Carbohydrates

Question 1

Give some properties of carbohydrates

Answer 1

Highly oxidisable
Storage form of energy
Structure and protection
Cell to cell communication (ABO blood group antigens)

Question 2

What are monosaccharides?

Answer 2

Sugars that can’t be hydrolysed to form a simple sugar.

Most commonly hexoses (6carbons)

Question 3

What are the 3 main hexoses in human biochemistry?

Answer 3

Glucose
Galactose
Fructose

Question 4

What are disaccharides?

Answer 4

2 monosaccharides linked together by glycosidic bonds

Question 5

What is a glycosidic bond?

Answer 5

Covalent bond from when hydroxyl group of one monosaccharide reacts with an anomeric carbon of another monosaccharide.

Question 6

What are the 3 main disaccharides of human biochemistry?

Answer 6

Maltose
Sucrose
Lactose

Question 7

What is an anomeric carbon?

Answer 7

Mirror images of each other

It is carbon 1 on the glucose residue. Stabilises glucose and is the only residue that can be oxidised

Question 8

What is the difference between homo- and heteropolysaccharides?

Answer 8

Homo- single monomeric species

Hetero- have 2 or more monomer species

Question 9

What makes glycogen more extensively branched than starch?

Answer 9

It has alpha 1-6 bonds every 8-12 residues instead of every 24-30

Question 10

What are glycoproteins?

Answer 10

Proteins that have carbohydrates covalently attached

Question 11

What are the functions of carbohydrates attached to proteins?

Answer 11

Increase protein solubility
Influence folding and conformation
Protect it from degradation
Act as communication between cells

Question 12

What are GAGs?

Answer 12

Glycosaminoglycans

Unbranched polymers made from repeating units of hexuronic acid and an amino sugar which alternate through the chains

Question 13

What are Proteoglycans?

Answer 13

GAGs that have a protein covalently attached.

They form part of many connective tissues in the body.

Question 14

What are Glycoproteins?

Answer 14

Very similar to proteoglycans but there is more protein present than carbohydrate.

Found in ECM, cell membrane, blood and within cells

Question 15

What catalyses the final digestion of carbohydrates in the jejunum?

Answer 15

Mucosal cell surface enzymes

Question 16

What are the main products of carbohydrate digestion?

Answer 16

Glucose
Galactose
Fructose

Question 17

What type of carbohydrates cannot be digested by the gut?

Answer 17

Oligosaccharides

E.g cellulose and hemicellulose

Question 18

What are the benefits of oligosaccharides not being digested?

Answer 18

Increase faecal bulk and decrease transit time

Question 19

What are some causes of disaccharide deficiencies?

Answer 19

Severe intestinal infection
Inflammation of gut lining
Drugs injuring gut wall
Surgical removal of intestine

Question 20

What characterises disaccharide deficiencies?

Answer 20

Abdominal distension and cramps

Question 21

What is the most common disaccharide deficiency?

Answer 21

Lactose intolerance

Question 22

Why does the lack of lactase give disaccharide deficiency symptoms?

Answer 22

Undigested lactose is broken down by gut bacteria causing a build up of gas and irritant acids

Lactose is osmotically active so draws water from gut into lumen causing diarrhoea

Question 23

Why is glucose immediately phosphorylated when it reaches the liver or certain cells?

Answer 23

Glucose-6-phosphate can’t diffuse out of cells as GLUT transporters can’t recognise it

Question 24

What enzymes catalyse the phosphorylation of glucose into glucose-6-phosphate?

Answer 24

Glucokinase (liver)

Hexokinase (other tissues)

Question 25

What does the low Km and Vmax of hexokinase mean for tissue cells?

Answer 25

Low Km- even at low glucose levels tissues can grab glucose effectively.

Low Vmax- tissues are easily satisfied so don’t keep grabbing glucose

Question 26

Give some fates of glucose-6-phosphate?

Answer 26

Pentode phosphate pathway
Glycolysis
Glycogen storage

Question 27

What is the role of glucose-6-phosphatase?

Answer 27

Catalyses the release of glucose into the blood by converting glycogen into glucose-6-phosphate and then glucose.

Question 28

What is von Gierke’s disease?

Answer 28

Liver, kidney and intestine glucose-6-phosphate deficiency

Question 29

What is McArdle’s disease?

Answer 29

Skeletal muscle phosphorylase deficiency

Question 30

Describe the process of glycolysis?

Answer 30

A catabolic pathway that saves potential energy from glucose by forming ATP through substrate level phosphorylation

Question 31

What is the first committing step of glycolysis?

Answer 31

Phosphorylation of F-6-P to F-1,6 bisP. F-1,6bisP is solely destined for glycolysis. Irreversible step

Question 32

Where does glycolysis occur?

Answer 32

In the cytosol

Question 33

What does substrate level phosphorylation require?

Answer 33

Soluble enzymes and chemical intermediates

Question 34

Why does NAD+ need to be replenished?

Answer 34

NAD+ needs to be regenerated as it is limited within the cell. It’s comes from niacin an essential vitamin.

Pyruvate replenishes this supply by oxidising NADH.

NAD+ required for metabolising various intermediates within glycolysis

Question 35

What is the Cori cycle?

Answer 35

When the liver repays the oxygen debt muscles run up by converting lactate into glucose during gluconeogenesis

Question 36

What are the 4 bypass reactions of gluconeogenesis?

Answer 36

1 - pyruvate to oxaloacetate
2- oxaloacetate to PEP
3- F-1,6bisP to F-6-P
4- Dephosphorylation of G-6-P to Glucose

Question 37

Where does the last step in gluconeogenesis take place?

Answer 37

In the lumen of the ER.

Requires G-6-P to be shuttled in and glucose is then shuttled out

Question 38

What is the name of the pathway that produces NADPH for all organisms?

Answer 38

Pentode phosphate pathway