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DENF 1521 Biochemistry

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Review of Carbohydrate Structure

Lesson 2.1
Basic nomenclature of carbohydrates

Instructions
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NAME:
Dental Biochemistry Brush
DB Bullet Lesson 2.1 Carbohydrate nomenclature

2.1 Review of Carbohydrate Nomenclature

2.1A Lesson objectives

The objectives of this lesson are to understand that:

  1. Carbohydrates are organic aldehydes or ketones with multiple hydroxyl groups
  2. Carbohydrates serve many biological functions
  3. Stereochemistry of carbohydrates is very important
  4. Sugars can form disaccharides and polysaccharides via the O-glycosidic bond
  5. Knowing the structures of common mono- and disaccharides is important

2.1B What are carbohydrates?

Carbohydrates are organic ketone or aldehyde molecules containing multiple hydroxyl groups. Carbohydrates function:

  1. As energy stores, fuels, and metabolic intermediates
  2. As structural molecules in RNA and DNA
  3. As structural components of cell walls
  4. By being linked to many proteins and lipids

Monosaccharides are the simplest of the carbohydrates being an aldehyde or ketone with two or more hydroxyl groups. These have the general formula (CH2O)n. The aldehyde with n=3 is glyceraldehyde; the ketone dihydroxyacetone. These are called trioses.

Glyceraldehyde has a single asymmetric carbon and therefore, two stereoisomers exist; D-glyceraldehyde and L-glyceraldehyde. The D and L prefixes refer to the absolute configuration around the C-2 carbon of trioses. These structures are shown in the following table.

The structure of the trioses

D-glyceraldehyde

An aldose

L-glyceraldehyde

An aldose

Dihydroxyacetone

A ketose

Aldoses which have 4 carbons are called tetroses; 5 carbons, pentoses; 6 carbons, hexoses. Glucose, an aldose, and fructose, a ketose, are two important hexoses which will reappear many times in the lessons to follow. Tetroses, pentoses, and hexoses also can occur in the D or L configuration and this refers to the absolute configuration around C-3, for tetroses, C-4, for pentoses, and C-5 for hexoses. The 2 tables below contain the stereochemical configurations of the D-aldoses and D-ketoses.

The term epimer refers to D-sugars which differ in configuration at only 1 asymmetric carbon. For example, D-glucose and D-galactose are epimers at C-4; D-glucose and D-mannose are epimers at C-2.

 

Dental Biochemistry Brush
DB Bullet Lesson 2.1 Carbohydrate nomenclature
Basic nomenclature and stereochemistry
Practice
Exercise 1:

A carbohydrate is an organic aldehyde or ketone with 2 or more of which of the following groups attached?

No Response
Phosphate
Hydroxyl
Sulfhydryl
Polypeptide


Practice
Exercise 2:

The basic structural formula for a carbohydrate is

No Response
(CH2O2)n
(C2H2O )n
(CH2O)n
(CHO)n



Practice
Exercise 3: 		Which of the following statements describes the relationship between D-glucose and D-galactose?

No Response
They are epimers
One is an aldose and one is a ketose
They differ in configuration about 3 carbons
They have different numbers of carbon atoms


Practice Exercise 4:

What molecule is represented by the structure shown below?

 Item 5 structure
No Response
D-mannose
L-galactose
D-glucose
D-ribose


Practice
Exercise 5:
Which of the following statements about the sugar D-fructose is NOT true?

No Response
It is a ketose
It is a monosaccharide
It is an epimer of glucose
It is a hexose



Dental Biochemistry Brush
DB Bullet Lesson 2.1 Carbohydrate nomenclature

2.1C Cyclization and conformation of glucose and fructose

Another property of sugars is the formation of the hemiacetal and hemiketal. These are the predominant forms of glucose and fructose in solution and occur when the open chain form described above cyclizes. An aldehyde can react with an alcohol to form a hemiacetal. The 6-member ring structure results from the reaction of the C-1 aldehyde with the C-5 hydroxyl in glucose. This ring formed by the cyclization of glucose is called a pyranose ring because of its similarity to pyran.

