This family includes protoglobin from Methanosarcina acetivorans C2A. It is also found near the N-terminus of the Haem-based aerotactic transducer HemAT in Bacillus subtilis (Swiss:O07621). It is part of the haemoglobin superfamily. Protoglobin has specific loops and an amino-terminal extension which leads to the burying of the haem within the matrix of the protein. Protoglobin-specific apolar tunnels allow the access of O2, CO and NO to the haem distal site [1]. In HemAT it acts as an oxygen sensor domain [2]. It can also recognise cyanide (Matilla et. al., FEMS Microbiology Reviews, fuab043, 45, 2021, 1. https://doi.org/10.1093/femsre/fuab043).

1: Archaeal protoglobin structure indicates new ligand diffusion paths and modulation of haem-reactivity. Nardini M, Pesce A, Thijs L, Saito JA, Dewilde S, Alam M, Ascenzi P, Coletta M, Ciaccio C, Moens L, Bolognesi M; EMBO Rep. 2008;9:157-163. PMID:18188182

2: Structure of the oxygen sensor in Bacillus subtilis: signal transduction of chemotaxis by control of symmetry. Zhang W, Phillips GN Jr; Structure. 2003;11:1097-1110. PMID:12962628