Fit for industry: Stereochemically pure 2‐O‐(α‐D‐glucopyranosyl)‐sn‐glycerol (αGG) was obtained in high yield from an efficient and selective biocatalytic process (see schematic outline). The sucrose phosphorylase catalyzed transfer of a glucosyl group from sucrose to glycerol unites the main advantages of transglycosidases, glycosyltransferases, and glycosynthases for glycoside synthesis and provides access to αGG as an industrial chemical.
Compatible solutes constitute a diverse class of low-molecular-mass organic molecules that are accumulated in high intracellular concentrations in response to the external stress of hyperosmolality or high temperature. Many of these compounds like alpha, alpha-trehalose are well known for their stabilizing effect on protein structure and could lead to development of more stable protein formulations. Negatively charged solutes like mannosylglycerate (R-2-O-alpha-D-mannopyranosyl-glycerate) are widespread among (hyper)thermophilic microorganisms and are thought to be exceptionally potent stabilizers of proteins under high-temperature denaturation conditions. To further inquire into the role of compound charge for protective function, we have compared two naturally occurring and structurally related solutes, glucosylglycerol (2-O-alpha-D-glucopyranosyl-sn-glycerol) and glucosylglycerate (R-2-O-alpha-D-glucopyranosyl-glycerate), as stabilizers of different enzymes undergoing inactivation through elevated temperature or freeze drying, and benchmarked their effects against that of alpha,alpha-trehalose. Glucosylglycerate in concentrations of >/=0.1 M was the most effective in preventing thermally induced loss of enzyme activity of lactate dehydrogenase, mannitol dehydrogenase, starch phosphorylase, and xylose reductase. alpha,alpha-Trehalose could usually be replaced by glucosylglycerol without compromising enzyme stability. Glucosylglycerol and glucosylglycerate afforded substantial (eightfold) protection to mannitol dehydrogenase during freeze drying.
The cDNA encoding trehalose phosphorylase, a family GT-4 glycosyltransferase from the fungus Schizophyllum commune, was isolated and expressed in Escherichia coli to yield functional recombinant protein in its full length of 737 amino acids. Unlike the natural phosphorylase that was previously obtained as a truncated 61 kDa monomer containing one tightly bound Mg2+, the intact enzyme produced in E. coli is a dimer and not associated with metal ions [Eis, Watkins, Prohaska and Nidetzky (2001) Biochem. J. 356, 757-767]. MS analysis of the slow spontaneous conversion of the full-length enzyme into a 61 kDa fragment that is fully active revealed that critical elements of catalysis and specificity of trehalose phosphorylase reside entirely in the C-terminal protein part. Intact and truncated phosphorylases thus show identical inhibition constants for the transition state analogue orthovanadate and alpha,alpha-trehalose (K(i) approximately 1 microM). Structure-based sequence comparison with retaining glycosyltransferases of fold family GT-B reveals a putative active centre of trehalose phosphorylase, and results of site-directed mutagenesis confirm the predicted crucial role of Asp379, His403, Arg507 and Lys512 in catalysis and also delineate a function of these residues in determining the large preference of the wild-type enzyme for the phosphorolysis compared with hydrolysis of alpha,alpha-trehalose. The pseudo-disaccharide validoxylamine A was identified as a strong inhibitor of trehalose phosphorylase (K(i)=1.7+/-0.2 microM) that displays 350-fold tighter binding to the enzyme-phosphate complex than the non-phosphorolysable substrate analogue alpha,alpha-thio-trehalose. Structural and electronic features of the inhibitor that may be responsible for high-affinity binding and their complementarity to an anticipated glucosyl oxocarbenium ion-like transition state are discussed.
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