Trimethylamine N-Oxide (TMAO): a Unique Counteracting Osmolyte?

Abstract

Cells of many organisms facing osmotic shrinkage or swelling undergo homeostatic volume regulation using osmolytes—inorganic ions (Na+, K+, Cl-) in transient disturbances, but special organic osmolytes in long-term disturbances. Neutral amino acids, methylamines and polyols are key examples. Widely termed 'compatible' cosolutes/cosolvents, they—unlike inorganic ions—do not perturb membrane potential nor (supposedly) macromolecules. Indeed, most enhance protein stability in part through preferential exclusion; i.e., they 'dissolve' poorly in proteins' hydration layer and reduce water availability for hydrating unfolding proteins. However, these concepts imply that organic osmolytes are all 'compatible' and interchangeable in this way. Instead, most have unique non-osmotic cytoprotective properties such as antioxidation, and some may have stabilizing features not universal among osmolytes. The latter is exemplified by trimethylamine N-oxide (TMAO), an osmolyte high in chondrichthyans (sharks and rays), and that increases with depth in many marine animals. First, TMAO is the strongest enhancer of protein folding among common osmolytes, but unlike most osmolytes, exhibits some preferential binding with proteins. Second, unlike other common osmolytes such as glycine, TMAO is not found in nature in the absence of a protein destabilizer—notably urea (primary osmolyte of chondrichthyans) and hydrostatic pressure, both counteracted by TMAO. Without a destabilizer, TMAO can over-stabilize proteins causing non-functional aggregates; i.e., it is not 'compatible'. Third, TMAO 'hardens' water structure and reduces water compressibility (again unlike other osmolytes). Under high pressure in the deep sea, these 'piezolyte' properties reduce both protein unfolding and cell volume compression.

Benefits of Taurisolo in Diabetic Patients with Peripheral Artery Disease

“…In normal circumstances, TMAO plays a crucial role in regulating fluid balance, stabilizing protective functions and aiding in preserving the integrity of cells [ 44 ]. However, TMAO has been identified as an inflammatory and proatherogenic molecule when present at a value over the physiologic range and impairs angiogenesis, repressing the CXCR4 axis [ 45 , 46 , 47 , 48 , 49 ].…”

Modulation of Biomolecular Liquid–Liquid Phase Separation by Preferential Hydration and Interaction of Small Osmolytes with Proteins

“…Methylamines such as betaine and TMAO are present in cells in many animals, including mammals. 7,17 In cartilaginous fish such as sharks, TMAO acts as a stabilizer. Several studies on protein conformational stability and functional activity have demonstrated that when urea destabilizes proteins and reduces their function, half to equivalent amounts of TMAO counteract the effect of urea.…”

“…Various molecules, such as cosolvents and osmolytes, are present in cells to regulate cell volume, structural stability and function of biomolecules, and molecular crowding effects. − Numerous studies have investigated the impact of crowder molecules, including polysaccharides, monosaccharides, glycerol, and osmolytes such as trimethylamine N -oxide (TMAO) and urea with respect to protein denaturation temperature and pressure. − For example, sucrose, sorbitol, glycerol, and TMAO increase denaturation temperatures by 2–4 °C/M and denaturation pressures by ∼2 kbar/M . One mechanism underlying protein stabilization is the excluded volume effect.…”

“…However, high-urea concentrations denature intracellular proteins; thus, some osmolytes must act as stabilizers. Methylamines such as betaine and TMAO are present in cells in many animals, including mammals. , In cartilaginous fish such as sharks, TMAO acts as a stabilizer. Several studies on protein conformational stability and functional activity have demonstrated that when urea destabilizes proteins and reduces their function, half to equivalent amounts of TMAO counteract the effect of urea. ,, Moreover, TMAO stabilizes the RNA tertiary structure and counteracts the denaturing effects of urea at the same ratios observed for proteins .…”

“…Second, the overstabilization of protein association by TMAO. The strong stabilizing potential of TMAO can inhibit protein flexibility and favor aggregates of intrinsically disordered proteins . Although the concentration of TMAO inside the droplets is unclear, at least the droplet surfaces are exposed to a high concentration of TMAO in the buffer, leading to irreversible aggregation of FUS molecules on the droplet surface.…”