Structure of a secreted lancelet carotenoid cleavage dioxygenase (CCD), from an ancestral CCD clade

Carotenoid cleavage dioxygenases (CCDs) are a superfamily of enzymes found in all living kingdoms. Scientists have recently discovered a new subfamily of CCDs, called beta-carotene oxygenase-like (BCOL) proteins, which are unique to chordates. These BCOL proteins are particularly prevalent in marine invertebrates like mollusks and lancelets.

Researchers have now determined the 3D structure of a BCOL protein from the lancelet Branchiostoma floridae, known as BCOLg. This enzyme is capable of asymmetrically cleaving beta-carotene into all-trans retinal. The team found six out of seven annotated BCOL transcripts in Branchiostoma gut tissue through RNA sequencing.

BCOLg is synthesized with four N-linked glycosylation sites, making it heavily glycosylated. Its structure belongs to the seven-bladed propeller fold category, which is typical of this enzyme family. Moreover, the protein exhibits a disulfide bond that contributes to its structural stability. Like other CCDs, BCOLg contains Asp substitutions at two of three Glu positions, which strengthens the three His residues responsible for Fe2+ coordination.

Interestingly, BCOLg does not possess the characteristic -PDPCK- motif that is present in the majority of metazoan CCDs.