Practice Questions

Chapter 9

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The following are practice questions designed to test your understanding of lecture and textbook material.

A. Provide a brief answer to the following questions. In some cases, a structure will suffice. In others, you may need a structure and/or several sentences in order to answer the question.

1. Draw a structure of a disaccharide of glucose having an a-(1-4) linkage.

2. How many different disaccharides of glucose are possible?

3. Based on the linkages between carbohydrates, discuss the different types of glucose residues are in amylopectin.

4. Discuss how the repeating unit of starch differs from that of cellulose.

5. The complete hydrolysis of melezitose, a non-reducing sugar found in honey, gives two molecules of D-glucose and one molecule of D-fructose. Enzymatic hydrolysis of melezitose gives D-glucose and sucrose. Exhaustive methylation of melezitose converts all free hydroxyl groups to methyl ethers (OCH3 groups) and gives 1,4,6-trimethyl-D-fructose, the structure of which is shown below. What is the structure of melezitose?

 

6. Trehalose is a disaccharide composed entirely of D-glucose. Exhaustive methylation of trehalose converts all free hydroxyl groups to methyl ethers (OCH3 groups) and enzymatic hydrolysis of this methylated product gives exclusively 2,3,4,6-tetramethyl-a-D-glucose, the structure of which is shown below. What structure would you propose for trehalose?

B. Select a response that best answers the question and place the letter in the blank in before the question.

_____ 1. Which of the following statements about raffinose is true?

(A) it is a reducing trisaccharide with one glucose residue

(B) it is a reducing trisaccharide with two glucose residues

(C) it is a non-reducing trisaccharide with no glucose residues

(D) it is a non-reducing trisaccharide with one glucose residue

(E) it is a non-reducing trisaccharide with two glucose residues

 

_____ 2. What statement best describes the following compound?

(A) it is a sugar phosphate

(B) it is an amino sugar

(C) it is a sugar alcohol

(D) It is a deoxy sugar

(E) it is a sugar acid

 

_____ 3. Which term best describes a-L-fucose?

(A) sugar phosphate

(B) amino sugar

(C) sugar alcohol

(D) deoxy sugar

(E) sugar acid

_____ 4. Which term best describes the relationship between D-lyxose and D-xylose?

 

 

(A) enantiomers

(B) diastereomers

(C) anomers

(D) tautomers

(E) none of the above

_____ 5. Which term best describes the relationship between D-gluconic acid and D-gulonic acid?

(A) enantiomers

(B) diastereomers

(C) anomers

(D) epimers

(E) none of the above

ANSWERS:

1. The following are both valid answers. Both display an a-(1-4) linkage but one has an a- and the other a b-hydroxyl group at the reducing terminus.

 

2. There are five hydroxyl groups in the first glucose that could form an acetal with the second. Four of these are simple hydroxyl groups at C-2, C-3, C-4 or C-6; the fifth is the hydroxyl group at C-1 that is part of the hemiacetal bond. This question reduces to the problem of the number of possible ways in which the hydroxyl groups at these five positions can be linked to the second glucose. Each linkage between one glucose and a second could involve either an an a- or b-linkage. In addition, each disaccharide formed using the C-2, C-3, C-4 or C-6 hydroxyl groups could have a reducing terminus in either an a- or b-configuration. Therefore, there are 4 isomers for each of these four or a total of 16 possible isomers from the C-2, C-3, C-4 and C-6 hydroxyl groups. The remaining fifth hydroxyl group at C-1 can participate in an acetal bond with the second glucose in four possible ways. Grand total is twenty possible disaccharides.

Four of the twenty possibilities are shown below in which the C-4 hydroxyl group of one glucose is linked to the hemiacetal center of the second.

 

5. Enzymatic hydrolysis gives sucrose and glucose. The key to this problem is to figure out where this second glucose molecule is attached to sucrose. The answer comes from the the permethylation produce, 1,4,6-trimethyl-D-fructose. In this structure, we see that one hydroxyl group is not methylated. It must be the place where the second glucose is attached in melezitose. We cannot tell, from the information given, whether this second glucose is attached in an a- (as drawn below) or b-configuration.

6.

 

Part B.

1. D

2. C

3. D

4. B

5. B