Carbohydrate Metabolism

Question 1

What is the general formula for a Carbohydrate and are they hydrophilic or hydrophobic?

Answer 1

(CH2O)n

Hydrophilic because they contain many OH groups and interact with water.

Question 2

How do monosaccharides enter cells?

Answer 2

They require transporters to pass across the cell membrane due to their hydrophilic nature.

Question 3

What is a monosaccharide?

Answer 3

It is a single sugar unit with 3-6 C atoms. It is partially oxidised and so requires less O2 for complete oxidation than a fatty acid with the same length carbon chain.

Question 4

How do monosaccharides form a disaccharide?

Answer 4

A glycosidic bond forms between the two units. -O-

Question 5

Give three examples of polysaccharides

Answer 5

Glycogen, Cellulose, Starch

Question 6

What is an Oligosaccharide?

Answer 6

It contains 3-12 sugar units and can also be called a Dextrin

Question 7

Why cannot cellulose be broken down?

Answer 7

It contains B glycosidic bonds which the GI enzymes cannot digest.

Question 8

How many stages are there in Catabolism and what happens at each stage?

Answer 8

1 - Large molecules are broken down into smaller building blocks which can be up taken by cells. This happens in the GI extracellularly.
2 - Building blocks are broken down into smaller molecules in this oxidative step (which releases some reducing power). Small amounts of ATP are produced
3 - TCA cycle. Acetyl is converted to carbon dioxide with the. Release of some energy as ATP.
4 - Mitochondrial step. H carriers are reoxidised and so this step requires oxygen. A large amount of ATP energy is released.

Question 9

What reaction does salivary amylase support?

Answer 9

Starch/ Glycogen —> Dextrins

Question 10

What is the enzyme which breaks down Dextrins and where does this occur?

Answer 10

Pancreatic amylase, this occurs in the small intestine and Dextrins are broken down into Monosaccharides.

Question 11

Disaccharases are found on the brush border in the small intestine. What are the four different enzymes which can be found here?

Answer 11

Lactase, Sucrase, Maltase (a1-4 bonds), Isomaltase (a1-6 bonds)

Question 12

How is glucose taken into cells from the blood stream?

Answer 12

Facilitated diffusion through GLUT1-5 transport proteins. This is hormonally controlled.

Question 13

What are the four cell types which have an absolute requirement for glucose?

Answer 13

Rbc, wbc, kidney, lens of eye.

Question 14

Where does glycolysis occur?

Answer 14

It is an intracellular pathway which occurs in the cytoplasm of all cells.

Question 15

What is the overall reaction for glycolysis?

Answer 15

Glucose (C6) + 2NAD+ + 2Pi + 2ADP –> 2 Pyruvate (C3) + 2NADH + 2H+ + 2H2O

Question 16

What is the function of glycolysis and what 3 useful things are produced?

Answer 16

To oxidise glucose.

ATP is synthesised from ADP, C6 and C3 intermediates are produced and NADH is produced.

Question 17

Which enzyme catalysts the reaction Glucose –> Glucose-6-phosphate?

Answer 17

Hexokinase. In the liver it is Glucokinase.

Question 18

What reaction does phosphofructokinase catalyse?

Answer 18

Fructose 6 phosphate –> Fructose 1,6 bis phosphate.

Question 19

In step 10 of glycolysis, ADP –> ATP. What is the enzyme which catalysts this step and what is the end product?

Answer 19

Pyruvate kinase and then end product is pyruvate.

Question 20

In glycolysis, what is the overall ATP synthesis?

Answer 20

At the beginning of the pathway, 2 moles of ATP are invested. After cleavage, 2 moles of ATP are produced for each pyruvate and so overall 2 moles of ATP is synthesised.

Question 21

What is the effect of a slower rate on an equilibrium reaction and a mono directional reaction?

Answer 21

In an equilibrium reaction it will increase the time it takes to reach equilibrium. In a mono directional reaction it will reduce the yield.

Question 22

Why does lactose intolerance cause diarrhoea?

Answer 22

There is a lack of lactase enzyme and so lactose is not broken down. It remains in the intestine, lowering the water potential drawing water in from the cells of the intestinal walls. This means there is more water in the colon and therefore diarrhoea.
Bacteria can break down this lactose producing gases which leads to bloating and discomfort.

Question 23

What is the additional enzyme needed for glycolysis to occur anaerobically?

Answer 23

Lactate Dehydrogenase. This reaction means that the H carrier NADH is reoxidised to NAD and so glycolysis can continue.

Question 24

What are the two fates of lactate?

