Kinetic Studies on the Enzyme Debranching of Guar Gum

Abstract

1. The degree of debranching of concentrated guar gum substrate (ca. 30% w/v) can be accurately controlled by varying the amount of α-D-galactosidase added to the guar flour. The reaction is allowed to continue until the enzyme becomes inactivated so that the degree of debranching is proportional to the amount of enzyme added.

2. The degree of debranching can also be accurately controlled by varying the concentration of guar gum used in the α-galactosidase reaction, keeping the amount of enzyme used constant. The degree of debranching achieved by this method is inversely proportional to the concentration of guar gum used.

3. Efficient mixing of the enzyme into the concentrated guar gum flour has been shown to be essential for good debranching. Little debranching was achieved when the α-galactosidase was only poorly mixed into the concentrated flour pastes, indicating that the free diffusion of the enzyme in the damp flour is very limited over long distances.

4. Kinetic parameters of the α-galactosidase acting on guar were determined. K_m was 0.02–0.06 M depending on the method of analysis used. V_max was 542–625 μ moles galactose released/mg enzyme/min.

5. Galactose is a product of the reaction and acts as an inhibitor of the α-galactosidase. Quantification of this inhibitory effect is difficult because it appears that the debranching reaction proceeds both by transfer of galactose to water to form free galactose, and also by direct transfer to free galactose (as the acceptor molecule) to form di-galactose and then larger oligosaccharides. This second reaction occurs especially in the initial stages of the debranching reaction and when concentrated guar substrates are used.

6. Attempts to minimize the colour of the enzyme-modified guar were unsuccessful. These were the use of lipoxygenase to degrade carotenoids present in the guar and the use of phosphatidyl choline as an inhibitor of the Maillard reaction between amino acids and the galactose released by the α-galactosidase.

Key Words:

• Guar gum
• α-D-galactosidase
• Debranching
• Enzyme-modified guar