Iteration of the HHE family ([2]) found it to be related to Hemerythrin. It also demonstrated that what has been described as a single domain ([1]) in fact consists of two cation binding domains. Members of this family occur all across nature and are involved in a variety of processes. For instance, in Nereis diversicolor Swiss:P80255 binds Cadmium so as to protect the organism from toxicity ([3]). However Hemerythrin is classically described as Oxygen-binding through two attached Fe2+ ions. And the bacterial Swiss:Q7WX96 is a regulator of response to NO, which suggests yet another set-up for its metal ligands ([4]). In Staphylococcus aureus P72360 has been noted to be important when the organism switches to living in environments with low oxygen concentrations ([4]); perhaps this protein acts as an oxygen store or scavenger. This domain can bind oxygen (Matilla et.al., FEMS Microbiology Reviews, fuab043, 45, 2021, 1. https://doi.org/10.1093/femsre/fuab043)

1: Structure of methemerythrin at 2.8-Angstrom resolution: computer graphics fit of an averaged electron density map. Stenkamp RE, Sieker LC, Jensen LH, McQueen JE Jr; Biochemistry 1978;17:2499-2504. PMID:678527

2: New Knowledge from Old: In silico discovery of novel protein domains in Streptomyces coelicolor. Yeats C, Bentley S, Bateman A; BMC Microbiol 2003;3:3-3. PMID:12625841

3: Structures of wild-type chloromet and L103N hydroxomet Themiste zostericola myohemerythrins at 1.8 A resolution. Martins LJ, Hill CP, Ellis WR Jr; Biochemistry 1997;36:7044-7049. PMID:9188702

4: The srhSR gene pair from Staphylococcus aureus: genomic and proteomic approaches to the identification and characterization of gene function. Throup JP, Zappacosta F, Lunsford RD, Annan RS, Carr SA, Lonsdale JT, Bryant AP, McDevitt D, Rosenberg M, Burnham MK; Biochemistry 2001;40:10392-10401. PMID:11513618