Answer 24

In the heart, it is converted to pyruvate and then oxidised to form CO2.
In the liver it undergoes gluconeogenesis and is converted back to glucose.
This can occur because these tissues both have good oxygen supplies.

Question 25

What is the consequence of increases lactate in the blood?

Answer 25

Elevations in plasma lactate can occur and this means that it exceeds the renal threshold and is excreted in the urine but it also affects the buffering capacity of the blood - lactic acidosis.

Question 26

What can lead to an increased plasma lactate?

Answer 26

Strenuous exercise, hearty eating, congestive heart disease, liver disease, thiamine deficiency, alcohol metabolism.

Question 27

Why is DHAP (dihydroxyacetone phosphate) and important intermediate in glycolysis?

Answer 27

It can be converted to glycerol phosphate by the enzyme Glycerol-3-phosphate dehydrogenase. Glycerol phosphate is very important in TAG and lipid synthesis. The liver can also phosphorylate glycerol.

Question 28

What is bisphosphoglycerate mutase?

Answer 28

It is an enzyme which acts on 1,3 bisphosphoglycerate and converts it to 2,3 bisphosphoglycerate which is an important regulator in the O2 affinity of haemoglobin.

Question 29

Why can bagged blood only be kept for a certain amount of time? (Think 2,3 BPG)

Answer 29

Blood for transfusions is fed with glucose, but as this is used up, 2,3 BPG will be converted back to 1,3 BPG and will continue down the glycolysis pathway to produce energy. This means that if the bloody was transfused it would have a high affinity for oxygen and so would not release oxygen at the tissues and so it would not be beneficial due to the lack of 2,3 BPG.

Question 30

What is an allosteric regulator?

Answer 30

It is a molecule which binds at a site which is not the active site on an enzyme to either activate or inhibit its action.

Question 31

What molecule inhibits the action of Hexokinase?

Answer 31

Glucose 6 phosphate.

Question 32

In muscle, what impact does a high ATP to AMP ratio have on the action of phosphofructokinase?

Answer 32

It inhibits its action.

Question 33

In the liver, what activates the enzyme phosphofructokinase?

Answer 33

A high insulin to glucagon ratio

Question 34

What effect does a high insulin to glucagon ratio have on pyruvate kinase?

Answer 34

It activates it by dephosphorylation of the enzyme.

Question 35

What are two common sites where lactate is produced?

Answer 35

RBC and skeletal muscle.

Question 36

List the steps for fructose to enter glycolysis.

Answer 36

Fructose --> Fructose1phosphate by fructokinase | Aldose catalysts reaction to Glyceraldehyde 3 phosphate - enters glycolysis.

Question 37

What are the clinical consequences of deficiency of Fructokinase or Aldolase?

Answer 37

A lack of fructokinase has no clinical symptoms and fructose is excreted in urine. A lack of Aldolase means that levels of fructose1phosphate build up and this causes damage to the liver. Treatment is a fructose free diet.

Question 38

What reaction does galactokinase catalyse?

Answer 38

Galactose --> Galactose 1 phosphate

Question 39

What is the enzyme which catalysts the reaction of Glactose 1 phosphate to glucose 1 phosphate?

Answer 39

Uridyl transferase.

Question 40

What is the enzyme which is commonly deficient in galactosaemia? And what are the clinical consequences of this?

Answer 40

Uridyl transferase. This means that both galactose and galactose 1 phosphate accumulate, and they enter other pathways. When galactose is converted to galactitol it depletes the supply of NADPH which is needed to maintain sulphide ions in their reduced form. Inappropriate disulphide bridge formation occurs (in lens of eye this is cataracts).

Question 41

What is known the treatment for galactosaemia?

Answer 41

Complete removal of lactose from the diet.

Question 42

Why is the reaction in the penthouse phosphate pathway irreversible?

Answer 42

There is loss of CO2.

Question 43

What is the function of the penthouse phosphate pathway?

Answer 43

It produces NADPH in the cytoplasm which is used to maintain SH residues and also is bio synthetic reducing power used in lipid synthesis.

Question 44

What useful precursor does the pentose phosphate pathway provide?

Answer 44

It provides nucleotides for nucleic acid synthesis.

Question 45

What is the consequence of Glucose6phosphate dehydrogenase deficiency?

Answer 45

This is the enzyme involved in the pentose phosphate pathway so this means that there is reduced NADPH and so some inappropriate disulphide bridges may form which can lead to decrease protein functionality. In RBC Heinz bodies form.

Question 46

What is the name of the enzyme which catalysts the reaction Galactose --> Galactitol?

Answer 46

Aldose reductase