Action of Alpha- Amylase on starch

Prior to a race, many marathon runners will try to increase their glycogen concentrations by loading up with foods with high starch content, such as pasta. α-Amylase secreted by the pancreas will digest the starch into which of the following major products?

A. Amylose, amylopectin, and maltose

B. Glucose, galactose, and fructose

C. Glucose, sucrose, and maltotriose

D. Limit dextrins, maltose, and maltotriose

E. Limit dextrins, lactose, and sucrose


Answer- The correct answer is-D- Limit dextrins, maltose, and maltotriose

Starch consists of two fractions.

About 20% is a water soluble material called Amylose. The majority of the starch is a much higher molecular weight substance, consisting of nearly a million glucose units, and called amylopectin.

(a) Amylose is a linear polymer of α-D-glucose, linked together by α 1→4 glycosidic linkages. It is soluble in water, reacts with iodine to give a blue color and the molecular weight of Amylose ranges between 50, 000 – 200, 000 (figure-a).


Figure- Structure of amylose and amylopectin- components of starch

(b) Amylopectin is a highly branched polymer, insoluble in water, reacts with iodine to give a reddish violet color. The molecular weight ranges between 70, 000 – 1 000, 000. Branches are composed of 25-30 glucose units linked by α 1→4 glycosidic linkage in the chain and by α 1→6 glycosidic linkage at the branch point (figure-b).

α-Amylase hydrolyzes α(1→4) glycosidic bonds present in starch (amylose and amylopectin) in a random fashion leaving primarily the disaccharide maltose, the trisaccharide maltotriose, and an oligosaccharide known as the α-limit dextrin, which is composed of 6 to 8 glucose residues with one or more α(1→6) glycosidic bonds.

As regards other options-

Amylose, amylopectin, and maltose- are not the hydrolytic products of starch. Maltose can be correct but amylose and amylopectin are the components of starch which are hydrolyzed by alpha amylase.

Galactose and fructose are not present in starch.

Similarly sucrose and lactose are also not present in Starch.

Hence the most suited option is – Limit dextrins, maltose, and maltotriose.

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