How can a protein influence the properties of a pigment by providing it withe a specific chemical context?

Biliverdin is a linear chain of four tetrapyrrole rings. Biliverdin does not bind in an extended conformation as retinol. Istead it binds in a compact conformation like heme. But in photosynthetic organelles, when biliverdin act as a photoreceptor, the protein binds in a different way. Alpha-helical protein with the globin fold replaces the up-and-down barrel structure(Branden &Tooze,1991).

A similar situation can be seen in bacteriorhodopsin and rhodopsin, where the transmembrane receptor proteins are composed of seven helix motif.