Complex Lipid Synthesis and Breakdown

Question 1

Triacylglycerol and Phospholipid Synthesis - What? Why? Where?

Answer 1

What - Using glycerol as the “backbone,” either 3 fatty acid groups are attached to form TAG OR 2 fatty acid groups plus a headgroup of phosphate and an alcohol form the phospholipids

Why - TAGs are the preferred storage form of fuel in adipose tissue, where they are nearly anhydrous globules in the cytosol of adipocytes

Phospholipids have several functions: major component of membranes (cells, organelles); reservoir for intracellular messengers; anchor some membrane-bound proteins; components of lung surfactants and bile

Where - TAGs: cytosol w/ final step in cytoplasmic face of ER; primarily in adipose and liver tissue (and also lactating mammary glands and intestinal mucosal cells)

Phospholipids: smooth endoplasmic reticulum - then to Golgi, various membranes, or secreted via exocytosis; occurs in all cells (except mature RBCs)

Question 2

Triacylglycerol and Phospholipid Synthesis - How?

Answer 2

Primary pathway for TAG and phospholipid synthesis is called de novo pathway & can be divided into four main steps

1. Backbone synthesis: glycerol 3-phosphate formed
2. two fatty acids attached -> Phosphatidate
3. To form TAG - phoshpate group is removed from phosphatidate and a third fatty acid is attached to C3
4. OR to form phospholipids - phosphatidate is activated by CTP addition to form CDP-diacylglycerol and appropriate alcohol headgroups attached

Question 3

Draw the pathway of Triacylglycerol and Phospholipid Synthesis

Question 4

Sphingomyelin and Glycolipid Synthesis - What? Why? Where?

Answer 4

What - Using sphingosine as a backbone, a fatty acid is attached through an amide bond to C2 to form ceramide; For sphingomyelin, a phosphorylcholine is attached to C1; Glycolipids have one or more sugars attached to C1 (but NO phosphate group)

Why - Sphingomyelin is an important component of the myelin of nerve fibers; Glycolipids are structural components of membranes (particularly nerve tissues) and generate lipid-signaling molecules, like phospholipids can

Where - Endoplasmic Reticulum and Golgi apparatus

Question 5

Sphingomyelin and Glycolipid Synthesis - How?

Answer 5

1. Formation of sphingosine: synthesis of the backbone
2. Ceramide formation: attache fatty acid via amide bond to C2
3. Form sphingomyelin (using CDP-choline) OR
4. Form glycolipids - attaching one or more sugars as polar head group

Question 6

Draw the pathway of Sphingomyelin and Glycolipid Synthesis

Question 7

Phospholipid and Sphingolipid/Glycolipid Degradation Pathways - What? Why?

Answer 7

What - the breakdown of phospholipids - either completely or ‘partially’ - to yield various fatty acids, alcohol headgroups, glycerol, etc

Why - membrane PL degradation plays important role in signaling: Arachidonic acid -> released by phospholipase A2 -> prostaglandin synthesis; IP3 and DAG -> released by phospholipase C (from phosphorylated phosphatidyl inositol); both are second messengers in signaling pathways

Continuous turnover in membranes to remove/replace oxidized lipids produced by ROS such as O2 and H2O2

Result: oxidized (broken) lipid -> leaky membrane

To alter the membrane composition to maintain proper fluidity or other membrane functions;

Where - many of the enzymes are in or associated with membranes in all cells

Question 8

Phospholipid and Sphingolipid/Glycolipid Degradation Pathways - How?

