Biochemistry- lipid catabolism

Question 1

What are essential fatty acids?

Answer 1

Polyunsaturated fatty acids which can not be made by the body but are required by the body and therefore must be consumed.

Question 2

What can deficiencies in essential fatty acids cause?

Answer 2

These can lead to membrane disorders, increased skin permeability and mitochondrial damage.

Question 3

Which vitamins are fat-soluble?

Answer 3

A, D E and K

Question 4

Give examples of simple lipids

Answer 4

fatty acids, triglycerides, waxes

Question 5

Give examples of compound lipids

Answer 5

phospholipids, glycolipids, lipoprotein

Question 6

Give examples of steroids?

Answer 6

Cholesterol, steroid hormones

Question 7

Are fatty acids aliphatic?

Answer 7

yes- this means they have no rings

Question 8

Do most fatty acids, and the ones in our bodies, contain an even or an odd number of carbon atoms?

Answer 8

Even- usually 2-20 but there can be more

Question 9

Do branched fatty acids, or fatty acids with an odd number of C atoms exist?

Answer 9

yes, but these are rare

Question 10

Are double bonds usually in the cis or trans configuration

Answer 10

Cis

Question 11

Explain what a cis fatty acid is

Answer 11

Cis: the adjacent hydrogen atoms are on the same side of the double bond. The rigidity of the double bond freezes its conformation and causes the chain to bend and restricts the conformational freedom of the fatty acid.
When a chain has main cis bonds, it becomes quite curved. The result is, when fatty acids are in phospholipids in a bilayer or TAG in a lipid droplet, the fatty acids can not be as closely packed.

Question 12

Explain what a trans fatty acid is

Answer 12

The two hydrogen atoms are bound to opposite sides of the double bond.
As a result, they do not cause the chain to bend much, and the shape is similar to straight fatty acids.

Question 13

What is the most common fatty acid in the body?
How many carbon atoms does it have?
How many double bonds does it have?

Answer 13

Palmitic acid

It has 16 carbon atoms and no double bonds

Question 14

How many carbon atoms and double bonds does stearic acid have?

Answer 14

It has 18 carbon atoms and no double bonds

Question 15

How many carbon atoms and double bonds does oleic acid have?

Answer 15

It has 18 carbon atoms and 1 double bond

Question 16

If carbon atoms in a fatty acid are labelled using the greek alphabet, which carbon atom is the alpha atom and which is the omega atom?

Answer 16

The carbon atom adjacent to the carboxyl group is the alpha atom, and the carbon atom furthest away is the omega atom, no matter how many atoms are in between.

Question 17

Fatty acids with up to how many carbon atoms are liquids at room temperature?

Answer 17

Up to 8 carbon atoms

Question 18

What is the effect of double bonds on the melting point of fatty acids?

Answer 18

It lowers the melting point

Question 19

What are the main products of fat digestion?

Which of these are readily absorbed by epithelial cells?

Answer 19

Glycerol (readily absorbed by epithelial cells)
Fatty acids
Monoglycerides

Question 20

What happens to chylomicrons are muscle and adipose tissue?

Answer 20

They are attacked by lipoprotein lipase which cleaves them. Free fatty acids are then taken into the cells and are either resynthesised to form TAG in adipose tissue for storage or are oxidised to produce energy (in muscle). This depends on how much is available.

Question 21

What is the name for the breakdown of lipids?

Answer 21

Lipolysis

Question 22

What initially happens to the stored fat in lipolysis?

Answer 22

It is cleaved by a hormone-sensitive lipase e.g. adrenaline-sensitive lipase

Question 23

What has to happen to fatty acids before they can be oxidised?
How much energy is required to do this?
Where does this occur?

Answer 23

They have to be activated by being linked to coA. This forms acyl coA. This requires 2 ATPs.
It occurs in the cytoplasm.

Question 24

What is acyl coA?

Answer 24

It is formed from the linkage of fatty acids to coA. It has an undefined number of carbons, which depends on the number of carbons in the fatty acid chain.

Question 25

Where does further oxidation of acyl coA occur?

Answer 25

In the matrix of the mitochondrion.

Question 26

How is the fatty acid from acyl coA transported into the mitochondrial matrix?

Answer 26

In the cytoplasm, fatty acids are transferred from acyl-coA to a carrier protein called carnitine, which can be transported across the inner mitochondrial matrix by a specialacyl-carnitine transporter, which also transports carnitine out of the matrix. In the matrix, the acyl group is cleaved off of carnitine and transferred onto another coA. The remaining carnitine is moved back out and the cycle can continue.

Question 27

What is beta oxidation?

Answer 27

This a cycle of reactions which occurs in the mitochondrial matrix. There are 4 steps in each cycle. Acyl coA which has been brought into the matrix is the substrate. The products are one 2-carbon acetyl coA, a new acyl coA which is 2 carbons shorter than the previous one, 1FADH2, 1 NADH+ H+ (as there are 2 oxidation reactions in the cycle).

Question 28

What is required for the oxidation of unsaturated fatty acids, odd-chain fatty acids and branched chain fatty acids?

Answer 28

Additional enzymes. The oxidation of these fatty acids usually yields less ATP.

Question 29

What happens to the glycerol that is produced after cleavage of TAG?

Answer 29

It is activated to glycerol-3-phosphate by glycerol kinase which is present in the liver and kidney, but absent from skeletal muscle, heart muscle and adipose tissue. It is then dehydrogenated to dihydroxyacetone phosphate which is an intermediate in glycolysis, and therefore it can enter mainstream metabolism.

Question 30

What are ketone bodies? What are they called? Where are they formed?

Answer 30

They are derived from acetyl coA and contain ketone groups. There are 3: acetoacetate, acetone and hydroxybutyrate. They are formed in liver mitochondria from acetyl coA from beta oxidation.

Question 31

What happens to ketone bodies after they have been formed in liver mitochondria?

Answer 31

They diffuse into the bloodstream and to peripheral tissues. Peripheral tissues can pick them up and convert them back to acetyl coA and they can enter the TCA cycle.

Question 32

Why are ketone bodies problematic in starvation and diabetes?

Answer 32

Normally fatty acid beta oxidation yields acetyl coA, which enters the TCA cycle if fat and carbohydrate degradation is balanced. Oxaloacetate is required for this, and this can be produced as a side product of glycolysis (hence why carbohydrate metabolism needs to be in balance with fat metabolism). In starvation or diabetes oxaloacetate is consumed for gluconeogenesis. Fatty acids are oxidised to produce energy and acetyl coA is converted into ketone bodies, so there are high levels in the blood which is too much for extrahepatic tissue. Ketone bodies are moderate acids and accumulation leads to severe acidosis . This impairs tissue function, particularly central nervous tissue function, which can lead to an acidotic coma and in some cases death. Acetone can be smelt on the breath.