A precise, concrete catalytic function has not yet been assigned to HemAT, although it is certain that HemAT is involved in methylchemostasis, thus sensing O 2 concentration and allowing cells to move toward the environment with a higher O 2 concentration. Ec DOS is a phosphodiesterase (PDE), whereas FixL is a histidine kinase (HK). The protein fold of the heme-binding domain of Ec DOS and FixL is the PAS (Per-Arnt-Sim) fold, whereas that of HemAT is the globin fold. Prokaryotes possess well-characterized heme-based oxygen-sensor enzymes such as Ec DOS, FixL, and HemAT. The properties of O 2 are much different from those of other gaseous molecules such as NO and CO, which are also important in various physiological and pathological processes.

The O 2 molecule is among the most abundant gas molecules in the environment and is important for numerous physiological functions thus, O 2-sensor proteins are physiologically important in living organisms. A number of representative gas-sensing proteins, including heme-based oxygen sensors such as the Escherichia coli Direct Oxygen Sensor ( Ec DOS, also known as Ec DosP), NO sensors such as soluble guanylate cyclase (sGC), and CO sensors such as CooA, have been reported. This second signal is then transduced to the C-terminal functional domain, switching on (or off) the associated activity, which may consist of phosphodiesterase (PDE), diguanylate cyclase (DGC), histidine kinase (HK), or transcription, among others. In this process, binding of the gas molecule is the first signal and the protein structural alteration near the heme-bound site becomes the second signal.

Association of the gas molecule to the heme iron complex or dissociation from it alters the protein structure in the heme-bound sensor domain.

The heme iron complex bound to the sensor domain acts as the gas sensor. In general, the heme-based gas-sensing protein is composed of two domains: an N-terminal sensor domain and a C-terminal functional domain ( Figure 1). In addition to these well-known prototypical heme proteins, is an emerging class of heme-based gas-sensing proteins. Heme proteins play important roles in O 2 storage (myoglobin ), O 2 transfer (hemoglobin ), O 2 activation (cytochrome P450, nitric oxide synthase), electron transfer (cytochromes), and many more functions.

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