Amylopectin phosphorylase action

Muscle also contains an amylo-1,6-glucosidase which hydrolyzes the l,6 -a-glucoside links of the limit dextrin obtained from phosphorylase action on glycogen or amylopectin. The products of the reaction are glucose and a dextrin which can further be partially phosphorolyzed by the phosphorylase. 

If ramified molecules exist in starch and in glycogen, it must be concluded that, under certain conditions, the attachment of the new D-glucose unit sometimes also takes place in the 6-position. In the case of amylopectin, the result is a polysaccharide with something like 5.5% 1,6-a-D-glucosidic linkages. Haworth, Peat and Bourne have shown that in addition to the phosphorylase discovered by Hanes (termed P-enzyme ) there is in the potato a second enzyme, termed Q-enzyme. The combined action of the two enzymes leads to the formation of branched-chain... 

In the method of Cori and Lamer, glycogen (or amylopectin) is completely digested by the concurrent action of muscle phosphorylase and amylo-(l 6)-glucosidase. n-Glucose (which arises only from residues attached to C6 of an adjacent residue) and n-glucosyl phosphate (which is obtained from all other residues) are determined, and the branch-point content is calculated from the proportion of D-glucose. Only 10-15 mg. of glycogen is required for each assay the method has been applied to more... 

The linkages of amylopectin and glycogen provide the cell with a glucose polysaccharide that upon action by degradative enzymes, such as amylases, isoamylases, debranching enzymes, or phosphorylases are easily converted into glucose and or a-Glc-1-P. 

Fig. 4.—Structures Around the Outermost Branch-points in the Limit Dextrins Remaining After Action of foeta-Amylase on Amylopectin and Glycogen. [Structure (b) is obtained uniquely by foeta-amylase action on phosphorylase limit-dextrins (see p. 315). For an explanation of the symbols, see footnote 49.]...

That this interpretation of the action of phosphorylase and the debranching enzyme is incorrect came later from the work of Walker and Whelan. They examined the structure of the limit dextrin prepared by the action of highly purified, muscle phosphorylase on amylopectin. Debranching of this dextrin by R-enzyme resulted in production of malto-tetraose. (Similar results were later obtained when glycogen -dextrin was debranched with pullulanase. ) It was therefore concluded that the unequal attenuation of the two chains by phosphorylase, proposed... 

Fig. 11.—Successive Actions of Phosphorylase, beta-Amylase, Isoamylase, and befa-Amylase on the Meyer Model of Amylopectin and Glycogen. [The symbolism is basically that of Fig. 9, modified to show the action of the various enzymes as follows - - - removed by pho horylase and heto>amylase , branch points hydrolyzed by isoamylase and-, immune to beta amy]ase after isoamylase action.]...

Fig. 12.—(a) Structure Proposed for Glycogen and Amylopectin by Whelan and Coworkers. [The symbols are as given in Fig. 9.] (b) Successive Actions of Phosphorylase, heia-Amylase, Isoamylase, and heta-Amylase on the Whelan Model. 

Star-shaped molecules possessing similar properties have also been obtained by the action of muscle phosphorylase on intact amylopectin. Both the synthesis and the degradation of starch are regulated by the level of inorganic phosphate. The highest rates of starch synthesis by spinach chloroplasts in the light occurred at <0.25 mmole of phosphate, whereas a ten-fold higher concentration of phosphate is required for the breakdown of starch in the dark. 

The majority of the carbohydrate chemists in this country have accepted the multibranched model for glycogen and amylopectin as the one representing a closer approximation to the true structure. Nevertheless, the Haworth and Hirst laminated model has been seriously considered as a possibility, inasmuch as it is consistent with practically all the data upon which Meyer and Bemfeld base their multibranched structure. Most of the experimental results can be interpreted so as to fit either model. By submitting glycogen and amylopectin to separate and consecutive action of phosphorylase and amylo-l,6-glucosidase, Lamer et oi. were able to examine the inner stmcture of these polysaccharides in... 

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