BIOCHEM - Lipid Metabolism

Question 1

Describe a triglyceride.

Answer 1

Most of the lipids (fats & oils) in the diet are in the form of triglycerides.

Triglycerides are tri-esters: glycerol + 3 x fatty acids

fatty acids are connected to glycerol backbone with ester bonds.

Question 2

What are the properties of fatty acids:
- in what ways can they differ from one another?

Answer 2

Fatty acids consist of a methyl/omega end and an acid group end.

differ in:

• chain length
• saturated/unsaturated (cis/trans)
• double bonds (how many/where they are)

Question 3

What’s the difference between esterified and non-esterified fatty acids?

Answer 3

Most of the time in the body FA’s are esterified.

Esterified FA’s are when they are present in form of triglyceride.
- otherwise they are free fatty acids.

Question 4

What are the properties of saturated fatty acids?

What are the properties of unsaturated fatty acids?

What are some example of important SFA’s, MUFA’s & PUFA’s in the diet?

Answer 4

Saturated fatty acids:
- carbon chain saturated with H
- straight chains pack closely together

Unsaturated fatty acids:
- cis & trans fatty acids.
- cis fatty acids: hydrogen on same side of double bond = bend in chain
- trans fatty acids: hydrogen on opposite side of double bond = straight chain (similar to sat fat)

SFA’s: butyric acid
MUFA: oleic acid
PUFA: linoleic/a-linoleic acid

Question 5

How are lipids digested and absorbed?

Answer 5

Digestion:

stomach: gastric lipase - small amount of lipid digestion
small intestine: bile produced by liver & pancreatic lipase aid digestion of dietary lipids into glycerol + free FA’s/ mono-acylglyceride.

Absorption:

• Bile acids attach to fatty acid and form micelle (dietary mixed micelle)
• Move close to microvilli of small intestine lumen and allow diffusion of lipid into mucosal cell. (unmixed water layer) - the dietary mixed micelles spontaneously disperse to allow absorption.
• Inside mucosal cell they are re-arranged to form triglycerides again.
• Incorporated into lipid transport particles (lipoproteins) e.g., chylomicrons* and enter lymph.

Question 6

What is a micelle?

Answer 6

A micelle consists of a core and a shell, where hydrophobic end groups form the core and hydrophilic head groups form the outer shell.

e.g., phospholipids (phosphate head – hydrophilic & two fatty acid tails – hydrophobic)

Question 7

What is a lipoprotein?

Answer 7

Lipoprotein = lipid + protein

Need lipoproteins to allow transport of lipids around body as lipids are hydrophobic and cannot be transported through blood as they do not dissolve in water.

5 types:
LDL, HDL, chylomicrons, VLDL, IDL
Lipoproteins are classified according to their density.

Question 8

What are the stages of fatty acid metabolism?

Answer 8

Digestion/absorption = free fatty acids & glycerol
Fatty acid links to Coenzyme A to become activated.

Carnitine Shuttle:
- Carnitine carries activated fatty acids from across the mitochondrial membrane from cytosol to inner mitochondrion.

Beta-oxidation:
- fatty acids are ‘cleaved’ into molecules of Acetyl CoA
- starts at beta carbon of fatty acid chain
- enzymes cleave 2-C at a time –> Acetyl CoA + NADH & FADH
- Acetyl CoA enters citric acid cycle and ETC