Intradomain Allosteric Regulation of Soluble Epoxide Hydrolase by Its Substrates.

Soluble epoxide hydrolase (sEH) is a bifunctional enzyme with epoxide hydrolase activity in the C-terminal domain (C-EH) and lipid phosphate phosphatase activity in the N-terminal domain (N-phos). The C-EH hydrolyzes bioactive epoxy fatty acids such as epoxyeicosatrienoic acid (EET). The N-phos hydrolyzes lipid phosphomonesters, including the signaling molecules of lysophosphatidic acid (LPA). Here, we report that the C-EH and N-phos are reciprocally regulated by their respective substrates. Full-length sEH (sEH-FL) showed positive cooperativity toward the substrate for each domain. Similar cooperativity was found when truncated enzymes having only C- and N-terminal domains, sEH-C and sEH-N, respectively, were used, suggesting an intra-domain nature of the cooperativity. In addition, the N-phos substrate LPA inhibited C-EH activity in sEH-FL and sEH-C equally. Similarly, the C-EH substrate EET inhibited N-phos activity. Structural and kinetic data suggest the presence of allosteric sites in each domain of the sEH enzyme, which share the binding of LPA and EET. Thus, each of the two sEH activities is regulated by a substrate of its own and by that of the other domain. This mechanism may explain why sEH has evolved to have two different enzyme activities, which possibly allows sEH to balance the metabolism of bioactive lipids.