Answer 8

Numerous specific phospholipases (PLA2, PLC, etc) which remove FA tails or headgroups; and glycosidases, which remove the sugar headgroups of glycolipids

Question 9

Clinical problems due to defects in lipid synthesis and degradation pathways

Answer 9

Obesity - apparent genetic contributions, as well as environmental and behavioral factors

Lipid storage diseases - due to defects in phospholipid or sphingolipid degradation

Question 10

Sphingolipid Storage Diseases of Humans - Tay-Sachs disease

Answer 10

Principal storage substance: Ganglioside G

Enzyme deficiency: Hexosaminidase A

Sign/symptoms: Mental retardation, blindness, cherry red spot on macula, death between second and third year

Question 11

Sphingolipid Storage Diseases of Humans - Gaucher’s disease

Answer 11

Storage substance: Glucocerebroside

Enzyme deficiency: Glucocerebrosidase

Signs/Symptoms: Liver and spleen enlargement, erosion of long bones and pelvis, mental retardation in infantile form only

Question 12

Sphingolipid Storage Diseases of Humans - Fabry’s disease

Answer 12

Storage Substance: Ceramide trihexoside

Enzyme deficiency: alpha-Galactosidase A

Signs/Symptoms: Skin rash, kidney failure, pains in lower extremities

Question 13

Sphingolipid Storage Diseases of Humans - Niemann-Pick disease

Answer 13

Storage Substance: Sphingomyelin

Enzyme deficiency: Sphingomyelinase

Signs/Symptoms: Liver and spleen enlargement, mental retardation

Question 14

Sphingolipid Storage Diseases of Humans - Globoid leukodystrophy (Krabbe’s disease)

Answer 14

Storage substance: Galactocerebroside

Enzyme deficiency: Galactocerebrosidase

Signs/Symptoms: Mental retardation, absence of myelin

Question 15

Sphingolipid Storage Diseases of Humans - Metachromatic leukodystrophy

Answer 15

Storage Substance: Sulfatide

Enzyme deficiency: Arylsulfatase A

Signs/Symptoms: Mental retardation, nerves stain yellowish brown with cresyl violet dye (metachromasia)

Question 16

Sphingolipid Storage Diseases of Humans - Generalized gangliosidosis

Answer 16

Storage substance: Ganglioside Gmi

Enzyme deficiency: Gmi ganglioside: beta-galactosidase

Signs/Symptoms: Mental retardation, liver enlargement, skeletal involvement

Question 17

Sphingolipid Storage Diseases of Humans - Sandhoff-Jatzkewitz

Answer 17

Storage substance: Gm2 ganglioside, globoside

Enzyme deficiency: Hexosaminidase A and B

Signs/Symptoms: Same as 1; disease has more rapidly progressing course

Question 18

Sphingolipid Storage Diseases of Humans - Fucosidosis

Answer 18

Storage Substance: Pentahexosylfucoglycolipid

Enzyme deficiency: alpha-L-fucosidase

Signs/Symptoms: cerebral degeneration, muscle spasticity, thick skin

Question 19

Respiratory Distress Syndrome

Answer 19

Premature babes develop RDS because of immaturity of their lungs resulting from a deficiency of pulmonary surfactant

Maturity of fetal lung can be predicted antenatally by measuring lecithin/sphingomyelin (L/S) ratio in the amniotic fluid

Critical L/S ratio is 2.0 or greater

risk of developing RDS when L/S ratio is < 2.0

Results unreliable if amniotic fluid specimen has been contaminated by blood or meconium

Determination of saturated palmitoylphosphatidylcholine (SPC), phosphatidylglycerol, and phosphatidylinositol have been found to be additional predicotors of RDS

Exogenous surfactant replacement therapy using surfactant from human and animal lungs is effective in prevention and treatment of RDS

Question 20

Cholesterol Synthesis - What? Why? Where?

Answer 20

What - use of 18 acetyl CoAs (2C units) to make cholesterol - a 27C, 4 ring structure

Why - Cholesterol is an essential component of membranes and is a precursor to steroid hormones and bile salts; can also be used to make Vitamin D

Where - Cytosol (and smooth ER for latter part of pathway) of all cells; NOT a dietary requirement

Question 21

Cholesterol Synthesis - How?

