Insight into 5-lipoxygenase biocatalysis and interaction with membrane components

Abstract

Inflammation is one of the innate defence mechanisms exerted by the human body for protection and to initiate the healing process. Chronic inflammatory reactions can lead to several disease conditions like asthma, allergic rhinitis and rheumatoid arthritis. A central role in these diseases is played by the Leukotriene (LT) family of the lipid mediators, one of family of pro-inflammatory lipid mediators derived from the 20-carbon fatty acid arachidonic acid. The key enzyme involved in LT biosynthesis is 5-Lipoxygenase (5LO) which initiates the leukotriene biosynthesis by catalysing the conversion of arachidonic acid (AA) to leukotriene A4 (LTA4). In this thesis, one aim is to elucidate the interactions of 5LO with the membrane and components therein which is the first step in the leukotriene biosynthesis and which could be one possible drug target. Furthermore, details of the LTA4 formation were investigated as well as LTA4 hydrolysis.

In Paper I & II, we demonstrated the calcium-mediated binding of 5LO with membranes in vitro using a membrane substitute called nanodisc. The selected nanodisc platform contained 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and the membrane scaffolding protein MSP1E3D1 which provides a bilayer surface of inner diameter around 10 nm. Paper I shows 5LO binds directly with nanodiscs in the presence of calcium. This calcium-mediated binding was shown by non-denaturing gel-electrophoresis. Due to the high binding affinity, the 5LO-nanodisc complex was visible in the gel in the absence of calcium ions during the gel run. In the presence of Ca2+, a titration varying the ratio of 5LO to nanodiscs showed that maximum two 5LO binds to the nanodiscs used. The binding of 5LO with nanodiscs in the presence of calcium aides in the product formation and the results are comparable with the similar membrane mimic, liposomes. The complex of 5LO with nanodiscs was visualized for the first time by non-denaturing gel-electrophoresis and negative stain transmission electron microscopy (TEM). Furthermore, our results indicated that a 5LO dimer can be formed and it is formed by intermolecular disulphide bonds between cysteines located on the protein surface. The dimeric 5LO does not bind to a membrane and has no enzymatic activity. We have visualized also the dimer of 5LO by TEM. This paper clearly demonstrates the advantages of the application of nanodiscs as a membrane substitute to study monotopic membrane proteins. Paper II specifically focuses on a method developed to directly visualize and optimize conditions for the interaction of peripheral membrane proteins with the membrane using TEM. This method utilizes the phenomenon of stacking of phospholipids by a negative stain called sodium phosphotungstic acid (NaPT). Nanodiscs were used as a membrane substitute and form long stacks when stained with NaPT. The absence of stacking can be interpreted as a positive interaction between a protein and membrane, i.e. the protein bound to the nanodisc membrane prevents it from being stacked by the NaPT. With the help of this method, we have visualized different binding modes of 5LO with the membrane.

Paper III unravels the mechanism of product formation of 5LO by kinetic isotope effect. The results shed light on regiospecificity, H-transfer and donor-acceptor regulations in the oxygenation of AA. Paper IV deciphers the secret behind dual product formation and reduced suicidal inactivation of Xenopus laevis (African clawed toad) leukotriene A4 hydrolase, xlLTA4H, by solving the structure by crystallography to a resolution of 2.3Å along with biochemical assays. So far only one approved drug for clinical use is available that directly blocks the leukotriene biosynthesis, Zileuton™, whereas other drugs block receptors for leukotrienes. One reason for the lack of inhibitors may be the lack of detailed knowledge of the first steps in the leukotriene biosynthesis, something this thesis project initially set out to clarify.