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.