Chromoprotein Explained

A chromoprotein is a conjugated protein that contains a pigmented prosthetic group (or cofactor). A common example is haemoglobin, which contains a heme cofactor, which is the iron-containing molecule that makes oxygenated blood appear red. Other examples of chromoproteins include other hemochromes, cytochromes, phytochromes and flavoproteins.^[1]

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.

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.

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]}

Notes and References

1. Book: An Introduction to Biochemistry . Fearon WR . 1940 . 131 . Elsevier . 9781483225395 .
2. Kanegae T, Hayashida E, Kuramoto C, Wada M . A single chromoprotein with triple chromophores acts as both a phytochrome and a phototropin . Proceedings of the National Academy of Sciences of the United States of America . 103 . 47 . 17997–18001 . November 2006 . 17093054 . 1693861 . 10.1073/pnas.0603569103 . free . 2006PNAS..10317997K .
3. Zagranichny VE, Rudenko NV, Gorokhovatsky AY, Zakharov MV, Balashova TA, Arseniev AS . Traditional GFP-type cyclization and unexpected fragmentation site in a purple chromoprotein from Anemonia sulcata, asFP595 . Biochemistry . 43 . 42 . 13598–13603 . October 2004 . 15491166 . 10.1021/bi0488247 .
4. Chang HY, Ko TP, Chang YC, Huang KF, Lin CY, Chou HY, Chiang CY, Tsai HJ . 6 . 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 . June 2019 . 30836182 . 10.1016/j.ijbiomac.2019.02.138 . 73497504 .
5. Liljeruhm J, Funk SK, Tietscher S, Edlund AD, Jamal S, Wistrand-Yuen P, Dyrhage K, Gynnå A, Ivermark K, Lövgren J, Törnblom V, Virtanen A, Lundin ER, Wistrand-Yuen E, Forster AC . 6 . Engineering a palette of eukaryotic chromoproteins for bacterial synthetic biology . Journal of Biological Engineering . 12 . 1 . 8 . 2018-05-10 . 29760772 . 5946454 . 10.1186/s13036-018-0100-0 . free .
6. Campbell RE, Tour O, Palmer AE, Steinbach PA, Baird GS, Zacharias DA, Tsien RY . A monomeric red fluorescent protein . Proceedings of the National Academy of Sciences of the United States of America . 99 . 12 . 7877–82 . June 2002 . 12060735 . 122988 . 10.1073/pnas.082243699 . 2002PNAS...99.7877C . free .
7. Bao L, Menon PN, Liljeruhm J, Forster AC . Overcoming chromoprotein limitations by engineering a red fluorescent protein . Analytical Biochemistry . 611 . 113936 . December 2020 . 32891596 . 10.1016/j.ab.2020.113936 . 221523489 . free .
8. Ahmed . F. Hafna . Caputo . Alessandro T. . French . Nigel G. . Peat . Thomas S. . Whitfield . Jason . Warden . Andrew C. . Newman . Janet . Scott . Colin . 2022-05-01 . Over the rainbow: structural characterization of the chromoproteins gfasPurple, amilCP, spisPink and eforRed . Acta Crystallographica Section D . 78 . Pt 5 . 599–612 . 10.1107/S2059798322002625 . 2059-7983 . 9063845 . 35503208. 2022AcCrD..78..599A .
9. Web site: Tsang . Jennifer . Chromoproteins: Colorful Proteins For Molecular Biology Experiments . 2024-10-28 . blog.addgene.org . en-us.