Biochemistry - lipid anabolism

Question 1

What is the name for the synthesis of new fatty acids?

Answer 1

Lipogenesis

Question 2

Which tissues can carry out lipogenesis?

Answer 2

Liver, kidney, mammary glands, adipose tissue, brain

Question 3

When does lipogenesis occur?

Answer 3

During periods of excess energy intake

Question 4

What happens when excess carbohydrate is taken in?

Answer 4

It is converted to fatty acids and TAG in the liver

Question 5

How are free fatty acids transported from the liver in the plasma?

Answer 5

bound to alubumin

Question 6

How is TAG transported from the liver in the plasma?

Answer 6

It is packaged into VLDL

Question 7

Is lipogenesis a reductive or oxidative process?

What does it therefore require?

Answer 7

It is reductive, therefore requires electrons.

Question 8

Give a brief overview of lipogenysis from starch.

Answer 8

Starch –> glucose –> pyruvate –>acetyl coA–> Fatty acids –> triglycerides

Question 9

What is the indirect precursor for the conversion of dietary carbohydrate into fatty acids?

Answer 9

Acetyl coA

Question 10

Where does lipogenysis occur?

Answer 10

In the cytoplasm

Question 11

Where does beta oxidation occur?

How is the acetyl coA transported into the cytoplasm?

Answer 11

In the mitochondrial matrix.
The inner mitochondrial membrane is impermeable to acetyl-coA but citrate can cross it. Acetyl coA is linked to oxaloacetate to form citrate. If a lot of citrate is present in the matrix it can be transported into the cytoplasm.

Question 12

What is the direct precursor for the synthesis of fatty acids?

Answer 12

Malonyl coA

Question 13

Describe fatty acid synthesis, starting from acetyl coA.

Answer 13

Acetyl coA is activated to malonyl coA by acetyl coA carboxylase. This carboxylates acetyl-coA and converts it into a 3 carbon malonyl coA. One ATP is required for each acetyl coA that is converted into malonyl coA.
Malonyl donates the carbons to the new fatty acid.

Question 14

Which enzyme catalyses the synthesis of fatty acids?

What is the only type of fatty acids it can synthesise?

Answer 14

Fatty acid synthase.

It can only synthesise saturated straight chain fatty acids.

Question 15

What does fatty acid synthase comprise of?

Answer 15

A dimer of identical polypeptides.
Each of the polypeptides that makes up the dimer has 7 distinct enzyme activities.
It contains an acyl-carrier protein (ACP) which carries the growing fatty acid chain during synthesis.

Question 16

How does fatty acid synthase synthesise a fatty acid?

Answer 16

Fatty acids are synthesised in a cycle of reactions.
In the 1st step, 1 acyl carrier protein is loaded with an acetyl group from acetyl coA, and the second acyl carrier protein (on the other half of the dimer) is loaded with a malonyl group from malonyl coA.
2 carbons from the malonyl are transferred onto the acetyl group to form a 4 carbon fatty acyl chain.
Then the second acyl carrier protein is loaded with another malonyl group and the cycle repeats itself.
Each cycle of reactions adds another 2 carbon atoms onto the fatty acyl chain.
The reactions are reductive, so NADPH is used as an electron donor.
The cycles repeat until the growing acyl chain has reached 16 carbon length. It is then released from the acyl carrier protein.

Question 17

a) What is the longest fatty acid chain that fatty acid synthase can make?
b) How are longer, unsaturated, oddly evened or branched fatty acids made?

Answer 17

a) 16 carbons (i.e. palmitic acid)

b) They require additional enzymes.

Question 18

How is fatty acid synthesis controlled?

Answer 18

By regulating acetyl-coA carboxylase at a systemic level (by hormones) and at the enzymatic level by allosteric regulation.

Question 19

Which three hormones regulate fatty acid synthesis?

What is their effect?

Answer 19

Insulin: stimulates acetyl coA carboxylase and fatty acid synthesis
Glucagon: inhibits acetyl coA carboxylase and fatty acid synthesis
Epinpherine (adrenaline): Inhibits acetyl coA carboxylase and fatty acid synthesis.

Question 20

Which molecules regulate acetyl coA carboxylase allosterically?

Answer 20

Citrate: stimulates acetyl coA carboxylase and fatty acid synthesis
Palmitoyl coA: Inhibits acetyl coA carboxylase and fatty acid synthesis (its concentrations are high when there are plenty fatty acids)

Question 21

As well as fatty acids, what is required for the synthesis of triglycerides?

Answer 21

glycerol-3-phosphate

Question 22

Where and how is glycerol-3-phosphate synthesised?

Answer 22

In the liver, by phosphorylating glycerol with glycerol kinase
In the adipose tissue: This can produce glycerol-3-phosphate through glycolysis. (there is not glycerol kinase present in the adipose tissue). Therefore this can only happen if there is plenty glucose in the bloodstream.

Question 23

How are the fatty acids and glycerol-3-phosphate joined?

Answer 23

Esterification