Location, location, location: compartmentalization of early events in leukotriene biosynthesis

Abstract

Leukotrienes (LTs), derived from arachidonic acid (AA) released from the membrane by the action of phospholipase A(2), are potent lipid mediators of the inflammatory response. In 1983, Dahlén et al. demonstrated that LTC(4), LTD(4), and LTE(4) mediate antigen-induced constriction of bronchi in tissue obtained from subjects with asthma (Dahlén, S. E., Hansson, G., Hedqvist, P., Björck, T., Granström, E., and Dahlén, B. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 1712-1716). 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.

FIGURE 1.

Initial events in LT biosynthesis. Ca²⁺ promotes membrane binding of both cytosolic phospholipase A₂ (cPLA2; gray) and 5-LOX (salmon). The AA (see box) generated by phospholipase A₂ is the substrate for 5-LOX. The 5-LOX product (LTA₄) is converted to either LTC₄ or LTB₄ depending on the location of 5-LOX. *, OH for the corresponding hydroxyeicosatetraenoic acid (5S-HETE), produced by reduction of 5S-HPETE. LTA₄H, LTA₄ hydrolase; LTC₄S, LTC₄ synthase.

FIGURE 2.

8R-LOX. The schematic shows 8R-LOX from P. homomalla (Protein Data Bank code 2FNQ). The membrane-targeting PLAT domain is in purple, and the catalytic domain is in white. Blue spheres mark the Ca²⁺ ions positioned by amino acids conserved in 5-LOX, which are included in stick rendering. White, carbon; red, oxygen; blue, nitrogen. Trp residues in the PLAT domain found in the putative membrane insertion loops common to 8R- and 5-LOXs are show as gold spheres. The catalytic iron is shown as an orange sphere, and the position of the terminal amino acid is in red. The arched helix, which shelters the active site, is in green.

FIGURE 3.

FLAP and LTC₄ synthase. a, schematic of a FLAP trimer (Protein Data Bank code 2Q7M). Monomers are in purple, blue, and pink. The inhibitor MK-591 is depicted in stick rendering. Yellow, carbon; red, oxygen; blue, nitrogen. b, superposition of a LTC₄ synthase monomer (beige; code 2UUH) onto FLAP. GSH is in sphere rendering. Magenta, carbon; orange, sulfur. The stick rendering corresponds to a detergent molecule and is presumed to indicate the binding site for LTA₄.

FIGURE 4.

LTA₄ hydrolase. a, thermolysin-like (M1 peptidase) core of LTA₄ hydrolase. Zn²⁺ is a gray sphere. b, LTA₄ hydrolase (Protein Data Bank code 1HS6). Purple, N-terminal domain; green, C-terminal domain. The arrow marks the approximate position of the active-site access channel.