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), is a member of the Class 1 alpha 1,2-mannosidase family conserved from yeast to mammals. Although Class 1 alpha 1,2-mannosidases are essential 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 its homology with the mammalian enzymes make it a good model to study the catalytic mechanism of this family of alpha 1,2-mannosidases. The stereochemical course of hydrolysis of Man(9)GlcNAc by the yeast enzyme was followed by proton nuclear magnetic resonance spectroscopy. It was observed that beta-D-mannose is released from the oligosaccharide substrate, thereby demonstrating that the enzyme is of the inverting type. (C) 1995 Academic Press, Inc.