0-Glucosidic linkages, enzymes hydrolyzing

Amylo-1 —> 6-glucosidase obtained by Cori and Larner218 from rabbit muscles, and R-enzyme isolated by Hobson, Whelan and Peat219 from potatoes and broad beans, are typical debranching enzymes, which will hydrolyze the 6 — 1-a-D-glucosidic linkage rather than the normal 4 —> 1-a-D linkage. These enzymes will therefore be particularly important in determinations of the fine structure of amylopectin, if they can be sufficiently well purified. 

A number of starch-converting enzymes belong to a single family termed the a-amylase family or family 13 hydrolases. This group of enzymes shares common characteristics such as an eight-stranded a/p barrel structure, the ability to hydrolyze 1,4-a-D-glucosidic linkages of attached polysaccharides in a-conformation, and conserved amino acid residues in the active sites of the enzymes (van der Maarel et al. 2002). 

The enzymes that hydrolyze (1 — 6)-a-D-glucosidic linkages in starch and glycogen and in related a- and /3-dextrins are called debranching enzymes. The nomenclature for this type of enzyme was confusing, but has been clarified. 

In contrast to the /S-amylase, the malt a-amylase (and other liquefying amylases) very rapidly lower the viscosity of starch pastes. When the relative viscosity has dropped to half the original value, only about 0.1% of the D-glucosidic linkages in the starch have been hydrolyzed. The malt a-amylase is a typical liquefying enzyme. 

Cellobiose differs from (-l-)-maltose in one respect it is hydrolyzed by the enzyme emulsin (from bitter almonds), not by maltase. Since emulsin is known to hydrolyze only jS-glucoside linkages, we can conclude that the structure of (+)-cellobiose dilfers from that of (+)-maltose in only one respect the D-glucose units... 

Two distinct groups of enzymes attack the (1— 4) a-n-glucosidic linkages the amylases, which hydrolyze them, and a number of transglucosylases... 

Glucoamylase was characterized in 1951 as an enzyme which hydrolyzed a-( 1 —> 4) glucosidic linkages in starch so as to remove successive glucose units from the non-reducing ends of the chains. Subsequently, it was demonstrated that the enzyme also hydrolyzes a-(l 6) and a-( 1 -> 3) glucosidic bonds, although at much slower rates (23). 

Lack of specificity of an enzyme for the linkage hydrolyzed in a mixed-linkage homopolysaccharide has already been mentioned (see p. 269), Rhizopus arrhizus laminaranase being able to split both (l->3)- and (l- 4)- -D-glucosidic linkages. The possibility that such an action may explain results difficult to account for on any other basis should always be borne in mind. 

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