Biosynthetic pathways of inositol and glycerol phosphodiesters used by the hyperthermophile Archaeoglobus fulgidus in stress adaptation

Archaeoglobus fulgidus accumulates di-myo-inositol phosphate (DIP) and diglycerol phosphate (DGP) in response to heat and osmotic stresses, respectively, and the level of glycero-phospho-myo-inositol (GPI) increases primarily when the two stresses are combined. In this work, the pathways for the biosynthesis of these three compatible solutes were established based on the detection of the relevant enzymatic activities and characterization of the intermediate metabolites by nuclear magnetic resonance analysis. The synthesis of DIP proceeds from glucose-6-phosphate via four steps: (i) glucose-6-phosphate was converted into L-Myo-inositol 1-phosphate by L-Myo-inositol I-phosphate synthase; (ii) L-MYO-inositol 1-phosphate was activated to CDP-inositol at the expense of CTP; this is the first demonstration of CDP-inositol synthesis in a biological system; (iii) CDP-inositol was coupled with L-MYO-inositol I-phosphate to yield a phosphorylated intermediate, 1,1'-di-myo-inosityl phosphate 3-phosphate (DIPP); (iv) finally, DIPP was dephosphorylated into DIP by the action of a phosphatase. The synthesis of the two other pollyol-phosphodiesters, DGP and GPI, proceeds via the condensation of CDP-glycerol with the respective phosphorylated polyol, glycerol 3-phosphate for DGP and L-MYO-inositol 1-phosphate for GPI, yielding the respective phosphorylated intermediates, 1X,1'X-diglyceryl phosphate 3-phosphate (DGPP) and 1-(1'X-glyceryl) myo-inosityl phosphate 3-phosphate (GPIP), which are subsequently dephosphorylated to form the final products. The results disclosed here represent an important step toward the elucidation of the regulatory mechanisms underlying the differential accumulation of these compounds in response to heat and osmotic stresses.