Carbohydrates

Question 1

Describe saccharides?

Answer 1

They are the most abundant biological molecules
(CH2O)n where n>3
Monomer = monosaccharides
Polymer = polysaccharide

Question 2

What are Fischer projections?

Answer 2

All vertical bonds point into the plane of the paper
All horizontal bonds point out of the plane of the paper

The assignment of D- or L- is based on the absolute configuration of the asymmetric centre farthest from their carbonyl group
D-sugars are the most biologically abundant form

Question 3

What are monosaccharides?

Answer 3

Aldehyde or ketone derivatives - containing at least 3 carbon atoms

If the C=O group is an aldehyde, the sugar = aldose
If the C=O group is a ketone, the sugar = ketose

Question 4

What are the names given to carbohydrates per number of carbons in the chain?

Answer 4

3C = trioses
4C = tetroses
5C = pentoses
6C = hexoses
7C = heptoses

Question 5

Describe aldoses

Answer 5

For aldohexose sugars all but two of its carbons are chiral
= 16 possible stereoisomers (2^4)

e.g. D-Glucose, D-Mannose and D-Galactose

Question 6

Describe ketoses?

Answer 6

For ketohexose sugars three of its carbons are chiral
= 8 possible stereoisomers (2^3)

e.g. D-Fructose

Question 7

What are epimers?

Answer 7

Differ only in the configuration around one carbon and as bonds need to be broken Interconversion can’t take place spontaneously

Question 8

How can monosaccharides be altered in a reaction?

Answer 8

They can react with alcohols

Alcohol + Aldehyde = Hemiacetal

Alcohol + Ketone = Hemiketal

Question 9

How can the alcohol reactions with monosaccharides also take place?

Answer 9

The hydroxyl group OH, within the monosaccharide can react with it’s own C=O group (intramolecularly) to form cyclic hemiacetal and hemiketals

Question 10

How are cyclic hemiacetals and hemiketals represented?

Answer 10

Haworth projections
The heavier ring bonds project in front of the plane of the paper
The lighter ring bonds project behind it

Question 11

What are the names given to the cyclic molecules formed after intramolecular reaction with OH?

Answer 11

Five-membered rings = furanoses
e.g. D-Fructofuranose

Six-membered rings = pyranoses
e.g. D-Glucopyranose

Question 12

What is an anomer?

Answer 12

Within a cyclised monosaccharide, the carbonyl carbon = anomeric carbon, is now the chiral centre with 2 configurations

The 2 stereoisomers differing at the anomeric carbon are anomers

Question 13

What are the 2 anomers?

Answer 13

alpha - OH groups:
Down, up, down, down

beta - OH groups:
Down, up, down, up

Question 14

What is significant about the two anomers of D-glucose?

Answer 14

They are diastereoisomers

They have slightly different physical and chemical properties (including optical rotations)
The anomers freely interconvert in aqueous solution = mutarotation

Question 15

Describe the structure of the rings?

Answer 15

Tetrahedrally (sp3) hybridised
Chair conformation (not planar)
The most stable form is the bulky substituent groups (OH and CH2OH) occupying equitorial positions - extending alternately above and below the ring

Question 16

How can sugars be modified and linked further?

Answer 16

1. Aldoses are oxidised, the aldehyde is oxidised to a carboxylic acid = aldonic acid (-onic acid at the end) e.g. D-Gluconic acid
2. Oxidation of primary alcohol group of aldoses = -uronic acids e.g. D-Glucuronic acid
3. Aldoses and ketoses can be reduced under mild conditions (e.g. NaBH4) yeilds polyhydroxy alcohols = alditols (-itol) e.g. ribitol
4. Monosaccharide units with one OH group replaced by H are known as deoxysugars
5. In amino sugars one or more OH groups can be replaced with NH2 - often then acetylated
   e. g. D-Glucosamine

Question 17

How are sugars linked to other compounds?

Answer 17

Glycosidic bonds - link the anomeric carbon to other compounds

The anomeric group can condense with an alcohol to form alpha or beta glycosides and releasing H2O

Question 18

Give an example of a glycosidic bond where a sugar is linked to another compound?

Answer 18

Formation of an N-glycosidic bond between the anomeric carbon and an amine

The link of D-ribose to purines and pyrimidines in nucleic acids

Question 19

Describe glycosidic bonds?

