Dextrins 0-amylase limit

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). 

When the substrate was amylopectin (3-amylase limit dextrin, a different pattern of products was formed, namely Gl, G2 and G3, with no G6, G7 or higher sized dex-trins.14 Reaction with the (3-limit dextrin indicated that Gl, G2 and G3 are formed from the chains between the o-( 1 —6) branch linkages as the outer chains were removed by the action of (3-amylase (see Section 7.2 for a discussion of the action of (3-amylase). It further indicated that G6 and G7 from amylopectin were formed exclusively from longer unbranched outer chains. It also indicated that the number of glucosyl units between the branch linkages of amylopectin were sufficiently few that they could not yield the larger G6 and G7 products, but could give the smaller Gl, G2 and G3 products. 

Maltose (3-amylases are primarily found in plants and have been isolated from sweet potatoes,27 soybeans,28 barley29 and wheat.30 Maltose (3-amylases are also elaborated by bacteria, e.g. by Bacillus polymyxa,31 B. megaterium,32 B. cereus33 and Pseudomonas sp. BQ6.34 These (3-amylases all produce (3-maltose and a high molecular weight (3-limit dextrin. The limit dextrins result when the enzyme reaches an a-(l—>-6) branch linkage, which it cannot pass. Approximately half of an amylopectin molecule is converted to (3-maltose the remaining half is the (3-limit dextrin. 

Figure 7.21 Determination of the structure of Bacillus amyloliquefaciens a-amylase limit dextrin, using enzymes no reaction with (3-amylase (b) reaction with pullulanase to give maltose + maltotriose (c) reaction of glucoamylase to give two tetrasaccharides, both ofwhich are eventually converted into panose + glucose. Analysis of the reactions can be made by thin layer chromatography239.

Formation of 3-Amylase Limit Dextrins of Amylopectin and Determination of their Fine Structure... 

In comparative studies of starch fractions, Wilson, Schoch and Hudson, and also Kerr, - showed that the amylose fraction of starch gave much higher yields of Schardinger dextrins (up to about 70 %) than did the amylopectin fraction. Starch modification or degradation products in general gave reduced yields especially with beta amylase limit dextrins or acid hydrolyzed starch sirups there was no detectable production of crystalline dextrins. 

The cross-reactivity of dextrans, glycogens, and synthetic polyglucoses in anti-Pn IX and XII was attributed to multiple n-glucose residues in SIX and SXII. From tbe cross-reactivity of /3-amylase limit dextrin and its parent glycogen, Heidelberger and his colleagues inferred that the cross-... 

D-galactose and L-rhamnose. Evidence for the presence of isomaltose residues in SXVIII accrued also from cross-reactions of various polysaccharides in anti-Pn XVIII. Polysaccharides which contained o-d-(1—>6)-linked D-glucose residues (dextrans, glycogen, -amylase limit dextrin) precipitated horse anti-Pn XVIII and, in general, the degree of reaction was related to the proportion of this linkage in the structure. 

At the concentration used, debranching of amylopectin and its beta-limit dextrin was incomplete, and there was no action on glycogen. First-stage, alpha-amylase limit-dextrins, those containing side chains of... 

The /3-amylases in the absence of the a-amylases are incapable of degrading whole starches completely. The hydrolysis proceeds rapidly until about 50 to 55 % of the theoretical amount of maltose is produced and then very slowly until a limit of about 61 to 68% is reached (101), The solution is still viscous and the residue, called a /3-amylase limit dextrin, is unfer-mentable. The limit dextrin arises from the inability of /3-amylase to act beyond a branch point in the randomly branched amylopectin molecule and may be envisaged as a pruned amylopectin structure. In the case of potato starch, the /3-limit dextrin includes all the associated phosphate. The limit dextrin contains one end group for every 10 to 12 D-glucose residues (102), in contrast to one in every 25 or 30 residues for the original amylopectin. The initial attack of /3-amylase on amylopectin is about 20 times as fast as on amylose (103), Maltose in amounts of 53 to 62 % of the theoretical have been reported from the action of /3-amylases on amylopec-tins separated from various starches (104). When the /3-limit dextrin is cleaved by acid hydrolysis or by the action of a-amylase, the structure is opened and new chain ends are made available which can be further acted upon by i3-amylase. 

Figure 11.1. Structures of pancreatic a-amylase limit dextrins. The arrows indicate positions of slow hydrolysis.

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... 

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.

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... 

---