Achromoprotein is a conjugated protein that contains apigmentedprosthetic group (or cofactor). A common example ishaemoglobin, which contains aheme cofactor, which is theiron-containing molecule that makesoxygenated blood appear red. Other examples of chromoproteins include otherhemochromes,cytochromes,phytochromes andflavoproteins.[1]
In hemoglobin there exists a chromoprotein (tetramer MW:4 x 16.125 =64.500), namely heme, consisting of Fe++ fourpyrrol rings.
A single chromoprotein can act as both aphytochrome and aphototropin due to the presence and processing of multiple chromophores. Phytochrome in ferns containsPHY3 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]
TheGFP 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-fluorescentAnemonia sulcata chromoprotein to a fluorescent chromoprotein.[4]
Sea anemones contain purple chromoprotein shCP with its GFP-like chromophore in thetrans-conformation. The chromophore is derived from Glu-63, Tyr-64 and Gly-65 and thephenolic group of Tyr-64 plays a vital role in the formation of aconjugated system with theimidazolidone moiety resulting a high absorbance in theabsorption 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 inE. coli forsynthetic biology.[5] However, chromoproteins bring high toxicities to theirE. coli hosts, resulting in the loss of colors. mRFP1, the monomericred 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.
Chromoproteins are valuable in synthetic biology, genetic engineering, and biotechnology as visible markers for tracking gene expression, assaying cellular functions and creating colorful biosensors.[8][9]
