Lecture 11: biosynthesis of triglycerides and cholestrol

Origins of glycerol

• the limiting substrate during TAG synthesis is the glycerol backbone.

  • More precisely, TAG synthesis begins with a molecule of glycerol-3-phosphate.

Sources of glycerol-3-phosphate

1. Recycling of the glycerol produced during lipolysis of TAG

   1. in adipose tissue, FA will be released from TAG

2. Reduction of dihydroxyacetone phosphate (DHAP) produced during glycolysis.

   1. 1 molecule of glucose goes into make DHAP

3. Reduction of dihydroxyacetone phosphate (DHAP) produced during glyceroneogenesis.

   1. Using gluconeogenesis to produce glycerol

Lipolysis

TAG and cleave FA to release them so it can be used as a source of energy

gluconeogenesis

formation of new glycerol-3-phosphate from anythin other than glucose

Origins of glycerol: Adipose tissues

• Adipocytes lack glycerol kinase and therefore, they rely on pathways involving DHAP (glycolysis or glyceroneogenesis)

• Glycerol produced by the b-oxidation is returned to the liver through the blood stream to be reused (using Glycerol kinase).

• Most of the glycerol 3-phosphate is produced through glyceroneogenesis.

• adipose tissue cant regulate own cholesterol

• glycerol being produced by adipose tissue can’t be converted to glycerol phosphate

Origins of glycerol: Liver

Glycerol is acquired using all three mechanisms

• adipose, DHAP glycolysis & glyconeogenesis

TAG synthesis

• Glycerol phosphate pathway: Liver, adipose tissues and intestine

• Monoacylglycerol pathway: Specific to the intestine

Glycerol phosphate pathway- DAG formation

1. Actions of GPAT and AGPAT

   1. use 2 enzymes to add a chain to carbon 1 and carbon 2

2. PAP enzymes are also called Lipin (family of three proteins)

   1. if you want to add a 3rd FA, it can’t because theres a phosphate on C3

Monoacylglycerol pathway- Source of monoacylglycerol

1. Action of pancreatic lipase leads to the production of free fatty acids (FFA) and 2- monoacylglycerol

   1. pancreatic lipase hydrolyzes FA on carbons 1 and 3 (on edge therefore easy to access)

2. Short and medium-chain FAs (as well as glycerol) pass directly from the intestine to the lymphatic system (not shown)

   1. mechanism: flip flop

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

Monoacylglycerol pathway

• deals with TAGs coming from diet

• no dephosphorylation

Monoacylglycerol pathway-DAG formation

1. Action of MGAT in the intestinal cells

TAG formation

• uses DGAT to turn DAG to TAG

Localization of TAG biosynthesis machinery

Endoplasmic reticulum (ER) and the mitochondria outer membrane

• enzymes are located at the mitochondria and the ER facing the outside (on surface)

• lots of different enzymes

TAG synthesis

• Phosphatidic acid (PA) is a key intermediate for TAG synthesis but can also be used in the biosynthesis of glycerophospholipids.

  • keep phosphate on carbon #3 and add something to it

What is the fate of TAGs?

1. Storage in Lipids droplets (LDs)

• Storage of energy

• LDs are highly hydrophobic and therefore they remain in the cells.

• FA can also be released from LD and access the bloodstream or travel to mitochondria to be used as immediate energy source (b-oxidation).

• These work on the surface of the oil droplet

  • ATGL = Adipose TriGlyceride Lipase

  • HSL = Hormone sensitive lipase

  • MGL = Monoacyglycerol lipase

• Release of FA is controlled by hormones

• glucagon = decrease glucose= release energy

• glucagon stimulates release of FA

  • increase lipolysis ((release of FA from TAG)

2. Redistribution of TAG throughout the body

   1. Chylomicrons (intestine)

   2. Very-Low-Density Lipoprotein (liver)’

      1. Coat TAG in phospholipids so that it can be distributed throughout the body

What is the effector of glucagon?

pKa

• it phosphorylates perilipin CGi-58 is released and attacks ATGL, ATGL releases FA

Biological functions of cholesterol

1. structural component of cell membranes, particularly the plasma membranes

2. constituent of lipoproteins

3. precursor for bile acids and steroid hormones

Cholesterol Biosynthesis

• Around 50-80% of the total cholesterol found in your body is coming from de novo biosynthesis

• The other 20-50% is from your diet and the recycled cholesterol (surplus is excreated, picked up and restored)

• Roughly 80% of total daily cholesterol production occurs in the liver and intestine.

• Cholesterol is also produced in the adrenal glands, and reproductive organs.

• All of cholesterol’s carbon atoms are derived from acetate

Where does acetate come from?

Acetyl-CoA

Tricarboxylate transport system

Too much ATP= slow down TCA cycle

• acetyl coa is diverted to something else (FA synthesis or cholesterol synthesis)

Three stages of cholesterol synthesis in the liver

1. condensation of acetate to form mevalonate intermediates (C6 unit)

2. polymerization of mevalonate to form squalene (C30 unit)

3. cyclization of squalene and further modifications to form cholesterol (C27 unit)

Cholesterol synthesis- stage 1

Condensation

• from acetyl-CoA (C2) to mevalonate (C6)

  • HMG-CoA reductase catalyzes the rate limiting step of cholesterol biosynthesis.

  • HMG-CoA reductase is an integral membrane protein

  • HMG-CoA reductase can be found in the ER and peroxisome

• from mevalonate (C6) to dimethylallyl pyrophosphate (C5) (Part 1)

  • lose 1 carbon

• from mevalonate (C6) to dimethylallyl pyrophosphate (C5) (Part 2)

  • adding phosphate facilitate decatrboxylation

Isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate are isoprenoids

• Isoprenoids are the oldest know biomolecules.

• Recovered from sediments which were roughly 2.5 billion years old.

• Family of around 30,000 known compounds

• Definition of isoprenoids: any compound biosynthesized from or containing isoprene units

Cholesterol synthesis- stage 2

Polymerization of 3 isoprenoids (C5) to form farnesyl pyrophosphate (C15)

• Reactions 1 and 2 are catalyzed by prenyltransferase

• Nucleophilic substitutions

• Final product is Farnesyl pyrophosphate (C15)

• use 1 dimethylallyl pyrophosphate, 1 isopentenyl pyrophosphate

Polymerization of farnesyl pyrophosphate (C15) to form. squalene (C30)

• Reactions 3 is catalyzed by squalene synthase

• This enzyme is located in the ER membrane

• Join two molecules of Farnesyl pyrophosphate in a head-to-head conformation

• Final product is Squalene

• use 1 NAPDH

Cholesterol synthesis- stage 3

Cyclization

From squalene (C30) to cholesterol (C27)

1. Cyclization of squalene to form lanosterol (C30)

2. Synthesis of cholesterol (C27) from lanosterol (C30)

Precursors

Cholesterol and isoprenoids are precursors of other compounds

Summary- The cholesterol biosynthesis pathway

• Building block for de novo cholesterol biosynthesis is acetyl-CoA

• HMG-CoA reductase catalyzes the rate limiting step

• Important intermediates are HMG-CoA, mevalonate, activated isoprenoids (IPP and DMAPP)

• Enzymes are located in peroxisome (pre-squalene) and ER