Betaines and Related Osmoprotectants. Targets for Metabolic Engineering of Stress Resistance1

Osmoprotectants (also termed compatible solutes) occur in all organisms from archaebacteria to higher plants and animals. They are highly soluble compounds that carry no net charge at physiological pH and are nontoxic at high concentrations. Osmoprotectants serve to raise osmotic pressure in the cytoplasm and can also stabilize proteins and membranes when salt levels or temperatures are unfavorable. Osmoprotectants therefore play important roles in the adaptation of cells to various adverse environmental conditions (Yancey, 1994). Chemically, there are three types: betaines and allied compounds, polyols and sugars (e.g. mannitol and trehalose), and amino acids such as Pro. This Updatewill focus on betaines and their biosynthetic pathways. Advances in polyol and Pro research are covered elsewhere (Delauney and Verma, 1993; Stoop et al., 1996). Betaines are amino acid derivatives in which the nitrogen atom is fully methylated, i.e. they are quaternary ammonium compounds. Figure1 shows the structures of the three best-known betaines from plants, Gly betaine, Pro betaine (stachydrine), and β-Ala betaine, as well as choline-O-sulfate and DMSP (Rhodes and Hanson, 1993). The last two compounds are strictly speaking not betaines because DMSP has a tertiary sulfonium in place of a quaternary ammonium group, and choline-O-sulfate has a sulfate ester instead of a carboxyl group—but they are close structural analogs of betaines and have similar chemical and physiological properties.