Protein matrix effects on glycan processing by mannosidase II and sialyl transferase from rat liver

Interactions of concanavalin A with glycoproteins. A quantitative precipitation study of concanavalin A with the soybean agglutinin

Certain oligomannose-type glycopeptides have been previously shown to be bivalent for binding to concanavalin A and capable of precipitating the lectin by forming homogeneous cross-linked lattices [L. Bhattacharyya, M. I. Khan, and C.F. Brewer, Biochemistry, 27 (1988) 8762-8767]. In the present study, the effect of protein environment on the binding properties of an oligomannose-type oligosaccharide has been examined through quantitative precipitation analysis of the interactions of concanavalin A (Con A) with the soybean (Glycine max) agglutinin (SBA), which is a tetrameric glycoprotein possessing a single Man9-oligomannose chain per monomer. The results showed that SBA forms two different types of cross-linked complexes with tetrameric Con A, depending on the relative ratio of the two molecules in solution. At a concentration of one equivalent or less, SBA forms a 1:1 complex with Con A. At concentrations exceeding one equivalent, SBA forms a 2:1 complex with Con A. However, SBA forms only 1:1 cross-linked complexes with dimeric forms of Con A, such as acetyl- and succinyl-Con A. The results demonstrated that the total valency of the carbohydrate of SBA is a function of both the quaternary structure of Con A, as well as the relative ratio of SBA to Con A. In addition, the individual Man9-oligosaccharide, which as a glycopeptide is bivalent for binding to Con A, expresses univalency when present on the protein matrix of SBA.

Regulation of glycan processing by Golgi enzymes from red kidney bean (Phaseolus vulgaris) seedlings

The effect of a protein matrix on the processing of glycoprotein glycans by Golgi enzymes from plant seedlings has been determined with an artificial glycoprotein system, comparing the processing rates of glycan-(biotinyl)Asn (or glycan-(biotinamidohexanoyl)Asn) substrates either free or bound to avidin. An analysis of the pooled glycoproteins from the seedlings suggested that the most common glycan structure is a complex one (GlcNAc-Man3GlycNAc2-protein), and consistent with this processing end-product, mannosidases I and II and GlcNAc transferases I and II were all found to be present in the seedling Golgi membrane preparations. The effect of the avidin matrix either in a proximal (biotinyl substrates) or distal (N-(biotinamido)hexonoyl substrates) association with the appropriate glycan substrate for these four enzymes was assessed from the direct comparison of the apparent first-order rate constants for the free and avidin-bound substrate-product conversions. All four plant enzymes were inhibited by the association of the glycan substrates with avidin, but the inhibition was much less pronounced than that observed with the corresponding enzymes from rat liver and hen oviduct. The rate effect shows a progression from 3- to 10-fold rate decreases in the proximal complexes and 2- to 3-fold in the distal complexes in going from the first (mannosidase I) to the fourth (GlcNAc transferase II) enzyme; with the mammalian and avian enzymes the largest effects were for the first ones and much larger absolute rate effects were observed. The results suggest that the nature of the processing enzymes in terms of this response to the avidin glycan substrates may differ in different organisms.