Member of the two-component regulatory system GcHK/Anae109_2439. Autophosphorylates in response to oxygen availability, and then transfers the phosphate group to a conserved Asp residue in the receiver domains of the cognate response regulator Anae109_2439, resulting in its activation. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Binds 1 heme b (iron(II)-protoporphyrin IX) group per subunit. Gas or ligand binding and heme iron redox state regulate the histidine kinase activity: heme-free AfGcHK and the heme Fe(II) complex are less active forms, whereas the heme Fe(III)-OH(-), Fe(III)-CN(-), Fe(III)-imidazole, Fe(II)-CO, and Fe(II)-O(2) complexes are active forms. The activation of the functional domain seems to be dependent on the formation of a six-coordinate low-spin heme iron complex. Mg(2+) ions, and to a lesser extent, Mn(2+) ions, enhance the autophosphorylation reaction, although the presence of a divalent metal ion does not appear to be essential for the reaction. Co(2+), Ni(2+), Zn(2+), and Cd(2+) totally inhibit autophosphorylation activity. kcat is 1.1 min(-1) for the autophosphorylation reaction with the Fe(III)-OH(-) bound form of the enzyme. kcat is 1.2 min(-1) with the Fe(III)-CN(-) bound form of the enzyme. kcat is 1.1 min(-1) with the Fe(III)-imidazole bound form of the enzyme. kcat is 1.0 min(-1) with the Fe(II)-O(2) bound form of the enzyme. kcat is 1.0 min(-1) with the Fe(II)-CO bound form of the enzyme. kcat is 0.4 min(-1) with the Fe(II) bound form of the enzyme. kcat is 0.3 min(-1) with the heme-free form of the enzyme. The presence of the cognate response regulator (RR) protein significantly reduces the enzyme affinity for ATP. Optimum pH is around 11.0 for the autophosphorylation reaction with the Fe(III)-OH(-) bound form of the enzyme. At pH 8.0, displays 75% of the optimal rate. Is inactive below pH 5.5 and above pH 12.7. Homodimer. Consists of two domains: an N-terminal heme-based oxygen-sensor domain, and a C-terminal catalytic domain that exhibits histidine kinase activity (PubMed:21852234). The coordination and oxidation state of the sensor domain heme iron profoundly affect the enzyme's catalytic activity (PubMed:26212354). Autophosphorylated.