Answer 21

Overall reaction:

18 acetyl CoA + 18 ATP + 14 (NADPH + H+) + O2 –> 1 cholesterol (27 carbons) + 18 (ADP + Pi) + 14 NADP+

Overview of cholesterol synthesis pathway:

Question 22

Draw the pathway of Cholesterol Synthesis

Question 23

Regulation of Cholesterol Metabolism - HMG-CoA Reductase

Answer 23

Catalyzes the committed step of cholesterol synthesis; thus major control point

Reaction: HMG-CoA -> mevalonate (redox reaction)

Phosphorylation state of HMG-CoA reductase: Phosphorylated HMG-CoA reductase is inactive; Dephosphorylated HMG-CoA reducatse is active

Four levels of regulation of HMG-CoA reductase: Hormonal regulation (outside cell); Regulation of gene expression of the enzyme; Regulation of protein breakdown of the enzyme; Inhibited by cholesterol, a cellular effector

Hormonal regulation: Insulin (+): activates a phosphatase to dephosphorylate HMG-CoA reductase -> activation; Glucagon (-): activates a kinase to phosphorylate HMG-CoA reductase; thus is inactivated

Regulation of gene expression of the reductase: Cholesterol (-): HMG-CoA reductase synthesis

Regulation of protein breakdown of the reductase: Cholesterol (+): HMG-CoA reductase degradation; ultimately a NEGATIVE EFFECT on cholesterol synthesis since removing enzyme from cell

Question 24

Regulation of Cholesterol Metabolism - Synthesis of the LDL receptor is also regulated

Answer 24

Cholesterol (-): LDL receptor synthesis

LDL primarily carries cholesterol, so if enough cholesterol, do not want to endocytose an LDL

Question 25

Regulation of Cholesterol Metabolism - Cholic acid

Answer 25

One of the bile acids and a breakdown product of cholesterol

Found in the liver and intestine

Cholic acid has same regulatory effects on HMG-CoA reductase as dose cholesterol

Question 26

Regulation of Cholesterol Metabolism Summary

Question 27

Products of Cholesterol

Answer 27

Vitamin D - Cholesterol + Vitamin D -> Vitamin D; still a vitamin, because generally does not produce enough

Steroid hormones include: Glucocorticoids; Mineral corticoids; Androgens; Estrogens

Bile acids and bile salts

Question 28

Bile Acid and Bile Salt Synthesis - What? Why? Where?

Answer 28

What - Bile acids are 24 carbon degradation products of cholesterol, with hydroxyl groups inserted at specific positions and the hydrocarbon chain shortened by 3 carbons. Bile salts are bile acids that are conjugated to either glycine or taurine before leaving the liver

Why - function: powerful detergents (emulsifying agents), essential for digestion and absorption of dietary lipids - breakdown lipid droplets and membranes and make them accessible to H2O soluble hydrolytic enzymes

Where - bile acids and salts are synthesized in the liver, then secreted directly from the liver to the intestines OR secreted into the gall bladder for storage (if not immediately used for digestion); Amounts: ~0.5g/day produced; Recycled: over 95% reabsorbed and reused; lose about 0.5g/day in feces so cholesterol is not effectively removed from the body

Question 29

Bile Acid and Bile Salt Synthesis - How?

Answer 29

Synthetic Pathway

Committed step: cholesterol -> 7-alpha-hydroxycholesetrol; Enzyme: cholesterol 7-alpha-hydroxylase

Primary bile acids: cholic acid and chenodeoxycholic acid

Common bile salts: glycocholic acid and taurocholic acid

Question 30

Bile Acid and Bile Salt Synthesis - Regulation

Answer 30

Cellular effectors: Cholesterol (+); cholic acid (-)

Hormonal: insulin (+): again, favors most synthetic pathways; glucagon (-)

Question 31

Draw the pathway of Bile Acid and Bile Salt Synthesis

Question 32

Clinical Problem of Bile Acid Synthesis

Answer 32

Gallstones (cholelithiasis): more cholesterol enters the bile than can be solubilized by bile salts and phosphatidylcholine; thus cholesterol precipitates in gall bladder leading ot gall stones

Due to: decreased bile acids in bile (several causes) and increased biliary cholesterol excretion (use of fibrates to decrease triacylglycerol levels in the blood)

Question 33

Lipid-Soluble Vitamins

Answer 33

General features: all are isoprene units; hydrophobic, so not readily excreted, thus be careful overdosing because vitamins are stored in tissues; complex functions, not all known

Lipid-soluble vitamins are: Vitamins A, D, E, and K