Nonreducing-end labeled

Figure 5. S3mthes is of nonreducing-end labeled branched mal to-dextrins by reaction of dextransucrase with labeled sucrose and nonlabeled maltodextrins reaction with (A) maltose, (B) maltotriose, and (C) maltotetraose. Symbols are the same as in Figure 4.

Figure 7. Synthesis of the smallest nonreducing-end labeled maltodextrin, maltopentaose, (A) by reaction of phosphorylase with nonlabeled maltotetraose and labeled a-glucopyranosyl-1-phosphate and dual labeled maltopentaose (B) by reaction with reducing-end labeled maltotetraose and labeled a-gluco-pyranosyl-1-phosphate. Symbols are the same as in Figure 2.

It should be noted that the aldehydic carbon atom of the triose enediol released from the reducing end originates from C-3 of the hexose, whereas the carbonyl group of the D-glyceraldehyde formed from the nonreducing end originates from C-4. Thus, in the absence of isomerization, hexoses labeled at either C-l or C-6 would mainly yield lactic-3-14C acid. However, extensive isomerization does occur, and lactic-2-14C acid and lactic-I-14C acid are also found in considerable proportion. The presence of lactic-2-14C acid is explained by the reaction of the dicarbonyl compound 78 that undergoes a benzilic... 

Fig. 27.6. Glucoamylase activity. Glucoamy-lase is an a-1,4 exoglycosidase, which initiates cleavage at the nonreducing end of the sugar. Thus, for malotriose, the bond labeled 1 will be hydrolyzed first, which frees up the bond at position 2 to be the next one hydrolyzed.

Maltodextrins and isomaltodextrins specifically labeled in (a) the reducing-end glucopyranosyl unit, (b) the nonreducing-end glucopyranosyl unit, (c) a specific number of labeled glucopyranosyl units at one or the other end or both of the chain ends, and (d) all of the glucopyranosyl units uniformly labeled, find uses in the study of the function of the dextrins and especially in the study of the mechanisms of enzymes that interact with the dextrins and related substrates. Branched maltodextrins and cyclodextrins can also be specifically labeled. All of the methods that will be discussed involve the use of specific enzymes that are commercially available or can be readily prepared in the laboratory. 

Figure 4. Synthesis of (A) reducing-end labeled isomalto-dextrins by reaction of dextransucrase with nonlabeled sucrose and labeled sucrose and labeled a-methyl-D-glucopyranoside synthesis of (B) nonreducing-end isomaltodextrins by reaction with labeled sucrose and nonlabeled isomaltose and synthesis of (C) dual labeled isomaltodextrins by reaction with labeled sucrose and reducing-end labeled isomaltose. The circle represents a glucopyranosyl unit and a circle with a slash represents a reducing glucopyranose unit a triangle represents the fructose unit black are labeled and white are unlabeled.

C labelled nonreducing end a substrata of the enzyme to As shown in Figure 7, radio-n disappeared quite rapidly, ylopectin molecule was hydro-The labelled hydrolysis... 

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