title: Further studies on the catalytic mechanism of human liver alpha-L-fucosidase.
abstract: Radiolabeling of human liver alpha-L-fucosidase (alpha-L-fucoside fucohydrolase, EC 3.2.1.51) with [1-3H]conduritol C trans-epoxide revealed that there are four active sites per tetrameric enzyme complex. Solvent isotope effect experiments give evidence for a proton transfer at the rate-limiting step in catalysis. Transglycosylase activity was observed using methanol as an alternative glycone acceptor to produce methyl alpha-L-fucoside, suggesting that alpha-L-fucose is formed when water is the acceptor. Initial burst kinetics experiments suggest that a glycosyl-enzyme intermediate is formed, although the magnitude of the burst is not stoichiometric with the number of active sites. These data, along with previous results, suggest a general acid-general base catalytic mechanism involving double inversion of stereochemistry at C-1 of fucose, as well as the formation of either a covalent glycosyl-enzyme intermediate or a tight ion pair between a charged active-site residue and a hypothetical fucosyl oxocarbonium ion intermediate.
mesh_terms: D000345:Affinity Labels; D001665:Binding Sites; D002384:Catalysis; D006031:Glycosylation; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D012997:Solvents; D005644:alpha-L-Fucosidase / Q000302:isolation & purification / Q000378:metabolism*
chemical_list: D000345:Affinity Labels; D012997:Solvents; D005644:alpha-L-Fucosidase