Globin-coupled sensors and the protoglobins.

摘要

The strategy for detecting oxygen, carbon monoxide, nitric oxide, and sulfides is predominantly through heme-based sensors utilizing either a globin domain or a PAS domain. Whereas PAS domains bind various cofactors, globins bind only heme. Globin-coupled sensors (GCSs) were first described as regulators of the aerotactic responses in Bacillus subtilis and Halobacterium salinarum. GCSs were also identified in diverse microorganisms that appear to have roles in regulating gene expression. Functional and evolutionary analyses of the GCSs have led to the identification of their ancestor protoglobin---the origin of the GCS N-terminal sensor globin---in both Bacteria and Archaea. Protoglobins from the hyperthermophilic aerobe Aeropyrum pernix (ApPgb) and the methanogenic obligate anaerobe Methanosarcina acetivorans (MaPgb) were cloned, expressed, purified, and analyzed biochemically. Both proteins contain b-type heme and can bind the ferrous ligands molecular oxygen (O2) and carbon monoxide (CO), as well as the ferric ligands cyanide (CN-), azide (N3-), and imidazole; both forms could bind nitric oxide (NO). An oxygen-transport or storage function is precluded in ApPgb and MaPgb due to extremely fast autoxidation rates of 4.3 and 3.6 minutes, respectively. Using structural and biochemical features, the physiological roles of the protoglobins in O2 and CO respiration and protection against oxidative stress are discussed in the context of their host biochemistries.