(A1), 11-cis-3,4-didehydroretinal (A2) is also found as a ligand in some vertebrates, such as freshwater fishes. A2-bound opsins have a shifted ?_max and absorption spectrum compared to A1-bound opsins.

Functionally conserved residues and motifs

The seven transmembrane ?-helical domains in opsins are connected by three extra-cellular and three cytoplasmic loops. Along the ?-helices and the loops, many amino acid residues

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