Which polysaccharide(s) can the enzyme alpha-amylase break?
A) glycogen
B) starch
C) cellulose
D) both glycogen and cellulose
E) both glycogen and starch
The correct answer and explanation is:
The correct answer is E) both glycogen and starch.
Alpha-amylase is an enzyme that catalyzes the breakdown of polysaccharides, specifically those made up of glucose units. It works by hydrolyzing the internal α-1,4-glycosidic bonds in polysaccharides. Both glycogen and starch are made up of glucose units linked by α-1,4-glycosidic bonds, making them substrates for alpha-amylase.
Glycogen is a storage polysaccharide in animals, primarily found in the liver and muscles. It has a highly branched structure, consisting of glucose molecules connected by α-1,4-glycosidic bonds with α-1,6-glycosidic bonds at the branch points. Alpha-amylase can break down the linear α-1,4-glycosidic bonds in glycogen, releasing smaller chains of glucose called dextrins. The enzyme acts on the non-reducing ends of glycogen and progressively cleaves the glucose units.
Starch, found in plants, is also composed of glucose molecules linked by α-1,4-glycosidic bonds. Starch exists mainly in two forms: amylose (a linear polymer) and amylopectin (a branched polymer). Alpha-amylase can break down both forms, although it acts more efficiently on amylopectin due to the branched structure that provides more accessible cleavage sites.
Cellulose, however, is made up of β-1,4-glycosidic bonds, which are structurally different from the α-1,4-bonds in glycogen and starch. Alpha-amylase cannot break down cellulose because it lacks the ability to hydrolyze β-1,4-glycosidic bonds. Cellulose is a major structural component of plant cell walls, and its breakdown requires enzymes like cellulase, not alpha-amylase.
Thus, alpha-amylase can only break down glycogen and starch, which have α-1,4-glycosidic bonds, and not cellulose, which has β-1,4-glycosidic bonds.