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Chapter 7 : The Physiological Role, Biosynthesis, and Mode of Action of Compatible Solutes from (Hyper)Thermophiles
Category: Applied and Industrial Microbiology; Environmental Microbiology
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Many thermophiles and hyperthermophiles (from now on designated (hyper)thermophiles) have been isolated from both fresh water and seawater sources. Compatible solutes must be highly soluble and they usually belong to one of the following groups of compounds: amino acids, sugars, polyols, betaines, and ectoines. In general, compatible solutes accumulate to high levels in the cytoplasm. The relative abundance combined with the low molecular mass of these compounds greatly facilitates the task of their molecular identification by resorting to two powerful analytical techniques: nuclear magnetic resonance (NMR) and mass spectrometry. The mannosyl-3-phosphoglycerate synthase (MPGS) characterized to date produce mannosyl-3-phosphoglycerate with the same anomeric configuration of the substrate and accordingly have been classified as members of glycosyltransferases family GT55, which comprises GDP-mannose: α-mannosyltransferases that retain the anomeric configuration of the substrate. The evolution of MG biosynthesis is a fascinating topic but a meaningful discussion would demand more ample data sets and reliable tools for genome analysis. Diglycerol phosphate (DGP) biosynthesis was investigated by the author and his team on Archaeoglobus fulgidus. In the case of solutes from hyperthermophiles, it was shown that the protecting effect was clearly dependent on the solute charge. The melting temperature of bovine ribonuclease A (RNase A) in the presence and absence of 2-a-O-mannosylglycerate (MG) depends on the ionization state of the solute. Due to the enhanced ability to stabilize biological materials, the application of hypersolutes in industrial applications was soon envisioned, and several industrial patents on their uses have been filed.
Compatible solutes primarily restricted to hyperthermophiles.
Distribution of trehalose (circles), mannosylglycerate (diamonds) and di-myo-inositol-1,1′-phosphate (stars) among (hyper)thermophiles. Tree of Life adapted from Blöchl et al., 1995 . The question marks indicate unknown positions of the branching points between the three domains.
The two pathways for the synthesis of mannosylglycerate in Rhodothermus marinus. Single-step pathway uses mannosylglycerate synthase (MGS), while the two-step pathway involves the actions of mannosyl-3-phosphoglycerate synthase (MPGS) and mannosyl-3-phosphoglycerate phosphatase (MPGP).
Unrooted phylogenetic tree based on known or putative sequences of mannosyl-3-phosphoglycerate synthase genes. The ClustalX and TreeView programs 5,7 were used for sequence alignment and to generate the phylogenetic tree. The significance of the branching order was evaluated by bootstrap analysis of 1000 computer-generated trees. Bar, 0.1 change per site. Abbreviations: Aper, Aeropyrum pernix; Apro, Archaeoglobus profundus; Aven, Archaeoglobus venificus; Deth, Dehalococcoides ethenogenes; Mgri, Magna-porthe grisea; Ncra, Neurospora crassa; Pfer, Palaeococcus ferrophilus; Paby, Pyrococcus abyssi; Pfur, Pyrococcus furiosus; Phor, Pyrococcus horikoshii; Rmar, Rhodothermus marinus; Tthe, Thermus thermophilus; Tlit, Thermococcus litoralis.
Genomic organization of mannosylglycerate biosynthesis via the two-step pathway. Black arrows indicate mpgS genes; dark grey arrows indicate the mpgP genes; light grey arrow represents the phosphomannose isomerase/mannose-1-phosphate guanylyltransferase; the white arrow indicates phosphomannomutase.
Dependence of the melting temperature of RNase A on pH in the absence of solutes (squares and thin line) and with 0.5 M mannosylglycerate (circles and thick line).
Effect of solutes on the melting temperature of staphylococcal nuclease (SNase) and pig heart malate dehydrogenase (MDH). Abbreviations: Tre, trehalose; MG, α-mannosylglycerate; MGA, α-mannosylglyceramide; DIP, di- myo-inositol-1,1′-phosphate; DGP, diglycerol phosphate; KCl, potassium chloride; Gly, glycerol; Ect, ectoine; Hect, hydroxyectoine.
Schematic representation of the preferential exclusion of solutes from the protein surface in the native and unfolded states. Upon denaturation, the zone of exclusion increases and, although the same amount of solute molecules is depicted, these become locally more concentrated, i.e., their chemical potential has increased. As a consequence, to achieve thermodynamic equilibrium, the unfolding reaction is displaced to the native state, hence the solute acts as a stabilizer.
Fluorescence decay times of staphylococcal nuclease as a function of temperature in the presence of 0.5 M mannosylglycerate (solid symbols) and in the absence of solutes (open symbols). Native protein: triangles; denatured states of the protein: squares and circles.
Distribution of compatible solutes of (hyper)thermophilic microorganisms
Biochemical properties of recombinant mannosyl-3-phosphoglycerate synthase from Archaea and Bacteria