218 IMMUNO-CATALYSIS 



AMYLASES 



2. Specificities of Amylases 



Starches can be separated into at least two fractions, amylase 

 (a-amylose) and amylapectin (^ff-amylose). Amylose consists of un- 

 branched chains of glucose residues combined in a-l,4-glucosidic link- 

 ages. Amylopectin consists of both unbranched and branched chains of 

 glucose units, and contains a-l,6-glucosidic linkages as well, which are 

 present at the point of branching. 



Glycogen, unlike starches, is homogeneous in the sense that it con- 

 sists only of branched chains. However, the a-l,4-glucosidic linkage 

 is the most important structural feature common to both starch and 

 glycogen. The enzyme ^S-amylase acting upon glycogen attacks the 

 outer branches composed of maltose units, yielding 47 per cent maltose 

 and 53 per cent dextrin resistant to jS-amylase. The outer branches of 

 amylopectin are likewise degraded by /^-amylase to the extent of 55 

 per cent to maltose, leaving residual dextrin resistant to this enzyme. 

 Dextrin is composed chiefly of units of hexasaccharides. After the 

 action of yS-amylase on starch ceases, the resulting dextrin is attacked 

 by a-amylase. According to Myrback (1947), ^S-amylase attaches itself 

 to free non-reducing end-groups of normally formed chains in which 

 the glucose residues are united by the a-l,4-glucosidic linkages. If, at 

 a certain point in the molecule, the structure deviates from this 

 pattern the enzyme action stops at this point. Since branching in amy- 

 lopectin occurs on the sixth carbon of some of the glucose units in the 

 chains (Meyer, 1943), these 1,6-linkages are probably responsible for 

 stopping the hydrolysis at or near the points of branching. Thus, 

 )8-amylase splits the exterior unbranched maltose chains of starch, ex- 

 posing branch points to the action of a-amylase. The action of the latter 

 enzyme makes branches of glucose units in a-l,4-glucosidic union again 

 available for the action of jS-amylase until a junction (branch point) 

 once more obstructs the decomposition. 



According to Myrback (1947), a-amylase is not only independent of 

 free end-groups, but its action is successfully hindered by the proximity 

 of end-groups. It has the capacity of attacking and rupturing any 

 maltose linkage in a chain molecule of starch type, but the velocity is 



