BCH 4053 Biochemistry I
Fall 2001
Dr. Michael Blaber


Lecture 12

Carbohydrates


Carbohydrate Nomenclature

 

Monosaccharides

Number of Carbons

(Generic monosaccharide name)

Aldose Functional Group

Ketone Functional Group

3

(Triose)

Aldotriose

Ketotriose

Triulose

4

(Tetrose)

Aldotetrose

Ketotetrose

Tetrulose

5

(Pentose)

Aldopentose

Ketopentose

Pentulose

6

(Hexose)

Aldohexose

Ketohexose

Hexulose

 

Stereochemistry

In addition to the general name of the monosaccharide, the stereochemistry about each chiral center is important.

Fischer Projection

The majority of saccharides in nature have the "D" isomer

Here are some examples of Fischer projections for some ketopentoses:

 

Stereochemistry terms:

 

Cyclic Structures and Anomeric Forms

Alcohols react readily with aldehydes to form hemiacetals:

Likewise, alcohols react with ketones to produce hemiketals:

 

Since aldoses and ketoses contain alcohol groups, in addition to their aldehyde or ketone groups, they have the potential to react to form cyclic forms.

Alternative ring structures

Ring geometry

Functional groups in the axial position are somewhat crowded together, whereas, the same functional groups in the equatorial position are separated from each other.

The preferred chair conformation is the one that minimizes close contacts between large functional groups - these will preferentially occupy the equatorial position

Derivatives of Monosaccharides

Oligosaccharides

The basic monomeric units that are used to build up complex oligosaccharides are essentially limited to:

Common disaccharides

Lactose

Sucrose

Higher Oligosaccharides

Plants produce various higher oligosaccharides. Humans may or may not have the enzymes necessary to break them down into useful monosaccharides (i.e. to break specific glycosidic linkages). However, bacteria in our intestine may have such enzymes. Therefore, if you eat some of such complex carbohydrates, you are simply nourishing the bacteria in your intestine. They will thank you by producing more of their own waste products - including certain gases.

Polysaccharides

The majority of carbohydrate material in nature occurs in the form of polysaccharides (also known as glycans). Other molecules that contain polysaccharide structures are also referred to as polysaccharides.

 

Glycosidic linkages involve only one anomeric carbon (keto or aldehyde) per monosaccharide. However, there are several potential hydroxyl acceptor groups on a given monosaccharide. Thus, polysaccharides can be branched as well as linear.

Polysaccharide Functions

Polysaccharides have historically been described as having functions related to storage, structure and protection. But, the full range of functions of polysaccharides have not yet been fully determined.

Storage.

 

Structure.

 


© 2001 Dr. Michael Blaber