Recent studies have revealed a new class of heme enzymes, the heme-based sensors, which are able to turn on or off cellular signal transduction pathways in response to environmental changes. One of these enzymes is the heme-regulated phosphodiesterase from Escherichia coli (EcDOS). This protein is composed of an N-terminal heme-containing PAS domain and a C-terminal functional domain. PAS is an acronym formed from the names of the Drosophila period clock protein (PER), 5 vertebrate aryl hydrocarbon receptor nuclear translocator (ARNT), 6 and Drosophila single-minded protein (SIM). The heme cofactor in its PAS domain can act as a sensor of the cellular redox state that regulates the adenosine 3′,5′-cyclic monophosphate (cAMP) phosphodiesterase activity. The crystal structures of its heme-containing PAS domain have helped clarify how the heme redox-dependent structural changes initiate intramolecular signal transduction. Here, we review recent findings on the structure-function relationships of EcDOS.
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