Abstract
Urea-induced unfolding of lipoxygenase-1 (LOX1) at pH 7.0 was followed by enzyme activity, spectroscopic measurements, and limited proteolysis experiments. Complete unfolding of LOX1 in 9 M urea in the presence of thiol reducing or thiol modifying reagents was observed. The aggregation and oxidative reactions prevented the reversible unfolding of the molecule. The loss of enzyme activity was much earlier than the structural loss of the molecule during the course of unfolding, with the midpoint concentrations being 4.5 and 7.0 M for activity and spectroscopic measurements, respectively. The equilibrium unfolding transition could be adequately fitted to a three-state, two-step model (N left arrow over right arrow I left arrow over right arrow U) and the intermediate fraction was maximally populated at 6.3 M urea. The free energy change (DeltaG(H(2)O)) for the unfolding of native (N) to intermediate (I) was 14.2 +/- 0.28 kcal/mol and for the intermediate to the unfolded state (U) was 11.9 +/- 0.12 kcal/mol. The ANS binding measurements as a function of urea concentration indicated that the maximum binding of ANS was in 6.3 M urea due to the exposure of hydrophobic groups; this intermediate showed significant amount of tertiary structure and retained nearly 60% of secondary structure. The limited proteolysis measurements showed that the initiation of unfolding was from the C-terminal domain. Thus, the stable intermediate observed could be the C-terminal domain unfolded with exposed hydrophobic domain-domain interface. Limited proteolysis experiments during refolding process suggested that the intermediate refolded prior to completely unfolded LOX1. These results confirmed the role of cysteine residues and domain-domain interactions in the reversible unfolding of LOX1. This is the first report of the reversible unfolding of a very large monomeric, multi-domain protein, which also has a prosthetic group.
Highlights
Lipoxygenases are a family of non-heme iron dioxygenases that catalyze the rate-determining step in the production of leukotrienes and lipoxins from membrane-derived arachidonic acid [1]
The results of the present study demonstrate that the modification of cysteine residue is essential for rapid refolding of LOX1, and none of the four free cysteine residues in LOX1 is essential for activity
From the limited proteolysis experiments and equilibrium unfolding measurements, we suggest a folding model for LOX1 molecule
Summary
Lipoxygenases are a family of non-heme iron dioxygenases that catalyze the rate-determining step in the production of leukotrienes and lipoxins from membrane-derived arachidonic acid [1]. These metabolites are the important group of signaling molecules that mediate a number of biological processes [2]. Atherosclerosis, inflammatory bowel disease, psoriasis, asthma, and other immune system disorders are all influenced by the activities and products of the lipoxygenase enzymes [3, 4]. It has been reported previously that the generation of truncated LOX1, wherein residues 2–140 is replaced by five residues, yielded an inactive enzyme [13], which suggested the importance of N-terminal domain for catalysis. From the limited proteolysis experiments and equilibrium unfolding measurements, we suggest a folding model for LOX1 molecule
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