Biosynthesis of Triacylglycerols (TAG)

Question 1

Triacylglycerol structure

Answer 1

Question 2

Glycerol must be in the form of [blank] for TAG synthesis to occur

Answer 2

Glycerol-3-phosphate

Question 3

Name 3 sources of glycerol-3-phosphate

Answer 3

1) Recycling of the glycerol produced during b-oxidation of fatty acids
2) Reduction of dihydroxyacetone phosphate (DHAP) produced during glycolysis.
3) Reduction of dihydroxyacetone phosphate (DHAP) produced during glyceroneogenesis.
-Using gluconeogenesis to produce glycerol.

NOTE: In top image, glycerol kinase uses 1 ATP and uses glycerol-3-phosphate shuttle where NADH is converted to NAD+, and e- are passed to FAD to produce FADH2

Question 4

In the liver, glycerol is acquired using (ignore card)

Answer 4

all 3 mechanisms

Question 5

In adipose tissues, glycerol is required through

Answer 5

Pathways involving DHAP (glycolysis or glycerogenesis) because they lack glycerol kinase

Question 6

Two pathways can be used to synthesize TAGS

Answer 6

1) Glycerol phosphate pathway: Liver, adipose tissues and intestine
2) Monoacylglycerol pathway: Specific to the intestine

Question 7

Glycerol-3-phosphate pathway Step 1 (formation of DAG)

Answer 7

Question 8

Glycerol-3-phosphate pathway Step 2 (Formation of DAG)

Answer 8

NOTE: PAP enzymes are also called Lipin

Question 9

Monoacylglycerol Pathway Steps 1 and 2

Answer 9

1. Action of pancreatic lipase leads to the production of free fatty acids (FFA) and 2-monoacylglycerol
2. Short and medium-chain FAs (as well as glycerol) pass directly from the intestine to the lymphatic system (not shown) not long chain FA

Question 10

Monoacylglycerol Pathway Steps 3 and 4

Answer 10

3. Free fatty acids can be reassembled into TAG through the glycerol 3-phosphate pathway (also called, the Phosphatidic acid pathway)
4. 2-monoacylglycerol molecules enter the enterocyte and then, are converted to TAG through the monoacylglycerol pathway before being released into the lymphatic system

Question 11

Monoacylglycerol pathway: Action of MGAT in intestinal cells

Answer 11

Notice that there is no phosphate on c3, it comes from hydrolysis of TAG in diet

Question 12

Conversion of DAG to TAG

Answer 12

Question 13

TAG Biosynthesis machinery is localized in what 2 places?

Answer 13

1. Endoplasmic Reticulum
2. Mitochondria Outer Membrane

Question 14

TAG Biosynthesis machinery is localized in what 2 places?

Answer 14

1. Endoplasmic Reticulum
2. Mitochondria Outer Membrane

Question 15

2 Roles of Phosphatidic acid

Answer 15

1. Key intermediate in TAG synthesis
   2) Used in the biosynthesis of glycerophospholipids

Question 16

What are glycerophospholipids

Answer 16

They are membrane components that can play specialized roles

Question 17

What are the 2 fates of TAG

Answer 17

1. They can be stored as lipid droplets
2. Redistributed throughout the body

Question 18

What are 3 characteristics of lipid droplets/

Answer 18

1.Store energy
2. Hydrophobic so remain in cells
3. Fatty acids can be released from lipid droplets and can access the bloodstream or travel to mitochondria to be use as immediate energy source (b-oxidation)

Question 19

What are 3 types of lipid droplets

Answer 19

1. ATGL = Adipose TriGlyceride Lipase
2. HSL = Hormone sensitive lipase
3. MGL = Monoacyglycerol lipase

Question 19

What are 3 types of lipid droplets

Answer 19

1. ATGL = Adipose TriGlyceride Lipase
2. HSL = Hormone sensitive lipase
3. MGL = Monoacyglycerol lipase

Question 20

What 2 molecules redistribute TAG throughout the body?

Answer 20

1. Chylomicrons (intestine)
2. Very-low-Density lipoproteins/VLDL (liver)

Question 21

EXAM QUESTION: When [14C]-glucose is added to the balanced diet of adult rats, there is no increase in the total amount of stored TAGs, but the TAGs become labelled with [14C].

Explain.

14 C = radioactive

Answer 21

Thefirstthingisthe connectionbetweencarbohydratesandlipids.Theconnectionisacetyl-CoA.Thebuildingblockoffattyacidisacetyl-CoA,whichiscomingfromdifferentplacesbutalsoglycolysis.Soifyou’reglucoseisradioactive,yougetradioactiveacetylCoA.AndacetylCoAisusedasbuildingblockforfattyacidswhichgetattachedtoTAG.TAGbecomesradioactive.Thesecondthingthatisimportantisthatit’sdynamic.TAGgetsdestroyedandreformedallthetime,sothere’sahugeturnover.Weretalkingaboutaratonabalanceddiet,itmeansthisratewillstoreTAG,willuseitandwhenthisrathasamealitwillcreatesomemore,it’sadynamicprocess.TAGgetsbreakdownedandformed.Carbohydratemetabolism,glycolysisproducedacetylCoAwhichcanbeusedto createfattyacidswhichareaddedtoTAGmolecule.

-Need 3.5 glucose molecules to produce 14 C-glucose (because need 7 acetyl CoA)
-All of the TAG is labelled, so the entire fatty acid will be radioactive, tags all acetyl coa

Summary:
1. Connection between carbohydrates and lipids metabolism.
2. Newly formed fatty acids (containing [14C]) are stored as TAG in the adipose tissues
3. Fatty acids storage is dynamic.