The telltale structures of epoxide hydrolases

Traditionally, epoxide hydrolases (EH) have been regarded as xenobiotic-metabolizing enzymes implicated in the detoxification of foreign compounds. They are known to play a key role in the control of potentially genotoxic epoxides that arise during metabolism of many lipophilic compounds. Although this is apparently the main function for the mammalian microsomal epoxide hydrolase (mEH), evidence is now accumulating that the mammalian soluble epoxide hydrolase (sEH), despite its proven role in xenobiotic metabolism, also has a central role in the formation and breakdown of physiological signaling molecules. In addition, a certain class of microbial epoxide hydrolases has recently been identified that is an integral part of a catabolic pathway, allowing the use of specific terpens as sole carbon sources. The recently available x-ray structures of a number of EHs mirror their respective functions: the microbial terpen EH differs in its fold from the canonical alpha/beta hydrolase fold of the xenobiotic-metabolizing mammalian EHs. It appears that the latter fold is the perfect solution for the efficient detoxification of a large variety of structurally different epoxides by a single enzyme, whereas the smaller microbial EH, which has a particularly high turnover number with its prefered substrate, seems to be the better solution for the hydrolysis of one specific substrate. The structure of the sEH also includes an additional catalytic domain that has recently been shown to possess phosphatase activity. Although the physiological substrate for this second active site has not been identified so far, the majority of known phosphatases are involved in signaling processes, suggesting that the sEH phosphatase domain also has a role in the regulation of physiological functions.