Biological Molecules 2: Carbohydrates Flashcards
Name three common monosaccharides.
Glucose
Fructose
Galactose
How many chiral centres does glucose contain?
Four
What is an aldose?
A sugar that contains an aldehyde group where the carbonyl is located
What is a ketose?
A sugar that contains a ketone group where the carbonyl is located
How do Fischer projections represent a sugar molecule?
Defines each chiral centre by arranging the -OH groups so that each horizontal group points outwards from the molecule
What are the two types of Fischer projection for glucose?
L and D
Only D-glucose is found in nature
What are Haworth projections?
Projections that represent the 3D structures of molecules
Particularly in their ring form
Which form of representation best depicts a 3D molecule?
Chair representations
How is the pyranose/furanose ring formed in a sugar molecule?
One of the -OH groups must react with the anomeric carbon (the one on the carbonyl group) at the top of the molecule to create a ring
This forms a hemiacetal group
If an OH group points to the right on the Fischer projection, where is it located in the Haworth projection?
It will point down
Describe the steps involved in identifying a sugar from its representation.
1) Identify the anomeric carbon - this will have two oxygens attached - and this is carbon-1
2) Go around the ring anti-clockwise to determine the position of the -OH groups (if lower than hydrogen, it points down)
3) Convert the Haworth projection into a Fischer projection - break the bond to release the carbonyl carbon
4) CH2OH will be at the bottom then work out which direction the -OH groups point
How do you determine if a pyranose ring is alpha or beta?
If the resulting -OH group formed from the carbonyl points up, it is beta,
If it points down, it is alpha
How do anomers exist in equilibrium?
They do not exist in equal amounts
For example, in ribose, the straight chain only accounts for about 0.1%, pyranose (alpha and beta together) about 80%, and furanose about 20%
What is ribose?
A furanose sugar
Key component of RNA
Deoxyribose is a key component of DNA
What monosaccharides make up lactose and which bonds join them?
D-Galactose and D-Glucose
Joined by beta-1,4 glycosidic bonds
What monosaccharides make up sucrose and which bonds join them?
D-Glucose and D-Fructose
Joined by alpha-1,2 glycosidic bonds
Describe how glycosidic bonds are formed.
One OH group on the ring of one monosaccharide reacts with the anomeric carbon on another monosaccharide, releasing water in a condensation reaction
What are the most common glycosidic linkages?
1,2
1,4
1,6
Other than carbohydrate polymers, what molecules can be joined by glycosidic bonds?
Glycoproteins
Glycopeptides
Structures of DNA and RNA
What is amylose?
A polymer of D-glucose
Joined by alpha-1,4 glycosidic bonds
It is a plant starch
Easily digested into glucose molecules to be used in metabolic processes
What is cellulose?
A polymer of D-glucose (in its beta pyranose form)
Joined by beta-1,4 glycosidic bonds
Also a plant starch
But is not digested by humans due to its ability to form strong fibres
What is glycogen?
A polymer of D-glucose
Joined by both alpha-1,4 and alpha-1,6 glycosidic bonds (which enables branching)
Glycogen is found in the muscles and liver
What is chitin?
A polymer of a glucose-derivative
Joined by beta-1,4 glycosidic bonds
Also contains nitrogen acetylation which makes the polymer more rigid than cellulose
Why are D-glucose and D-fructose favoured energetically to sucrose?
Because they have improved biochemical availability
