THE SACCHAROMYCES-CEREVISIAE PROCESSING ALPHA-1,2-MANNOSIDASE IS AN INVERTING GLYCOSIDASE

The alpha 1,2-mannosidase from Saccharomyces cerevisiae, which removes one specific alpha 1,2-linked mannose residue from Man(9)GlcNAc(2), i s a member of the Class 1 alpha 1,2-mannosidase family conserved from yeast to mammals. Although Class 1 alpha 1,2-mannosidases are essentia l for the maturation of N-linked oligosaccharides in mammalian cells, nothing is known about their mechanism of action. The availability of sufficient quantities of recombinant yeast alpha 1,2-mannosidase and i ts homology with the mammalian enzymes make it a good model to study t he catalytic mechanism of this family of alpha 1,2-mannosidases. The s tereochemical course of hydrolysis of Man(9)GlcNAc by the yeast enzyme was followed by proton nuclear magnetic resonance spectroscopy. It wa s observed that beta-D-mannose is released from the oligosaccharide su bstrate, thereby demonstrating that the enzyme is of the inverting typ e. (C) 1995 Academic Press, Inc.