Starch hydrolysis 3-limit dextrin

In this connection it may be mentioned that many ordinary yeasts ferment, generally very slowly, isomaltose and trisaccharides (limit dextrins) with one maltose and one isomaltose linkage. - Dried yeasts ferment starch and limit dextrins mth a fairly high velocity. This fermentation is probably not due to a hydrolysis of the saccharides to D-glucose or maltose, but to a phosphorolysis to the Cori ester (a-D-glucopyranose 1-phosphate). 

P-Amylase Hydrolysis of a-l,4-glucan bonds in polysaccharides (starch, glycogen, etc.), yielding maltose and beta-limit dextrins. 

Now as the limit dextrins are stable toward the amylases, it seems sound to assume that they are built according to another scheme than are those parts of the starch which 3ueld maltose. In order to explain the incomplete saccharification and the formation of the limit dextrins, the author has assumed - that, although the starch molecules are built substantially according to the maltose scheme, there are at certain intervals anomalies of one kind or other which constitute hindrances to the enzyme action. Since in most cases 70 to 80% of maltose is formed from starch, it must be concluded that the normal action of the amylases is the opening of 1,4-a-D-glucosidic links. When the constitution of the chain differs in some way from that of maltose, the enzyme cannot attach itself to the anomalous part of the chain and no hydrolysis occurs. The author assumed that when, for instance, /5-amylase acts on starch, the... 

In a second experiment with potato starch, the hydrolysis was carried further so that only 6% of the starch was recovered as limit dextrins. About one-third of these dextrins were trisaccharides. 

It was mentioned above that salivary amylase hydrolyzes normal a-dextrins, for instance, a malto-he.xaose (and the corresponding acid) at practically the same velocity as it hydrolyzes starch. This means that the affinity of the enzyme for long and for short chains is about the same. Consequently, the curve for the hydrolysis by salivary amylase has not the peculiar form of the curve for malt a-amylase but is continuous and has no sharp break. The a-dextrins which are formed at first are saccharified very rapidly. The normal a-dextrins are saccharified completely. The anomalous a-dextrins form the limit dextrins. 

As some fractions of the limit dextrins produced by the common amylases have a much higher phosphorus content than starch, the author assumed that the substitution with phosphoric acid is one of the anomalies postulated as the cause of the limit dextrin formation. That the phosphoric acid acts in this way is evident from the investigations of Posternak, who isolated a tetraose phosphate from a mixture of limit dextrins. On acid hydrolysis this gave D-glucose 6-phosphate. 

During the incubation of starch a-dextrins with maltase-containing saliva for 2-3 months, small proportions of panose are formed. This probably represents the action of the maltase impurity on true tetra- and pentasaccharide a-limit dextrins, rather than a third stage of hydrolysis. 

Some methods available offer the use of am-yloglucosidase alone. Work done with com and potato starches illustrated that even vmder ideal conditions amyloglucosidase does not fully convert starch to dextrose, although the shortfall is small. The limit dextrin was more noticeable in corn starch hydrolysis. Proof was obtained by studying the reaction kinetics, and analyzing the hydrolysates by ion chromatography using pulsed amperometric detection. There remained always a small amount of a limit dextrin, and in the case of potato starch some other low molecular mass residues. 

Figure 1 Expanded chromatograms from Dionex ion chromatograph of enzyme hydrolysates of corn and potato starch, the use of a single enzyme, amyloglucosidase (AMG), and a mixture of AMG, a-amylase (a-A), and pullulanase (PU), to show the effect on hydrolysis of limit dextrin. Anion exchange column AS6, with pulsed amperometric detection. Gradient flow solvents (1) ISOmmolT NaOH and (2) ISOmmoll NaOH + 500mmol I NaOOCCHs. Postcolumn addition of 0.3mmoll NaOH.

The results of a detailed comparison of the relative efficiencies of different procedures for solubilizing the membrane-bound a-o-glucosidases of porcine intestinal mucosa have been reported. Procedures for the selective solubilization of certain members of the complex group of a-D-glucosidases have b en developed. The selective solubilization led to the conclusion that separate a-D-glucosidases (isomaltase and limit dextrinase respectively) are involved in the hydrolysis of isomaltose and the a-limit dextrins formed from starch by a-amylase. 

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. 

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