In hemoglobin there exists a chromoprotein (tetramer MW:4 x 16.125 =64.500), namely heme, consisting of Fe++ four pyrrol rings.
A single chromoprotein can act as both a phytochrome and a phototropin due to the presence and processing of multiple chromophores. Phytochrome in ferns contains PHY3 which contains an unusual photoreceptor with a dual-channel possessing both phytochrome (red-light sensing) and phototropin (blue-light sensing) and this helps the growth of fern plants at low sunlight.[2]
The GFP protein family includes both fluorescent proteins and non-fluorescent chromoproteins. Through mutagenesis or irradiation, the non-fluorescent chromoproteins can be converted to fluorescent chromoproteins.[3] An example of such converted chromoprotein is "kindling fluorescent proteins" or KFP1 which was converted from a mutated non-fluorescent Anemonia sulcata chromoprotein to a fluorescent chromoprotein.[4]
Sea anemones contain purple chromoprotein shCP with its GFP-like chromophore in the trans-conformation. The chromophore is derived from Glu-63, Tyr-64 and Gly-65 and the phenolic group of Tyr-64 plays a vital role in the formation of a conjugated system with the imidazolidone moiety resulting a high absorbance in the absorption spectrum of chromoprotein in the excited state. The replacement of Tyrosine with other amino acids leads to the alteration of optical and non-planer properties of the chromoprotein. Fluorescent proteins such as anthrozoa chromoproteins emit long wavelengths [4]
14 chromoproteins were engineered to be expressed in E. coli for synthetic biology.[5] However, chromoproteins bring high toxicities to their E. coli hosts, resulting in the loss of colors. mRFP1, the monomeric red fluorescent protein,[6] which also displays distinguishable color under ambient light, was found to be less toxic.[7] Color-changing mutagenesis on amino acids 64–65 of the mRFP1 fluorophore was done to acquire different colors.
^Zagranichny VE, Rudenko NV, Gorokhovatsky AY, Zakharov MV, Balashova TA, Arseniev AS (October 2004). "Traditional GFP-type cyclization and unexpected fragmentation site in a purple chromoprotein from Anemonia sulcata, asFP595". Biochemistry. 43 (42): 13598–13603. doi:10.1021/bi0488247. PMID15491166.
^ abChang HY, Ko TP, Chang YC, Huang KF, Lin CY, Chou HY, et al. (June 2019). "Crystal structure of the blue fluorescent protein with a Leu-Leu-Gly tri-peptide chromophore derived from the purple chromoprotein of Stichodactyla haddoni". International Journal of Biological Macromolecules. 130: 675–684. doi:10.1016/j.ijbiomac.2019.02.138. PMID30836182. S2CID73497504.