09. Structure and functions of carbohydrates

Question 1

Straight chain fructose can form which 2 structures?

Answer 1

1. Pyranose (6-membered ring)
2. Furanose (5 membered ring)

Question 2

What is an anomeric carbon?

Answer 2

Anomeric carbon is the carbon atom that bears the aldehyde / ketone group in the straight chain form

• for glucose / galactose : carbon 1
• for fructose : carbon 2

Question 3

What are anomers?

What are the 2 kinds of anomers?

Answer 3

A type of stereoisomer that differs in configuration at the hemiacetal or acetal carbon, a specific type of epimer

At carbon 1/2 (anomeric carbon),
- if - OH group point downwards : α epimer
- if - OH group point upwards : β epimer

Question 4

What are epimers?

Answer 4

A pair of carbohydrates which vary in one position for the placement / configuration of the -OH group at a chiral carbon.

(except for the 5th carbon, must be on same side. If on different side, it becomes D and L sugar)

Question 5

Why are chair conformations preferred over boat conformations?

Answer 5

Chair conformations are sterically less bulky as compared to boat conformations

• in chair conformation, bulky groups are placed in equatorial position
• However, in boat conformation, bulky groups are placed in axial position

Question 6

Out of all the monosaccharides and disaccharides, which one of them is NOT a reducing sugar?

Answer 6

Sucrose

Both of its anomeric carbons are involved in glycosidic bond.

Question 7

in cells, why cant free glucose molecules be stored, and need to be stored as large polymers (polysaccharides)?

Answer 7

Free glucose molecules can disrupt the osmotic balance of the cell, while polysaccharides are not osmotically active.

Question 8

In animals, what is glucose stored as, and most commonly in which organs?

What kinds of glycosidic bonds are present?

Answer 8

Glycogen, mostly stored in muscle and liver.

α-1,4 glycosidic bonds

Question 9

In plants, what is glucose stored as and what kind of glycosidic bonds are present ?

Answer 9

Starch

a. amylose : α-1,4 glycosidic bonds
b. amylopectin : α-1,4 glycosidic bonds and α-1,6 glycosidic bonds in between layers

Cellulose
- β -1,4 glycosidic bonds

Question 10

What are glycoproteins?

Answer 10

Covalent attachment of a carbohydrate group to a protein

Question 11

What kind of linkages can glycoproteins have and how do these links form?

Answer 11

1. O-linkage
   - when C2, anomeric C, of sugar binds to the –OH group in side chain of Serine and Threonine residues (note: though tyrosine has –OH in side chain, it is not involved in O linkages)
2. N-linkage
   - when C2, anomeric C, of sugar molecule binds to –NH2 group in side chain of asparagine

*Note: sugar can refer to monosaccharides like glucose, or their derivatives like N-acetylglucosamine (amino sugar)

Question 12

What are the 2 types of glycoproteins and what are their major components?

Answer 12

1. Proteoglycan
   - major component : carbohydrate (~95%)
2. Mucin
   - major component : carbohydrate (~80%)

Question 13

In the 2 types of glycoproteins, proteoglycan and mucin, carbohydrates make up the major component (95%/80%). This means that by mass, the carbohydrate component is heavier than the protein component. True or False?

Answer 13

FALSE. Although carbohydrates may be present in higher percentage, the protein component still has a larger mass than the carbohydrate component.

Question 14

Glycoproteins are components of ___?
What are some functions of glycoproteins?

Answer 14

cell membrane

Functions of glycoprotein (non-exhaustive):
- cell signaling : act as receptors for hormones or drugs to bind to
- lubricant and protective agent (mucin)
- hormones

and many more

Question 15

In proteoglycans, protein is attached to a polysaccharide known as?

What is this polysaccharide made of?

Answer 15

Glycosaminoglycan

It is a polymer made of repeating units of disaccharides, and each disaccharide is made of:

• a sugar molecule, where –OH group on C2 to is bonded to
• Carbon molecule of an amino sugar (sugar molecule like glucose, with one –OH group changed into amino group)

Sugar and amino sugar held together by O-glycosidic bond

Question 16

What are some functions of proteoglycans? [3]

Answer 16

1. Structural components
2. Lubricants
3. Mediate adhesion of cells to extracellular matrix

Question 17

What is the specific example of a type of proteoglycan and its function?

Answer 17

aggrecan, a key component of cartilage which acts an a shock absorber.

• It can cushion against compressive force, and absorption of water allows it to spring back after getting deformed by force.

Question 18

What is the carbohydrate moiety that is usually present in mucins?

Answer 18

N-acetyl galactosamine, an amino sugar

Question 19

What is the function of mucins?

Answer 19

It is the key component of mucus (for lubrication)

Question 20

How does protein glycosylation occur at the ER?

Answer 20

• Ribosomes are attached onto the cytoplasm of ER membrane to synthesize proteins.
• As protein is synthesized by the ribosomes, it is translocated across or into the ER membrane into the lumen (the interior) of the ER.
• Once inside the ER lumen, the protein can undergo post-translational modifications such as glycosylation, where carbohydrate molecules are added to the protein.

Question 21

Where does N/O-glycosylation of proteins occur?

Answer 21

• N-glycosylation (to form N linked glycoproteins) : begins in Endoplasmic Reticulum (ER) and continues in Golgi Complex
• O-linked glycosylation : takes place exclusively in the Golgi Complex.