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A 50-year-old female presents with severe abdominal pain. Her serum amylase and lipase levels are abnormally elevated and she is diagnosed with pancreatitis. Which linkage between glucose residues is cleaved by amylase?

A. α- 1, 4

B. α- 1, 6

C. β-1, 4

D. α 1, 2

E. β-1, 6

Answer- A) alpha – 1, 4 is the right answer.

Amylase acts on starch and glycogen to cleave α- 1, 4 glycosidic linkages.

Dietary polysaccharides (starch and glycogen) are digested first by salivary amylase to form dextrins that are further shortened by pancreatic amylase to form maltose. Maltose is further digested by maltase to form two glucose residues (figure-1).

Both starch and glycogen are polymers of glucose. The two main constituents of starch are amylose (13–20%), which has a nonbranching helical structure, and amylopectin (80–85%), which consists of branched chains composed of 24–30 glucose residues united by α-1,4 linkages in the chains and by α-1 ,6 linkages at the branch points (figure-2).

Glycogen, a more highly branched structure than amylopectin contains chains of 12–14 α-D-glucopyranose residues (in α-1, 4 glucosidic linkage) with branching by means of α-1, 6 glucosidic bonds (figure-2).

Amylase acts on alpha – 1, 4 glycosidic linkages only, the branch point containing α- 1, 6 linkage cannot be cleaved by amylase. Both starch and glycogen are substrates for amylase, having the structural similarities.

Oveview of digestion of carbohydrates

Figure-1- An overview of digestion of carbohydrates. Salivary digestion is limited due to shorter duration of stay of food in the oral cavity. The polysaccharides are mainly digested by Amylases (salivary and pancreatic, that are isoenzymes, they catalyze the same reaction, but differ from each other in their physical properties)

As regards other options

Cellulose a polymer of glucose consists of β-D-glucopyranose units linked by β-1, 4 bonds to form long, straight chains strengthened by cross-linking hydrogen bonds. Mammals lack any enzyme that hydrolyzes the, β-1, 4 bonds, and so cannot digest cellulose (figure-3).

α 1,2 linkage, more precisely- O-α-D-glucopyranosyl-(1,2)-β -D-fructofuranoside linkage is found in sucrose that is hydrolyzed by sucrase.

Β-1, 6 linkages are not common in nutrients.

Amylose, amylopectin and glycogen structures

Figure-2- Structure of Amylose, amylopectin and glycogen. Amylopectin and glycogen are structurally alike, except for the fact that glycogen is more branched than amylopectin. Both components of starch and glycogen contain alpha 1, 4 glycosidic linkages, hence they are ideal substrates for Amylase.

Comparison of  strcutures of starch, glycogen and cellulose

Figure-3- Comparison of structures of starch, glycogen and cellulose. Starch and glycogen are similar structurally whereas cellulose, despite being a polymer of glucose cannot be digested by human beings as it contains beta 1, 4 glycosidic linkages. Cellulose is digested by Cellulase enzyme.


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