Properties of Neoglycoproteins

The heat stability of glutaminase-asparaginase modified with glu-taraldehyde-activated glycopeptides was slightly greater than that of the native enzyme, as was its susceptibility to trypsin.108 This modification decreased the pi by several pH units, and altered the association behavior the glycosylated enzyme had a much wider distribution of molecular weights as determined by sedimentation equilibrium. 

Asparaginase modified with lactose by reductive animation was more stable to heat and tryptic digestion than the native enzyme.64 The degree of stability increased as the number of glycosyl groups attached was increased. The modified enzyme consisted of two populations with respect to pi one with the same pi ( 5) as the native enzyme, and one with a slightly lower pi. The attachment of 3 -0-(N-ace- 

A number of enzymes to which soluble dextran had been attached after activation by cyanogen bromide have been characterized by Marshall and coworkers.7 All of the conjugated enzymes were found to be more stable than the native enzymes to heat, proteases, denatur-ants, and the absence of calcium. The attachment of dextran appeared to have stabilized the conformation of the enzyme. It was suggested that the attachment of the enzyme to the dextran polymer at several points fixed its conformation in much the same way as do intramolecular, disulfide bridges.7 

Different methods of preparation produce neoglycoproteins that manifest different properties. Ideally, neoglycoproteins should possess the same physicochemical properties as the unmodified proteins, especially if the effect of the newly attached carbohydrate is to be examined. 

Modification reactions that neutralize charges or introduce hydro-phobic residues usually lower the enzymic activity. The attachment of monosaccharides to alpha amylase by diazo coupling lowered the activity.12 This enzyme was stable to the reaction conditions for diazo coupling (pH 10,15 min at 0°) if the diazonium salts were not included in the solution. Inclusion of maltose in the reaction mixture to protect the active site lessened, but did not eliminate, the loss of activity, suggesting that the incorporation of hydrophobic structures, or the modification of a critical residue distant from the active site, was at least partly responsible for the loss of activity. 

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