title: Metabolic mechanism of mannan in a ruminal bacterium, Ruminococcus albus, involving two mannoside phosphorylases and cellobiose 2-epimerase: discovery of a new carbohydrate phosphorylase, β-1,4-mannooligosaccharide phosphorylase.
abstract: Ruminococcus albus is a typical ruminal bacterium digesting cellulose and hemicellulose. Cellobiose 2-epimerase (CE; EC 5.1.3.11), which converts cellobiose to 4-O-β-D-glucosyl-D-mannose, is a particularly unique enzyme in R. albus, but its physiological function is unclear. Recently, a new metabolic pathway of mannan involving CE was postulated for another CE-producing bacterium, Bacteroides fragilis. In this pathway, β-1,4-mannobiose is epimerized to 4-O-β-D-mannosyl-D-glucose (Man-Glc) by CE, and Man-Glc is phosphorolyzed to α-D-mannosyl 1-phosphate (Man1P) and D-glucose by Man-Glc phosphorylase (MP; EC 2.4.1.281). Ruminococcus albus NE1 showed intracellular MP activity, and two MP isozymes, RaMP1 and RaMP2, were obtained from the cell-free extract. These enzymes were highly specific for the mannosyl residue at the non-reducing end of the substrate and catalyzed the phosphorolysis and synthesis of Man-Glc through a sequential Bi Bi mechanism. In a synthetic reaction, RaMP1 showed high activity only toward D-glucose and 6-deoxy-D-glucose in the presence of Man1P, whereas RaMP2 showed acceptor specificity significantly different from RaMP1. RaMP2 acted on D-glucose derivatives at the C2- and C3-positions, including deoxy- and deoxyfluoro-analogues and epimers, but not on those substituted at the C6-position. Furthermore, RaMP2 had high synthetic activity toward the following oligosaccharides: β-linked glucobioses, maltose, N,N'-diacetylchitobiose, and β-1,4-mannooligosaccharides. Particularly, β-1,4-mannooligosaccharides served as significantly better acceptor substrates for RaMP2 than D-glucose. In the phosphorolytic reactions, RaMP2 had weak activity toward β-1,4-mannobiose but efficiently degraded β-1,4-mannooligosaccharides longer than β-1,4-mannobiose. Consequently, RaMP2 is thought to catalyze the phosphorolysis of β-1,4-mannooligosaccharides longer than β-1,4-mannobiose to produce Man1P and β-1,4-mannobiose.
authors.affiliation: Research Faculty of Agriculture, Hokkaido University, N-9, W-9, Sapporo 060-8589, Japan.
authors.forename: Ryosuke
authors.initials: R
authors.lastname: Kawahara
authors.forename: Wataru
authors.initials: W
authors.lastname: Saburi
authors.forename: Rei
authors.initials: R
authors.lastname: Odaka
authors.forename: Hidenori
authors.initials: H
authors.lastname: Taguchi
authors.forename: Shigeaki
authors.initials: S
authors.lastname: Ito
authors.forename: Haruhide
authors.initials: H
authors.lastname: Mori
authors.forename: Hirokazu
authors.initials: H
authors.lastname: Matsui
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D001441:Bacteroides fragilis / Q000201:enzymology; D002238:Carbohydrate Epimerases / Q000737:chemistry / Q000378:metabolism*; D002474:Cell-Free System / Q000201:enzymology; D007527:Isoenzymes / Q000737:chemistry / Q000378:metabolism; D008351:Mannans / Q000737:chemistry / Q000378:metabolism; D006005:Phosphorylases / Q000737:chemistry / Q000378:metabolism*; D045857:Ruminococcus / Q000201:enzymology*
chemical_list: D001426:Bacterial Proteins; D007527:Isoenzymes; D008351:Mannans; C014647:mannobiose; D006005:Phosphorylases; D002238:Carbohydrate Epimerases