-cysteinyl-LT LTB 4 is synthesized. The Reaction PathwayThe biosynthesis of LTs is initiated by the action of 5-lipoxygenase (5-LOX), which promotes the transformation of AA to LTA 4 . The action of 5-LOX is triggered by calcium-dependent membrane binding, which targets it to the nuclear membrane. There it acquires its substrate, liberated from membrane phospholipids by the action of Ca 2ϩ -stimulated cytosolic phospholipase A 2 , from the transmembrane protein FLAP (five-lipoxygenase-activating protein). Like all LOXs, 5-LOX catalyzes the regio-and stereospecific peroxidation of a polyunsaturated fatty acid (1, 2). The enzyme transforms the substrate AA to the 5S-isomer of hydroperoxyeicosatetraenoic acid (5S-HPETE). However, in contrast to other members of this superfamily, 5-LOX can further metabolize the hydroperoxy product to an allylic epoxide, specifically LTA 4 . Studies have shown that both in vitro (3) and in vivo (4), the efficiency of LTA 4 production is improved with membrane binding, i.e. 5-LOX successfully carries the two-step reaction to completion, and the ratio of LTA 4 to inter...

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