Lipid Catabolism

Question 1

What is a key structural difference of lipids compared to carbohydrates?

Answer 1

• lipids are more reduced than carbohydrates

- they therefore produce more energy when oxidised

Question 2

What are the 3 classes of lipids? Give examples for each

Answer 2

1- fatty acid derivatives
-fatty acids (fuel molecules)
-triacylglycerides (fuel & insulation)
-phospholipids (plasma membranes)
2-hydroxy-methyl-glutaric acid derivatives (C6 compounds)
eg Ketone bodies (C4), cholesterol (C27)
3-vitamins 
-A,D,E,K

Question 3

Describe the structure of triacylglycerols & give some properties of them.

Answer 3

• glycerol backbone and x3 fatty acid chains
• hydrophobic, stored in adipose, used in starvation & during pregnancy for extra energy
• storage & mobilisation under hormonal control

Question 4

What is the first step of lipid (triacylglycerol) metabolism?
Where do the fatty acid go to be utilised & where do they go to be stored?

Answer 4

• hydrolysis in small intestine by pancreatic lipase, breaks down triacylglycerols into fatty acids & glycerol
• they’re then recombined into TAG in the SI as chylomicrons by lipoproteins.
• released into circulation via lymphatics & carried to 1.adipose tissue to be stored as TAG & released as fatty acids when needed OR 2. Carried to consumer tissues on albumin to be oxidised into energy

Question 5

Give the structure & properties of fatty acids.

Answer 5

• saturated or unsaturated
• amphipathic (hydrophobic & hydrophilic groups)
• certain polyunsaturated FA are essential in diet
• chain length = 14-18

Question 6

How are fatty acids activated and transported across the membrane where they’re oxidised? Where does this happen?
Is this a oxygen dependant process?

Answer 6

• happens in mitochondria
• FA activated by linking to coenzyme A via ENZYME fatty acyl CoA synthase
• is then transported across inner mitochondrial membrane using a carnitine shuttle (regulated, so is the control of FA oxidation)
• then FA cycles through sequence of oxidative reactions, with C2 removed each cycle. (in the process, NAD-> NADH)
• YES needs O2

Question 7

Explain how glycerol is metabolised.

Answer 7

• transported in the blood to liver where it’s metabolised.
• glycerol phosphorylated to glycerol phosphate via ENZYME glycerol kinase
• can then either form DHAP which enters glycolysis OR triacylglycerol synthesis

Question 8

What are ketone bodies, where are they produced and what is the normal and abnormal body concentration ?

Answer 8

• water soluble molecules, alternative substrate, acetone may be excreted from lungs
• made by liver
• normally <1 mM, starvation/untreated diabetes= >10mM is then released into urine (ketonuria)

Question 9

How is ketone body production in the liver controlled ? hint: Krebs cycle

Answer 9

• if fatty acids build up, NAD will all be converted to NADH and so the enzymes in the cycle will be inhibited.
• cycle unable to continue so citrate builds up, and Acetyl CoA diverts and produces ketone bodies instead.
• these provide spare glucose in early starvation to brain & muscles, then later generate glucose from muscles to provide to brain

Question 10

How is ketone bodies synthesis regulated by insulin/ glucagon ratio?

Answer 10

• fed state (insulin/glucagon ratio high), lyase is inhibited, reductase is activated to allow cholesterol synthesis
• starvation state (insulin/glucagon ratio low), lyase activated (forms acetyl coA), reductase inhibited therefore ketone body synthesis.

Question 11

Name the three ketone body produced in the body. Where are they synthesised?

Answer 11

• acetoacetate, acetone & Beta-hydroxybutyrate

- all made in the liver