In molecular biology the orange carotenoid N-terminal domain is a protein domain found predominantly at the

N-terminus The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...

of the Orange carotenoid protein (OCP), and is involved in non-covalent binding of a

carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, and Fungus, fungi. Carotenoids give the characteristic color to pumpki ...

chromophore. It is unique for being present in soluble proteins, whereas the vast majority of domains capable of binding carotenoids are intrinsic membrane proteins. Thus far, it has exclusively been found in

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

, among which it is widespread. The domain also exists on its own, in uncharacterized cyanobacterial proteins referred to as "Red Carotenoid Protein" (RCP). The domain adopts an

alpha-helical The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues e ...

structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...

consisting of two four-helix bundles. Orange carotenoid-binding proteins (OCP) were first identified in

cyanobacterial Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

species, where they occur associated with phycobilisome in the cellular thylakoid membrane. These proteins function in photoprotection, and are essential for non-photochemical quenching (NPQ). In full-length OCP, the NPQ activity is regulated by photoactivation by strong blue-green light. OCP seems to act as a homodimer, and binds one molecule of 3'-hydroxyechinenone (a

ketocarotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...

) and one chloride ion per subunit. The carotenoid binding site is lined with a striking number of

methionine Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical ro ...

residues Residue may refer to: Chemistry and biology * An amino acid, within a peptide chain * Crop residue, materials left after agricultural processes * Pesticide residue, refers to the pesticides that may remain on or in food after they are appli ...

. The N-terminal domain of OCP is usually accompanied by a C-terminal domain which belongs to the NTF2 superfamily and helps bind the carotenoid. OCP can be proteolytically cleaved into a red form (RCP), which lacks 15 residues from the N-terminus and approximately 150 residues from the C terminus. This domain is implicated in binding the phycobilisome complex, which thereby facilitates thermal dissipation (quenching) of excess absorbed light energy.

References

{{InterPro content, IPR015233 Protein domains

See also

References