title: General acid-base catalysis of alpha-glucan phosphorylases: stereospecific glucosyl transfer from D-glucal is a pyridoxal 5'-phosphate and orthophosphate (arsenate) dependent reaction.
abstract: D-Glucal, containing a highly reactive double bond, can replace glucose 1-phosphate as the glucosyl donor in phosphorylase-catalyzed glucosyl transfer to a suitable oligo- or polysaccharide acceptor: D-glucal + Pi + (glucose)Pi leads to n 2-deoxy-alpha-D-glucosyl(glucose)n in equilibrium 2-deoxy-alpha-D-glucose-1-P + (glucose)n. This reaction is catalyzed by alpha-glucan phosphorylases from rabbit skeletal muscle, potato tuber, and Escherichia coli. D-Glucal is only measurably consumed by alpha-glucan phosphorylases when orthophosphate or arsenate is present. With saturating concentrations of these anions and a glucosyl acceptor, the D-glucal reaction proceeds at rates comparable with the rates of glucosyl transfer from glucose 1-phosphate and of phosphorolysis or arsenolysis of poly- or oligosaccharides. Furthermore, for the reaction to proceed, the enzyme must be in the active conformation containing the cofactor pyridoxal 5'-phosphate in its dianionic form. On the basis of proton nuclear magnetic resonance spectra, it is proposed that protonation at C-2 of D-glucal gives rise to a hypothetical 2-deoxy-beta-D-glucose intermediate, yielding as a final product (2-deoxy-alpha-D-[2(e)-2H]glucose)n alpha (1 leads to 4) saccharides. These 2-deoxy-alpha-D-glucose oligo- or polysaccharides are degraded by alpha-glucan phosphorylases by phosphorolysis or arsenolysis like natural linear and branched alpha-glucans. The absolute requirement of the D-glucal reaction for phosphate (or arsenate) and its dependency on the dianionic form of the pyridoxal 5'-phosphate bound to phosphorylase are rationalized in terms of a proton transfer relay involving juxtaposed phosphates. Phosphate--phosphate interactions were postulated by Withers et al. [Withers, S. G., Madsen, N. B., Sykes, B. D., Takagi, M., Shimomura, S., & Fukui, T. (1981) J. Biol. Chem. 256, 10759-10762].
mesh_terms: D001149:Arsenates / Q000378:metabolism*; D001151:Arsenic / Q000378:metabolism*; D055598:Chemical Phenomena; D002621:Chemistry; D003847:Deoxyglucose / Q000378:metabolism / Q000031:analogs & derivatives; D005947:Glucose / Q000031:analogs & derivatives* / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D006005:Phosphorylases / Q000378:metabolism*; D011732:Pyridoxal Phosphate / Q000378:metabolism*
chemical_list: D001149:Arsenates; C001289:d-glucal; D011732:Pyridoxal Phosphate; D003847:Deoxyglucose; D006005:Phosphorylases; D005947:Glucose; D001151:Arsenic; C025657:arsenic acid