Lipids

Question 1

Name the three classes of lipids.

Answer 1

Fatty acid derivatives
Hydroxy-methyl-glutaric acid derivatives
Vitamins

Question 2

How are triacylglycerides formed?

Answer 2

They are formed from a glycerol backbone and three fatty acid molecules. They are a product of esterification reactions.

Question 3

Give properties of TAG and state where in the body it can be found.

Answer 3

TAG is a hydrophobic molecule which is stored in its anhydrous form in adipose tissue.

Question 4

What are the products of TAG breakdown and where are they metabolised?

Answer 4

Glycerol –> Liver. Metabolised by glycolysis.

Fatty Acids –> Adipose / Target Tissues. Fatty Acid oxidation releases energy.

Question 5

What enzyme in the small intestine catalyses breakdown of TAG?

Answer 5

Pancreatic lipase.

Question 6

What are the two tissues/ cells where Fatty acid oxidation cannot be used to release energy?

Answer 6

Red Blood Cells (these have no mitochondria) and the brain (cannot pass the blood-brain barrier)

Question 7

Name two hormones which can activate fat mobilisation.

Answer 7

Adrenaline and Glucagon.

Question 8

Explain why when there is low Extracellular glucose, FA are oxidised.

Answer 8

When glucose is low, there is less substrate for TAG synthesis (glycerol-1-phosphate) being produced from glycolysis, so this leads to an accumulation of FA in the tissues. These are oxidised to release energy.

Question 9

Why are some fatty acids essential?

Answer 9

Fatty acids with a double bond after C9 cannot be synthesised by the body and therefore they must be sourced from the diet.

Question 10

Where does activation of fatty acids occur and what is the enzyme responsible for this?

Answer 10

Activation occurs in the cytoplasm.

Fatty acyl coA synthase.

Question 11

What transporter protein is used to transport activated fatty acids into the mitochondria for B oxidation?

Answer 11

Carnitine shuttle

Question 12

Name the substance which inhibits the Carnitine shuttle to prevent breakdown of newly synthesised FA.

Answer 12

Malonyl~coA

Question 13

When a Cn fatty acid enters B oxidation, what is the product?

Answer 13

A C(n-2) molecule and also a C2 which is acetyl~coA and enters the TCA cycle.

Question 14

What is the enzyme which converts glycerol from TAG breakdown into glycerol phosphate?

Answer 14

Glycerol kinase.

Question 15

Acetyl~coA is a useful intermediate for anabolism and catabolism. What anabolic processes is it a precursor for?

Answer 15

Hydroxymethylglutaric acid derivatives including Ketone Bodies and Cholestrol.
Fatty Acids

Question 16

Name three ketone bodies

Answer 16

Acetoacetate, acetone and B hydroxybuturate.

Question 17

How do statin drugs lower Cholestrol?

Answer 17

Statin drugs lower Cholestrol formation by inhibiting HMG~coA reductase which is a step in the metabolic pathway for Cholestrol synthesis.

Question 18

How is production of ketone bodies regulated?

Answer 18

Ketone body production is regulated because the starting molecule is acetyl~coA which is diverted from the TCA cycle. When NADH is high this leads to product inhibition, and so acetyl~coA is diverted to alternative pathways.

Question 19

What is the name of the enzyme which catalyses the reaction from HMG~coA to acetoacetate?

Answer 19

Lyase. This is inhibited when Insulin to Glucagon ratio is high as HMG~coA is used for Cholestrol synthesis. When this ratio is low, lyase is activated and ketone bodies form.

Question 20

How does muscle protein breakdown provide glucose for the brain?

Answer 20

Protein - amino acids, Pyruvate, gluconeogenesis glucose

Question 21

In prolonged starvation what fuel molecules do muscles use and why?

Answer 21

Fatty acids and ketone bodies. The prevent usage of glucose which is reserved for the brain.

Question 22

What is a common condition seen in starvation and also in type one diabetes?

Answer 22

Ketoacidosis. This occurs because acetoacetate and B hydroxybuturate are both relatively strong acids. Ketouria will also be seen because ketone bodies are above the renal threshold.

Question 23

What happens during fatty acid oxidation if the carbon chain has an odd number of carbons?

