Lecture 4- Lipids To Cholesterol Flashcards
What organic molecule in the body contains the largest source of energy?
Lipids
Where does the digestion of lipids begin? what catalyzes it? And what are the primary targets?
Begins in the stomach
Lipase
Targets Triacylglycerols (TAGs)
Where does the emulsification of dietary lipids take place?
Duodenum (small intestine)
What does emulsification of dietary lipids achieve? And what does the emulsifying?
It increases the surface area of the hydrophobic lipid droplets so that the digestive enzymes can act effectively.
Bile salts do it
What degrades dietary lipids?
Pancreatic enzymes
What degrades TAG (triacylglyceride)?
Pancreatic lipase
What degrades cholesterol esters?
Cholesteryl esterase
What degrades phospholipids?
Phospholipase A2
Lysophospholipase
Why are TAGs unable to efficiently be taken up by the mucosal cells of the intestinal villi?
TAGs are too large
What acts on TAGs to make them smaller, what is being removed and what is the end product?
Pancreatic lipase
Removes the fatty acids (FAs) from C1 and C3
Forms 2-monoacylglycerol
What form are most dietary cholesterol present in?
Free form (nonesterified)
What enzyme hydrolyzes cholesterol esters, what factor affects the enzyme, and what is the final product?
Cholesteryl esterase hydrolyzes the ester bond and releases a fatty acid.
It’s activity is greatly increased in the presence of bile salts.
Cholesterol is the final product
What type of backbone do phospholipids have?
Glycerol backbone
What enzyme removes a FA from the C2 of a phospholipid?
Phospholipase A2
What enzyme removes a FA from C1 of a phospholipid?
Lysophospholipase
What is the product when phospholipase A2 acts on a phospholipid?
A lysophospholipid
What is the product when lysophospholipase acts on a lysophospholipid?
Glycerophosphoryl base
What are two hormones produced by gut cells that aid in lipid digestion and what are their responses?
Cholecystokinin (CCK)- causes pancreas to secrete pancreatic enzymes, gall-bladder to secrete bile, and makes the stomach feel fuller longer
Secretin- causes the pancreas to secrete bicarbonate
What is the name of an intestinal mucosal cell?
Enterocyte
What forms mixed micelles and what are they?
Lipids, bile salts and fat soluble vitamins
Disk shaped clusters of amphipathic lipids that coalesce with their hydrophobic groups on the inside
Where is the primary site for lipid absorption?
The brush border membrane of the enterocytes
Where are the absorbed lipids in enterocytes resynthesized back to complex lipids (original forms)?
Endoplasmic reticulum of the enterocytes
What can lipid malabsorption result in?
Steatorrhea- lipid in feces
Caused by disturbances in lipid digestion and/or absorption
Chylomicron
A lipid droplet composed of an aggregate of resynthesized
TAGs and
cholesteryl esters, surrounded by phospholipids
Unesterified cholesterol, and
Apolipoprotein
How and where are chylomicrons released from enterocytes?
Released by exocytosis into the lacteals (lymphatic vessels originating in the villi of the small intestine)
What is the path of chylomicrons and its component parts?
Exits the enterocyte and enters into the lymph, then enters blood via the thoracic duct into the sublavian vein, TAGs are drawn off my muscle and fat cells, chylomicron remnants are removed from the blood by the liver.
(Don’t enter blood directly cuz too large)
TAGs contained in chylomicrons are broken down primarily where and by what?
In skeletal muscle and adipose tissue by
Lipoprotein lipase
Explain fatty acids
Produce energy
Resynthesized to TAGs which are stored until FAs are needed
Explain glycerol use in the body
Used almost exclusively by liver to produce glycerol-3-phosphate which can enter glycolysis or gluconeogenesis by oxidation to DHAP
Explain what happens to remaining chylomicron components
Taken up by liver and hydrolyzed to their component parts
Some can be recycled by the body
Fatty acid forms found in the body
Free fatty acids
Fatty acyl esters in TAGs
What tissues can particularly oxidize fatty acids to produce energy?
Liver
Muscle
Note which carbon in a fatty acid chain is the alpha carbon
C2, not the 1st carbon in the carboxyl group
What does a fatty acid consist of?
A hydrophobic hydrocarbon chain with a terminal hydrophilic carboxyl group
- amphipathic molecule
At pH of 7, is the hydrophilic carboxyl group of a fatty acid ionized or not ionized?
Ionized
What form are the majority of fatty acids in when found in plasma?
90%+ are fatty acid esters (primarily TAG, cholesteryl ester and phospholipid)
Unesterified (free) fatty acids are transported in the circulatory system in association with what?
Albumin (water soluble protein)
How long are fatty acids?
Typically between 12 and 20, but could be longer
How are fatty acids numbered?
Starting with the carboxylate end
Saturated vs unsaturated fatty acid
Saturated = no double bonds (completely reduced) Unsaturated = at least 1 double bond
The double bonds in fatty acids are most commonly (cis/trans)? And which type packs tighter? And which type causes a “kink”?
Cis is more common
Trans pack tighter
Cis cause a kink
Which fatty acids have a lower melting point, cis or trans?
Cis have lower melting point
What happens to the melting point as the hydrocarbon chain lengthens?
Melting point increases
Multiple double bonds in a fatty acid exhibit what type of spacing?
Spaced at 3-carbon intervals
Arachidonic acid
20:4 (5,8,11,14)
Explain what it means
20 carbons long
4 C=C between C5-6, C8-9, C11-12, C14-15
Another way to write 18:2 (9,12) and general class name
18:2w-6
Omega-6 fatty acid
w = omega
What are the essential fatty acids?
Linoleic acid
a-linolenic acid (extra dbl bond)
What are lipids?
Heterogenous group of hydrophobic organic molecules
What are linoleic acid and its derivatives referred to as?
Omega-6 fatty acids
What are a-linolenic acid and its derivatives referred to as?
Omega-3 fatty acids
What happens to carbohydrates and proteins obtained from the diet that are in excess of the body’s needs?
Converted to fatty acids and stored as triacylglycerols
In adults, where does fatty acid synthesis occur?
Liver (mainly)
Lactating mammary glands
Adipose tissue (to a lesser extent)
Where does fatty acid synthesis synthesis occur?
Cytosol
During fatty acid synthesis, where do the carbons come from that are incorporated into the growing fatty acid chain?
Carbons from acetyl- CoA
by using ATP and NADPH
How are fatty acids synthesized?
By the repetitive addition of two-carbon units to the growing end of the hydrocarbon chain
How is acetyl-CoA in the mitochondrion transported to the cytosol?
Acetyl-CoA binds to oxaloacetate via citrate synthase to form citrate which crosses the membrane and is then reconverted to oxaloacetate and acetyl-CoA
During fatty acid synthesis, what two products are acetyl-CoA converted to and by what enzymes?
To acetyl-ACP (acyl carrier protein) by transacylase
To malonyl-CoA by acetyl-CoA carboxylase
What is acetyl-ACP converted to and by what enzyme?
To acetyl-synthase by
Transacylase
What is malonyl-CoA converted to and by what enzyme?
Malonyl-ACP by
Malonyl transacylase
What is the first 4 carbon unit formed during fatty acid synthesis?
Acetoacetyl-ACP
Final product in fatty acid synthesis and what enzyme releases it?
Palmitate
Released by FAS (fatty acid synthase)