Dextrins limit

Limit dextrins Limiting amino acid Limiting current... 

Reaction Mechanisms. Pathways. Bioreacllons. and Bioreactors Chap 7 Starch - a-dextrin —> Limit dextrin - Maltose... 

In contrast, amylopectin contains 1—>6 glycosidic branch points at regular intervals. These result in an open, treelike stracture. Amylopectin is more readily hydrolyzed by amylase, and this results in a mixture of small, branched maltodextrin molecules that are often referred to as limit dextrins. Limit dextrins are further hydrolyzed by maltase. 

Amylase occurs in many plants, such as barley, wheat, rye, soy beans, and potatoes, where it is generally accompanied by some a-amylase. [ -Amylase initiates hydrolysis at the nonreducing end of an amylose or amylopectin chain, and removes maltose units successively until the reducing end of the molecule is encountered in amylose or a branch is met in amylopectin. ( -Amylase is used commercially in the preparation of maltose symps. After P-amylase hydrolysis of amylopectin there remains a P-amylase limit dextrin. ( -Amylase has been used as a probe of the fine stmcture of amylopectin (43-46). 

LimitDextrins. Limit dextrins are oligosaccharides containing one or more 1,6-a-Hnkages (see Carbohydrates). 

The glycogen phosphorylase reaction degrades glycogen to produce limit dextrins, which are further degraded by debranching enzyme, as already described. 

The only example of this technique applied to the amylose component is that already described, of the action of Z-enzyme on the /3-limit dextrin. In the case of amylopectin, enzymic methods enable a distinction to be made between the proposed laminated and highly ramified structures (I and III, in Fig. 1, page 352). The method used by Peat and coworkers101 involves the successive action of /3-amylase and R-enzyme on waxy maize starch. /3-Amylolysis will degrade A-chains down to two or three units from the 6 —> 1-a-D interchain linkages. These latter linkages will protect the... 

B-chains until they are acted on by R-enzyme, when maltose or malto-triose will be produced from the residual A-chain, and linear dextrins from the B-chains. The amount of maltose or maltotriose liberated on treating the /3-limit dextrin with R-enzyme will be a measure of the number of A-chains in the molecule, and from these data, the ratio of A B chains in the molecule can be calculated.220 Peat concluded that multiple branching is an intrinsic part of the amylopectin structure, as the observed yield of these sugars was greater than expected for a singly-branched structure. It should be noted that glycogen has been shown by similar enzymic methods to possess a truly random structure.221... 

Table X 4 summarizes similar data for the hydrolysis by maltase-free malt alpha amylase of beta dextrins obtained from arrowroot starch by the action of beta amylase. The beta dextrins were precipitated with alcohol from the reaction mixture of arrowroot starch after it had reached a limit in the hydrolysis at 60% theoretical maltose. The beta dextrins were hydrolyzed extensively by malt alpha amylase. Glucose was liberated in very small amounts even in the later stages of the hydrolysis of these beta dextrins maltose was liberated in appreciable amounts and, at equivalent hydrolyses, appeared to be formed somewhat more rapidly from the beta dextrins (Table X) than from the untreated starch (Table IX). Upon hydrolysis with malt alpha amylase the molecular weights of the beta dextrins dropped appreciably but not as extensively as when arrowroot starch was hydrolyzed directly by malt alpha amylase.

Fig. 6.—13C-N.m.r. Spectra of A, /3-Limit Dextrin of Rabbit-Liver Glycogen (aqueous solution ambient temperature chemical shifts based on tetramethylsilane) and B, Waxy-barley Amylopectin [in D,0 at 70° chemical shifts (8C) based on external tetra-methylsilane].

It is noteworthy that most of the chemical shift values for all three polymers may be closely approximated ( ) by calculations based on data for monomeric reference compounds. These findings illustrate, therefore, the general validity of studies on low molecular weight model compounds for einalysis of spectra of carbohydrate polymers. Many examples of equally satisfactory comparisons of this kind are to be found in studies on other polysaccharides (11,23). These polymers include glucans (l6), mannans (2k, 2 ), limit dextrins (26), lichenin (2j), agarose (28) and various polysaccharides of fungal and microbial orgins (e.g., 7,8,29-31). Observed departures from expectation have been attributed to specific conformational influences ( 8). 

This enzyme [EC 3.2.1.10] (also referred to as oUgo-1,6-glucosidase, sucrase-isomaltase, and limit dextrinase) catalyzes the hydrolysis of l,6-o -D-glucosidic linkages in isomaltose and dextrin products generated from starch and glycogen via a-amylase. See also Sucrase... 

Figure 6-5. Glycogenolysis. Degradation of glycogen occurs stepwise by hydrolysis of one glucosyl unit at a time from the nonreducing ends by phosphorylase. The limit dextrin occurs as indicated in the second step when there are four glucosyl units remaining to a branch point. Once debranching enzyme has resolved the limit dextrin, degradation by phosphorylase can resume.

Xu, A., Seib, P. A. (1997). Determination of the level and position of substitution in hydroxypropylated starch by high- resolution IH-NMR spectroscopy of alpha-limit dextrins. J. Cereal ScL, 25,17-26. 

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