Glycobiology building blocks III + IV

Question 1

what is a carbohydrate?

Answer 1

• Molecules containing carbon and water, with the general formula Cn(H2O)n and so also called “hydrated carbon”
• Plants make carbohydrates from carbon dioxide and water via the process of photosynthesis
• Carbohydrates can either be monosaccharides (single sugar monomer) or polysaccharides (chain of sugars)

Question 2

what is a monosaccharide?

Answer 2

• A carbohydrate that cannot be hydrolysed into a simpler form
• Glucose, fructose, galactose and ribose are examples
• Many monosaccharides bonded together form a chain called a polysaccharide
• In aqueous solutions monosaccharides with 5+ Cs in the backbone typically occur as cyclic structures

Question 3

What nomenclature can be used to describe monosaccharides?

Answer 3

• ‘ose’ at the end of the name indicates that the molecule is a sugar
• Prefixes e.g. ‘tri’-, ‘tetr-‘, ‘pent-‘ indicate the number of carbons in the sugar e.g. triose, pentose, hexose
• An additional prefix can be added to indicate whether the monosaccharide contains an aldehyde group (aldo-) or a ketone group (keto-) e.g. glucose can be described as aldo-hexose

Question 4

what is an aldose?

Answer 4

A monosaccharide that has the C=O group at the end of the carbon chain (known as an aldehyde group)

Question 5

what is a ketose

Answer 5

A monosaccharide that has the C=O on an internal carbon (not at the ends) - known as a ketone group

Question 6

What is a pyranose?

Answer 6

A six-membered ring formed of 5 carbons and 1 oxygen

Question 7

What is a furanose?

Answer 7

A five-membered ring formed from 4 carbons and 1 oxygen

Question 8

What are stereoisomers?

Answer 8

• Same molecular formula, atoms bonded in the same way but there is a difference in the arrangements of the atoms in space
• Includes: diastereomers, epimers, enantiomers and anomers

Question 9

What are structural isomers?

Answer 9

Molecules that have the same molecular formular but the atoms are arranged in a different order e.g. fuctose and glucose/galactose

Question 10

What is meant by asymmetric carbon (chiral centre)?

Answer 10

A carbon with 4 bonds, each of which is different

Question 11

What are diastereomers?

Answer 11

Two sugars that have the same chemical formula and differ only by the configuration of the hydroxyl groups on their asymmetric carbon

Question 12

What are epimers?

Answer 12

Diastereomers that are identical except for having one hydroxyl configured differently e.g. glucose and galactose differ only in the configuration of C4

Question 13

What are enantiomers?

Answer 13

two sugars which have the exact opposite configurations for all the hydroxyls, making them mirror images of eachother e.g. D-glucose (left) and L-glucose (right)

Question 14

What is an anomeric carbon?

Answer 14

• The new asymmetric carbon formed when a sugar converts from a straight chain structure into a ring structure
• The anomeric carbon can have the hydroxyl (-OH) in two different positions, referred to as α (below the ring) and beta (above the ring)
• N.B. not the same as chiral carbon!!!

Question 15

What are anomers?

Answer 15

Sugars that only differ in the configuration of the anomeric carbon e.g. alpha-D-glucose and beta-D-glucose

Anomers = cyclic epimers

Question 16

What is a disaccharide?

Answer 16

Two monosaccharides joined together via a condensation reaction

Question 17

What happens during a condensation reaction and what is the name of the bond formed?

Answer 17

The OH group of one monosaccharide combines with the H of another, releasing a molecule of water and forming a covalent bond known as a glycosidic bond

Question 18

What is trehalose?

Answer 18

Disaccharide formed of 2 glucose molecules
requires enzyme trehalase

Question 19

What is lactose?

Answer 19

Disaccharide formed of one galactose and one glucose

Glycosidic bond is hydrolysed by lactase

Question 20

What is sucrose?

Answer 20

Disaccharide formed of one fructose and one glucose
hydrolysed by sucrase

Question 21

What is a polysaccharide?

Answer 21

• A long chain of monosaccharides linked by glycosidic bonds
• Homopolysaccharides contain only a single type of monosaccharide
• Heteropolysaccharides contain two or more different kinds of monosaccharide
• can be branched or unbranched

Question 22

What is an oligosaccharide?

