L17 Triacylglycerols

Question 1

What does pancreatic lipase do and when/where does it act?

Answer 1

pancreatic lipase breaks down TGs after or during a meal inside the small intestine

Question 2

How can charged FA (charged head group) traverse the non-polar bilayer?

Answer 2

It is possible that within the pinocytic vesicles which transport FAs into the cell, the pH causes FAs to become protonated and thus maintain a neutral charge during diffusion across the membrane

Question 3

What does lipoprotein lipase (LPL) do and when/where does it act?

Answer 3

It anchors lipoproteins (chylomicrons, VLDLs) and hydrolyzes the TGs into monoglycerides and free FAs. This occurs in the blood vessels at the surface of muscle and adipose tissues which uptake the monoglycerides and the free FAs

Question 4

In what kind of tissues are lipid droplets usually found?

Answer 4

Adipose and liver tissue
Very little in muscle tissue

Question 5

What is the role of lipid droplets in adipocytes

Answer 5

storing triacylglycerols

Question 6

What is the role of lipid droplets in liver

Answer 6

they serve as transient buffer reservoir of esterified (stable and less soluble) FAs and esterified cholesterol

they are smaller than the lipid droplets found in adipocytes

Question 7

Under what conditions does lipolysis of lipid droplets occur?

Answer 7

fasting or during exercise (last stage)

Question 8

How many enzymes are involved in lipolysis of lipid droplets? Which ones and in what order?

Answer 8

3

adipose triglyceride lipase ATGL, hormone-sensitive lipase HSL, monoacylglycerol lipase MGL

Question 9

TF: lipolysis of lipid droplets is hormone regulated

Answer 9

true

Question 10

TF: insulin stimulates lipolysis

Answer 10

false, insulin inhibits lipolysis

lipolysis occurs during fasted state, and insulin is released during fed state

Question 11

What is the end product of lipolysis following the 3 enzyme reactions?

Answer 11

glycerol

Question 12

Explain how lipolysis is controlled by hormonal regulation?

Answer 12

hormones such as epinephrine will bind to receptors on the plasma membrane, activating adenylyl cyclase to make cAMP, which activates PKA.

PKA will then phosphorylate HSL and perilipin (proteins that coat the surface of lipid droplets).

perilipin phosphorylation allows enzymes to enter the lipid droplet and HSL phosphorylation activates its enzymatic activity

TAG is converted to free FAs and glycerol which will be transported and diffused respectively into the blood to be transported to other tissues.

In the blood, free FAs and glycerol bind to albumin for transport.

Question 13

What are the 2 steps to TAG (re)synthesis inside enterocytes

Answer 13

1) activation of FA acyl chain by CoA via Acyl-CoA Synthase (high energy intermediate Acyl-CoA to add to monoglyceride)

*FA acyl chain = hydrocarbon tail

2) Acyl-CoA (activated FA acyl chain high energy intermediate) is added to 2-monoacylglycerol (from diet) to eventually make triacylglycerol

Question 14

TF: TG can be synthesized in adipose tissues

Answer 14

true

Question 15

What are the 4 steps to de novo TAG synthesis (aka glycerol-3-phosphate pathway) in adipose and liver?

Answer 15

1. generate glycerol-3-phosphate (not G3P, glyceraldehyde-3-phosphate) from either DHAP synthesis or phosphorylation of glycerol (liver only since adipocytes lack glycerol kinase)
2. add 2 acyl-CoAs to glycerol-3-phosphate
3. dephosphorylation by lipin (of the glycerol + 2 acyl chains from acyl-CoA)
4. add one more acyl-CoA to make TG from DAG synthesized in the first 3 steps

*glycerol in liver is from lipolysis of lipid droplets (adipose and liver)

Question 16

Which enzyme is the key regulatory enzyme for TG de-novo synthesis? Why is it the key regulatory enzyme?

Answer 16

lipin because it is at the crossroad for TG and PL (phospholipid) synthesis

Question 17

TF: glycerol-3-phosphate is a product of glycolysis?

Answer 17

True,

it is also a product of fructose metabolism (but not galactose metabolism)

Question 18

TF: insulin stimulates the conversion of glucose to free fatty acids

Answer 18

True,

glucose then phosphorylates lipin, increasing TG synthesis and stopping TCA

*unphosphorylated lipin would convert phosphatidic acid into DAG which leads to PL synthesis (phosphatidic acid for TG synthesis, DAG for LP synthesis)

Question 19

TF: insulin promotes lipogenesis and does so via transcriptional control

Answer 19

True, it promotes lipogenesis

It also does so via transcription control by upregulating enzymes involved in TG synthesis