Osmolyte Explained

Osmolytes are low-molecular-weight organic compounds that influence the properties of biological fluids. Osmolytes are a class of organic molecules that play a significant role in regulating osmotic pressure and maintaining cellular homeostasis in various organisms, particularly in response to environmental stressors.^[1] Their primary role is to maintain the integrity of cells by affecting the viscosity, melting point, and ionic strength of the aqueous solution. When a cell swells due to external osmotic pressure, membrane channels open and allow efflux of osmolytes carrying water, restoring normal cell volume.

These molecules are involved in counteracting the effects of osmotic stress, which occurs when there are fluctuations in the concentration of solutes (such as ions and sugars) inside and outside cells. Osmolytes help cells adapt to changing osmotic conditions, thereby ensuring their survival and functionality.^[2] Osmolytes also interact with the constituents of the cell, e.g., they influence protein folding.^{[3] [4]} Common osmolytes include amino acids, sugars and polyols, methylamines, methylsulfonium compounds, and urea.

Case studies

Natural osmolytes that can act as osmoprotectants include trimethylamine N-oxide (TMAO), dimethylsulfoniopropionate, sarcosine, betaine, glycerophosphorylcholine, myo-inositol, taurine, glycine, and others.^{[5] [6]} Bacteria accumulate osmolytes for protection against a high osmotic environment.^[7] The osmolytes are neutral non-electrolytes, except in bacteria that can tolerate salts. In humans, osmolytes are of particular importance in the renal medulla.^[8]

Osmolytes are present in the cells of fish, and function to protect the cells from water pressure. As the osmolyte concentration in fish cells scales linearly with pressure and therefore depth, osmolytes have been used to calculate the maximum depth where a fish can survive. Fish cells reach a maximum concentration of osmolytes at depths of approximately 26900abbr=offNaNabbr=off, with no fish ever being observed beyond 27349abbr=offNaNabbr=off.^{[9] [10]}

Further reading

• Rose GD, Fleming PJ, Banavar JR, Maritan A . A backbone-based theory of protein folding . Proc. Natl. Acad. Sci. U.S.A. . 103 . 45 . 16623–33 . November 2006 . 17075053 . 1636505 . 10.1073/pnas.0606843103. 2006PNAS..10316623R . free .
• Holthauzen LM, Bolen DW . Mixed osmolytes: the degree to which one osmolyte affects the protein stabilizing ability of another . Protein Sci. . 16 . 2 . 293–8 . February 2007 . 17189473 . 2203298 . 10.1110/ps.062610407 .
• Harries. Daniel. Rösgen, Jörg. A Practical Guide on How Osmolytes Modulate Macromolecular Properties. Meth. Cell Bio.. Methods in Cell Biology. 2008. 84. 679–735. 10.1016/S0091-679X(07)84022-2. 17964947. 9780123725202.
• Book: Hochachka, P.W.. Somero, G. N. Biochemical Adaptation. Mechanism and Process in Physiological Evolution. 2002. Oxford University Press. Oxford.

Notes and References

1. Organic osmolytes as compatible, metabolic and counteracting cytoprotectants in high osmolarity and other stresses. Paul H. Yancey. Journal of Experimental Biology. 2005. 208. 15. 2819–2830. 10.1242/jeb.01730. 16043587. free.
2. Review of Medical Physiology, William F. Ganong, McGraw-Hill Medical, .
3. Bolen DW, Baskakov IV . The osmophobic effect: natural selection of a thermodynamic force in protein folding . . 310 . 5 . 2001 . 955–963 . 10.1006/jmbi.2001.4819 . 11502004.
4. Book: Su, Zhaoqian . Roles of cosolvents on protein stability. 2017. 1245504372.
5. Neuhofer . W. . Beck . F. X. . Survival in Hostile Environments: Strategies of Renal Medullary Cells . 10.1152/physiol.00003.2006 . Physiology . 21 . 3 . 171–180 . 2006 . 16714475 .
6. Arakawa T, Timasheff SN . The stabilization of proteins by osmolytes . . 47 . 3 . 1985 . 411–414 . 1435219 . 10.1016/s0006-3495(85)83932-1 . 3978211. 1985BpJ....47..411A.
7. Csonka LN . Physiological and genetic responses of bacteria to osmotic stress . . 53 . 1 . 1989 . 121–147. 10.1128/mr.53.1.121-147.1989 . 372720 . 2651863.
8. Gallazzini . M. . Burg . M. B. . 10.1152/physiol.00009.2009 . What's New About Osmotic Regulation of Glycerophosphocholine . Physiology . 24 . 4 . 245–249 . 2009 . 19675355 . 2943332 .
9. Marine fish may be biochemically constrained from inhabiting the deepest ocean depths . PNAS . 2014 . 111 . 12 . 4461–4465 . 10.1073/pnas.1322003111 . 24591588. Yancey PH, Gerringer ME, Drazen JC, Rowden AA, Jamieson A. 2014PNAS..111.4461Y . 3970477 . free.
10. News: Lu . Donna . 3 April 2023 . Scientists find deepest fish ever recorded at 8,300 metres underwater near Japan . The Guardian . London . 25 May 2023.