Answer 22

A SHORT chain of sugars (up to 10)

Question 23

What is glycogen? where is it stored

Answer 23

• A branched polymer of glucose monomers
• The storage form of glucose in humans and other vertebrates
• a 1,4 glycosidic bonds in chain hydrolysed by glycogen phophorylase, a 1.6 glycosidic bonds in branch points hydrolysed by debranching enzyme

stored in liver and muscle cells:
-in liver, when blood glucose low glucagon is released to breakdown glycogen via hydrolysis
- in skeletal muscle, adrenaline can cause glycogen breakdown

Question 24

What is starch?

Answer 24

• A mixture of 2 homopolysaccharides of glucose (amylose and amylopectin) which differ only in that amylopectin has additional glucose branches linked by alpha-1,6-linkages
• Plants store excess glucose made during photosynthesis as starch
• Starch is broken down by alpha-amylase into maltose, maltotriose and a1,6 linkages left as dextrins: further broken down by maltase and isomaltase, maltae acts on a1,4 linkages of maltoe and maltotriose, isomaltase acts on remaining a1,6 in the a limit dextrins

Question 25

what is cellulose?

Answer 25

• Major component of plant cell walls
• Unbranched chain of glucose monomers linked by beta-1,4-glycosidic bonds
• Every other glucose is flipped resulting in long, straight, non helical chains
• Chains cluster together to form parallel bundles and are held in place by hydrogen bonds
• Cellulose is processed by bacteria in the foregut of ruminants and hindgut of others e.g. horse

Question 26

What is the process of fermentation in ruminants?

Answer 26

• Hydrolysis of polysaccharides to glucose.
• Glucose is broken down to pyruvate by glycolysis (by the microbes).
• Pyruvate converted to Volatile Fatty Acids (VFAs) by species of bacteria in anaerobic conditions e.g
  ○ Acetate (2C)
  ○ Propionate (3C)
  ○ Butyrate (4C)
  and gases (methane and CO2)
• Animal absorbs the VFAs and other useful compounds made by the bacteria such as vitamins and amino acids.
  VFA can be also used to make acetyl CoA and glucose.

Question 27

what is chitin?

Answer 27

• Similar to cellulose with beta 1,4 glycosidic bond
• Monomers of N-acetyl-glucosamine
  Found in exoskeletons of invertebrates

Question 28

What are glycoproteins are their roles in the body?

Answer 28

-proteins modified with carbohydrate component
-protein with one or more oligoaccharides
- can be found in the outer surface of the plasma membrane, in the ECM and in the blood
- viruses can use glycoproteins to form a protective coat around themselves to hide from the immune system
- some hormones are glycoproteins e.g. equine chorionic gonadotrophin
- glycoproteins such as prothrombin and fibrinogen are involved in blood clotting
- o-linked glycosylation or N-linked glycoslylation

Question 29

What are proteoglycans and their roles in the body?

Answer 29

• carbohydrate modified with a protein component
• They are macromolecules of the cell surface or extracellular matrix
• The basic proteoglycan units consisting of a ‘core protein’ with covalently attached glycosaminoglycans
• major components of connective tissue such as cartilage
• proteoglycan aggrecan is major component of cartilage
• heparin which is an anti-coagulant and often added to blood tubes

Question 30

What is a glycosaminoglycan?

Answer 30

• Long unbranched polysaccharides consisting of a repeating disaccharide unit
  These polymers provide viscosity, adhesiveness and tensile strength to the extracellular matrix
• Have a strong negative charge meaning they attract water (hydrophillic) and so they can resist huge compressive forces which is crucial in parts of the body such as as the joints

Question 31

what are glycolipids?

Answer 31

• Cell membrane lipids in which the hydrophilic head groups are polysaccharides
  
  • The polysaccharides act as specific sites for recognition by carbohydrate binding proteins
    glycolipids: endotoxin
    Eg: Lipo-polysaccharide - the dominant surface feature of the outer membrane of gram-negative bacteria such as Escherichia coli

Question 32

what are glycoproteins

Answer 32

protein with one or more oligoaccharides
- found on outer surface of plasma membrane, in ECM and blood
- o-linked glycosylation or N-linked glycoslylation

Question 33

function of glycoproteins

Answer 33

• found on surface of lipid bilayer
• hydrophillic nature
• hormones can be glycoproteins
• blood clotting
• reproduction
• immune system recgonition

Question 34

example of glycosaminoglycan

Answer 34

heparin- anticoagulant

Question 35

what disorder might rewuire heparin

Answer 35

• cardic thrombosis
• pulmonary thrombois