C-terminal middle region of Androglobins (Adgbs) and related proteins; including permuted globin domain and IQ motif
Androglobin (Adgb, also known as Calpain-7-like protein, CAPN7L) is a large multidomain protein consisting of an N-terminal peptidase C2 family calpain-like domain, an IQ calmodulin-binding motif, and an internal, circularly permuted globin domain. The canonical secondary structure of hemoglobins is an 3-over-3 alpha-helical sandwich structure, where the eight alpha-helical segments are conventionally labeled, A-H, according to their sequential order; Adgbs differ from this in having helices C-H followed by A-B. Adgbs and other phylogenetically ancient globins, such as neuroglobins and globin X, form hexacoordinated heme iron complexes. Globins contain various highly conserved residues of the heme pocket: including a Phe in the interhelical position CD1 (Phe CD1, first position in the loop between the helices C and D) that is packed against the heme, a His at the 7th position of the E-helix (His E7) that binds the heme iron distally, and a His at the 8th position of the F-helix (His F8) that binds the heme iron proximally. Unlike other hexacoordinated globins, Adgbs have an E7 Gln; their hexacoordination scheme is [Gln]-Fe-[His]. In mammals, Adgb is mainly expressed in the testes and may play an important role in spermatogenesis. Arthropod Adgbs have degenerate globin domains (DOI:10.3389/fgene.2020.00858). This model spans the permuted globin domain, the IQ motif, and a conserved region of about 200 amino acid residues located C-terminal to the globin domain; it does not include the N-terminal protease domain or the large uncharacterized C-terminal domain of approximately 500 residues.
Comment:the hexacoordination scheme is [Gln]-Fe-[His]: Gln E7 binds the heme iron distally, Phe CD1 (first position in the loop between helices C and D) is packed against the heme, and His F8 binds the heme iron proximally
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