Identifying Specific Carbohydrate Structures

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Biochemistry › Identifying Specific Carbohydrate Structures

Questions 1 - 10
1

The molecule sucrose is shown below

Sucrose

What is the full name for this molecule?

-D-glucopyranosyl-(12)--D-fructofuranoside

-D-glucopyranosyl-(12)--D-fructofuranoside

-D-glucopyranosyl-(14)--D-fructofuranoside

-D-glucopyranosyl-(12)--D-fructopyranoside

None of these

Explanation

Sucrose is the molecule shown below(in Haworth projection).

Sucrose

We recognize that it's a disaccharide, and that the first molecule is -D-glucopyranose. The bond to the second monosaccharide (fructose) is tricky because the glycosidic bond is going to the anomeric carbon of fructose. Therefore, this is a 1-2 linkage. Finally, since the anomeric carbon of fructose is pointing in the same direction as the 6th carbon, this is a -D-fructopyranose.

2

Which of the following enzymes is correctly paired with its products and substrates?

Explanation

An easy way to distinguish the substrate and the product from the enzyme is that the enzyme will end in "ase". The following are the common substrates, enzymes and the associated products.

3

Identify the carbohydrate class of the given molecule.

D ribulose

Ketopentose

Ketoquintos

Ketohexose

Aldopentose

Aldohexose

Explanation

The structure is a ketopentose. Keto- refers to the ketone at the second carbon and pentose refers to a carbohydrate with five carbons.

4

Which of the following is a storage polysaccharide of plants?

Starch

Cellulose

Glycogen

Maltose

Amylopectin

Explanation

Amylopectin & amylose are joined to make starch. Cellulose is found in the cell wall of plant cells. Glycogen and maltose are not found in plants.

5

Identify the given structure.

D glucose ring

D-glucose

D-fructose

D-galactose

L-glucose

L-galactose

Explanation

This ring structure is of D-glucose. The hydroxyl group on the first carbon (the carbon to the bottom right of the oxygenin the ring) can face either down (the alpha conformation) or up (the beta conformation). Note that the beta conformer is more thermodynamically stable since all the hydroxyl groups on the ring would be in the equitorial position, thus minimizing steric hinderance and intramolecular electrostatic repulsion.

6

Which of the following is the correct structure of a disaccharide?

Maltose = glucose + glucose

Lactose = galactose + fructose

Lactose = galactose + galactose

Sucrose = glucose + glucose

Maltose = sucrose + fructose

Explanation

The correct structures of the disaccharides are:

Maltose = glucose + glucose

Sucrose = glucose + fructose

Lactose = glucose + galactose

7

Which of the following enzymes is correctly paired with its products and substrates?

Explanation

An easy way to distinguish the substrate nd the product from the enzyme is that the enzyme will end in "ase". The following are the common substrates, enzymes and the associated products.

8

Identify the carbohydrate class of the given molecule.

D ribose

Aldopentose

Aldoquintos

Aldohexose

Ketohexose

Ketopentose

Explanation

The structure is an aldopentose. Aldo- refers to the aldehyde at the first carbon and pentose refers to a carbohydrate with five carbons.

9

Glucose and galactose share a chemical formula and are known as epimers.

What is the difference between glucose and galactose?

They differ in configuration around the fourth carbon

Galactose is spread further apart than glucose

Glucose is bigger than galactose

Galactose is a ketose

Explanation

As a pair of epimers, this means that both glucose and galactose are the same in configuration in all carbons except for one non-carbonyl carbon. To differ in the configuration of the carbonyl carbon makes an anomer not an epimer. Both glucose and galactose are aldoses.

10

What is the difference between two carbohydrates that are epimers and two carbohydrates that are anomers?

The location of the change in configuration

The shape of the ring

The number of carbons that have differences in configuration

The number of rings

Explanation

A set of anomers will differ in the configurations of their carbonyl carbons only while a set of epimers will differ in configuration in only one carbon that is not the carbonyl carbon.

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