A ketone can react with an alcohol to form a hemiketal. The 5-member ring structure results from the reaction of the C-2 aldehyde with the C-5 hydroxyl in fructose. This ring formed by the cyclization of fructose is called a furanose ring because of its similarity to furan. The ring structures of glucose and fructose are shown below.

The ring structures of glucose and fructose
alpha-D-Glucopyranose ring
beta-D-Glucopyranose ring
alpha-D-Fructofuranose ring

The 6-member pyranose ring (glucose) cannot exist as a planar structure due to conformational constraints. The substituents on the ring may exist as an equatorial substituent or an axial substituent. Equatorial bonds are almost parallel to the plane of the ring while axial bonds are almost perpendicular to the plane of the ring. This results in 2 conformations for the ring, the boat and the chair forms. The presence of large substituents in the equatorial positions favor the chair form energetically. The predominant physiological form of beta-D-glucopyranose is the chair form because in this form all of the axial positions are hydrogens.

Chair form of beta-D-glucopyranose chair
Dental Biochemistry Brush
DB Bullet Lesson 2.1 Carbohydrate nomenclature

 

Sugar conformation
Practice
Exercise 6:

A pyranose ring is a six-membered ring.

No Response
True
False
Practice
Exercise 7:

The most stable form of beta-D-glucopyranose is the boat form.

No Response
True
False



Practice
Exercise 8:

The formation of a hemiacetal occurs between an aldehyde and a

No Response
Ketone
Hydroxal group
Glucopyranose
Phosphate
Dental Biochemistry Brush
DB Bullet Lesson 2.1 Carbohydrate nomenclature

2.1D The O-glycosidic bond

The formation of a bond between the C-1 of glucose and the oxygen atom of methanol or another sugar is called an O-glycosidic bond. In this way sugars are linked together to form disaccharides and polysaccharides.

Disaccharides are 2 sugar molecules linked together by an O-glycosidic bond. Some examples of common disaccharides are sucrose (glucose and fructose), lactose (galactose and glucose), and maltose (2 glucose molecules). Sucrose is table sugar, lactose is the sugar found in milk, and maltose results from the digestion of starch. The specific structures of these disaccharides are shown in the following table.

Some common disaccharides

Sucrose
(Glucose-alpha(1---2)-fructose)

sucrose

Lactose
(Galactose-beta(1---4)-glucose)

lactose

Maltose
(Glucose-alpha(1---4)-glucose)

maltose

In order to be absorbed in the gut, sucrose, lactose, and maltose must be digested to monosaccharides. There are specific enzymes for this purpose, the disaccharidases sucrase, lactase, and maltase. These enzymes are found on the outer surface of the epithelial cells which line the intestinal tract. Many adults are deficient in the enzyme lactase and this leads to the condition known as lactose intolerance.

Dental Biochemistry Brush
DB Bullet Lesson 2.1 Carbohydrate nomenclature
Disaccharides
Practice
Exercise 9: 		An O-glycosidic bond forms between two sugar molecules to form a disaccharide.

No Response
True
False


Practice
Exercise 10: 		The disaccharide lactose is composed of glucose and which of the following sugars?

No Response
Mannose
Galactose
Maltose
Fructose
Dental Biochemistry Brush
DB Bullet Lesson 2.1 Carbohydrate nomenclature

2.1E Summary

After completing this lesson you should understand the following about the basic structure and nomenclature of the carbohydrates important to metabolism.

  1. Carbohydrates are organic aldehydes or ketones containing multiple hydroxyl groups
  2. Carbohydrates have many biological functions including serving as energy stores, fuels, and metabolic intermediates, being components of RNA and DNA, and serving as structural components of cell walls.
  3. Carbohydrates can also be linked covalently to many proteins and lipids
  4. The stereochemistry of carbohydrates is very important
  5. Sugars can form disaccharides and polysaccharides via the O-glycosidic bond

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Final Instructions


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End Lesson 2.1
Basic nomenclature of carbohydrates

 

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