Answer 23

In this case, the cycle continues until 3 carbons remain. Carboxylase catalyses carboxylation and the production of methylmalonyl~coA which is then rearrange to succinyl~coA, a Kreb cycle intermediate.

Question 24

When glucose is low, Pyruvate dehydrogenase is inhibited. How does this help to maintain glucose?

Answer 24

Inhibition of Pyruvate dehydrogenase slows acetyl~coA synthesis, and therefore acetyl~coA –> citrate which I inhibits phosphofructokinase. This leads to a build up of Glucose 6 phosphate which acts as product inhibition on hexokinase and so maintains glucose concentration.

Question 25

What is glycogen?

Answer 25

Glycogen is a glucose store which is used between meals to maintain glucose plasma concentration.

Question 26

What is the advantage of the branched structure of glycogen?

Answer 26

It means it can be broken down quickly when required for utilisation.

Question 27

What is the action of phosphoglycomutase and in which pathway can this be seen?

Answer 27

It converts glucose 6 phosphate to glucose 1 phosphate which can then be utilised to make glycogen in glycogenesis.

Question 28

Name the two enzymes responsible for adding UDP-glucose to Glycogen.

Answer 28

Glycogen synthase - (a1-4) bonds

Branching enzyme - (a1-6) bonds

Question 29

When glycogen is broken down, what does it initially form? What enzymes assist in this process?

Answer 29

It forms glucose 1 phosphate. Debranching enzyme and glycogen phosphotase are the two enzymes responsible for this breakdown.

Question 30

What is the difference between glycogen breakdown in the muscles compared to the liver?

Answer 30

In the muscles, it is converted to glucose 6 phosphate and this enters glycolysis. In the liver the enzyme glucose 6 phosphatase acts on glucose 6 phosphate and converts it back to glucose. This enzyme is not present in muscle.

Question 31

How is glycogenolysis and glycogenesis regulated?

Answer 31

The rate limiting enzymes are glycogen synthase and glycogen phosphorylase.
Glucagon phosphorylates both of these which inhibits synthase and activates phosphorylase. Insulin has the opposite action.

Question 32

In muscle stores, glucagon has no effect. In this case what is the activator of glycogen phosphorylase?

Answer 32

AMP which is a low energy signal.

Question 33

Are lipids soluble or insoluble?

Answer 33

Lipids are insoluble molecules made from C, H and O.

Question 34

Why do glycogen storage diseases arise and what are the consequences?

Answer 34

They occur due to deficiency or dysfunction of enzymes in glycogen metabolism. Excessive storage leads to tissue damage, whilst low glycogen stores lead to exercise intolerance and hypoglycaemia.

Question 35

Where does gluconeogenesis occur?

Answer 35

It occurs in the liver and in the kidney cortex.

Question 36

What are precursors for gluconeogenesis?

Answer 36

Lactate from anaerobic glycolysis, glycerol from TAG breakdown and amino acids (mainly alanine).

Question 37

Step 10 of glycolysis is irreversible

PEP – Pyruvate. What is the reverse of this in gluconeogenesis?

Answer 37

Pyruvate is converted to oxaloacetate by Pyruvate carboxylase. This is then converted to PEP by PEPCK.

Question 38

Describe how the action of PEPCK is regulated.

Answer 38

With increase Glucagon/ cortisol there is increased PEPCK and so this reaction is stimulated. With increased insulin, amount decreases and so does reaction rate.

Question 39

What is the action of fructose1,6, bisphosphotase? How is it regulated with glucagon/ insulin?

Answer 39

This converts fructose 1,6 bis phosphate into fructose 6 phosphate. Glucagon stimulates and increases activity. Insulin inhibits.

Question 40

Define Hyperplasia

Answer 40

Increase in the number of cells.

Question 41

Define hypertrophy

Answer 41

Increase in cell size.

Question 42

What is lipogenesis?

Answer 42

Fatty acid synthesis. Dietary glucose is used as a source of carbon.

Question 43

What enzyme is involved in lipogenesis?

Answer 43

Fatty acid synthase. It builds them by sequential addition of 2 carbon units provided by malonyl~coA.

Question 44

What enzyme catalyses the reaction of acetyl~coA to malonyl~coA? How is it regulated?

Answer 44

Acetyl~coA carboxylase.
It is activated by increased insulin or citrate
It is inhibited by glucagon/adrenaline or AMP