Answer 19

They hydrolyse very slowly under physiological conditions but fast at a low pH
If an anomeric carbon is involved in a glycosidic bond it cannot convert between a and b forms via linear form = aldehyde group is not available to react = non-reducing sugar

Saccharides with anomeric carbons, that have not formed glycosides = reducing sugars

Question 20

Describe polysaccharides?

Answer 20

Consist of monosaccharides linked by glycosidic bonds
AKA Glycans
Classified as homopolysaccharides and heteropolysaccharides, consisting of one type or more than one type of monosaccharide
Form branched and linear polymers

Question 21

What are disaccharides?

Answer 21

The simplest polysaccharides - many occuring as the hydrolysis of larger molcules

Examples:
b-galactose and glucose form lactose (found naturally in milk)
a-glucose and b-fructose form sucrose (highly abundant in plants, table salt)

Question 22

How do we systematically name sugars?

Answer 22

For Lactose:

1. Name the two component monosaccharides: galacto- gluco-
2. Describe the ring types: pyranosyl, pyranose (the glucose is still a reducing sugar = ose at the end)
3. What are the anomeric forms: b (galactose), glucose can be a or b so leave out
4. How are they linked: 1→4

Therefore = O-b-D-galactopyranosyl-(1→4)-D-glucopyranose

Question 23

What are some polysaccharides?

Answer 23

Structural:
Cellulose and Chitin

Storage:
Starch and Glycogen

Question 24

Describe the structure of cellulose?

Answer 24

Linear polymer of up to 15,000 D-glucose residues linked by b(1->4) glycosidic bonds
Cellulose fibres are chains of flat ribbons in which successive glucose rings are flipped 180 ° relative to the previous glucose ring - allowing H2O removal
C3-OH of each glucose forms a H-bond with the ring oxygen (O5) of the next glucose
Parallel cellulose chains form sheets with interchain H-bonds - microfibril
Contains lignin – a plastic-like phenolic polymer, to form a matrix of the cell wall = withstand large stress

Question 25

Describe the functional role of cellulose?

Answer 25

The primary component of a plant cell wall
Very strong and rigid as a result of the microfibril, allowing it to prevent the cell bursting by exerting an inward pressure

Insoluble - due to interlinking H bonds, it cannot adjust its conformation to minimise the hydrophobic contact with water
Essential as water continually passes through the cell wall and therefore doesn’t erode

Question 26

What are cellulases?

Answer 26

Enzymes (secreted by symbiotic microorganisms) within the digestive tract of herbivores
Hydrolyse the b(1->4) linkages of cellulose but it is very slow

Question 27

Describe the role and structure of Chitin?

Answer 27

The main structural component the exoskeleton of invertebrates - pretects the soft tissue underneath
Present in the cell wall of many algae and most fungi

A homopolysaccharide of b(1→4)-linked N-acetyl-D-glucosamine
Similar structure to cellulose (apart from acetyl-amine group on C2)

Question 28

Describe the structure of starch?

Answer 28

A mixture of glycans: a-amylose and amylopectin

A-amylose is a linear polymer of thousands of glucose residues linked by a(1->4) bonds
A-amylose forms a helically coiled formation
Amylopectin has mainly of a(1→4) linked glucose units but with a(1→6) branch points every 24 to 30 glucose units on average
There can be 10^6 glucose units in amylopectin – one of the largest molecules in nature

Question 29

Describe the functional role of starch?

Answer 29

As it is insoluble, it allows high concentrations of glucose to be stored without creating a high intracellular osmotic pressure
Starch is the primary source of carbohydrate in the human diet
It’s broke down by amylase in the mouth and small intestine, before a debranching enzyme is used
It is a reducing sugar - but only has one residue lacking the glycosidic bond = reducing end

Question 30

Describe the stucture of glycogen?

Answer 30

It is a storage polysaccharide in animals - prevalent as cytoplasmic granules in skeletal muscle and in the liver

Comprises of alpha glucose residues joined by 1,4 and 1,6 glycosidic bonds
It is highly branched with branch points occuring every 8-14 glucose residues

Glycogen’s a(1->4) bonds are cleaved for metabolic use by glycogen phosphorylase
Glycogen’s a(1->6) bonds are cleaved by glycogen debranching enzyme