Lipids Flashcards
What are lipids?
Heterogeneous, hydrophobic organic molecules
- insolubility in water results in compartmentalization
Functions of lipids
- major source of energy
- structural component of cell membranes
- may serve as fat-soluble vitamins
- structural component of prostaglandins and steroid hormones
Deficiencies in lipid metabolism lead to…
atherosclerosis, diabetes, obesity
Five classes of lipids
- Free Fatty Acids
- Triacylglycerols (TAG)
- Phospholipids
- Glycolipids
- Steroids
Fatty Acids
- chains of hydrocarbons that terminate in COOH
- simplest lipids
- most common fuel source
- also building blocks for membranes
- can contain double bonds and vary in length
What are fatty acids ionized at
pH 7
Naming Fatty Acids
- C of COOH is C 1
- final methyl group is w
- double bond is represented by a delta superscript w/ numbers rep. the position of the bond from the end
Essential amino acids for diet
Linoleate, Linolenate
- body cannot produce large double bonds (omega 3) (don’t have the enzyme)
Unsaturated
contains double bonds
- can be cis or trans double bonds
- trans bad and unnatural
- decreases the M.P
- longer HC, greater M.P
Saturated
contains no double bonds
Too much saturated and trans unsaturated fats are bad
- elevated cholesterol and cardiovascular disease
- biochemical basis unknown but trans-unsaturated fats trigger immune response
Essential fats w-3 and w-6
- important precursors to hormones
- w-3 fats have cardiovascular protective properties
Triacylglycreols (TAGs)
- Glycerol backbone w/ three fatty acid chains
- primary storage molecule
- FFA are strong acids, low pH
- 1 gram of TAG = 6x the energy of 1 gram of glycogen
Adipose Tissue
TAG are stored in adipose tissue in fat droplets “oil within cell”
Examples of how TAG is stored
- stored in hummingbirds for energy
- stored in polar bears for warmth
Phospholipids
Major class of membrane molecules
- amphipathic
Glycolipids
- sugar containing lipids
- found in cell membranes
- role in cell-cell interacions
- sugar always on extracellular side
Steroids
Lipids w/ a steroid nucleus
Functions of Steroids
- hormones, facilitate digestion of lipids (bile salts), fluidity of membrane (dec.)
Dietary Lipids
- dietary lipid intake for an adult is 60-150g/day
- 90% TAG
- 10% cholesterol, cholesterol esters, phospholipid, unesterified FFA
Where does lipid digestion begin
stomach
What is lipid digestion catalyzed by
- lingual lipase- released from glands behind the tongue
- gastric lipase- released from mucosal cells lining the stomach
What does digestion of lipids degrade?
degrade short/med length TAGs to glycerol and FFA
Importance of lipid digestion
- important for lipid digestion in infants where milk (high in TAGs) is primary source of energy
- important in CF patients who have pancreatic insufficiency
What does mechanical mixing in the stomach begin w/?
emulsification of fats (oil in water)
What happens in the small intestine?
- dietary lipids undergo further emulsification when mixed w/ bile salt
What 2 mechanisms does emulsification help digestive enzymes work effectively w/?
- Mixture w/ bile salts- inc. SA of lipids available to enzymes
- Mechanical mixing by peristalsis
Degradation by pancreatic enzymes
TAG digestion
What is TAG digestion?
- TAG molecules too large for mucosal cellular uptake
- pancreatic lipase preferentially removes fatty acids from C 1 and 3
- primary products are two FFA and 2-monoacylglycerol
Can FFA freely transport into blood and cytoplasm?
No, need chylomicrons to make hydrophobic for transport
TAG re-synthesis and transport
In intestinal cell lipid digestion products are resynthesized into TAG and packaged for transport
What are Lipoproteins?
Lipoproteins are complexes of lipids and apolipoproteins found in the blood and plasma
Four main classes of lipoproteins
- Chylomicrons (CM)
- Very Low Density Lipoproteins (VLDL)
- Low Density Lipoproteins (LDL)
- High Density Lipoproteins (HDL)
Functions of Lipoproteins
- Maintain lipids in soluble form in blood
- Deliver lipids to tissues
Medical relevance of Lipoproteins?
Dietary condition or genetic defects in cholesterol metabolism lead to plaque formation and result in atherosclerosis
Composition of Lipoproteins?
- A core of TAG and cholesteryl esters
- a shell of cholesterol, phospholipids, apolipoproteins
How are TAG and cholesterol obtained?
obtained from diet or de novo synthesis (mix. of fat sources), and packaged into inner core and carried by the particle in blood
Size and density of CM?
- largest, lowest density
- highest lipid, lowest protein content
Size and density of VLDL and LDL?
- more dense than CM
- higher content of protein and lower content of lipid
Size and density of HDL?
- smallest and most dense
- greatest content of protein, lowest content of lipid
Composition of CM?
- mostly TAG
- carry dietary fats to rest of body
Composition of VLDL
- mostly TAG
- carry de novo synthesized fats from liver to rest of body
Composition of LDL
- very rich in cholesterol and cholesteryl esters (bad cholesterol)
- after TAG is delivered to tissues from VLDL, LDL is what remains
Composition of HDL
- acceptor of free cholesterol
- very rich in phospholipids and apolipoproteins
- carrier of good cholesterol
- cholesterol scavanger
What are Apolipoproteins?
Protein component of lipoproteins
Functions of Apolipoproteins
- Structural components of lipoproteins
- Recognition site for cell-surface receptors
- Activators or coenzymes for lipoprotein metabolic enzymes
What do CM and VLDL release?
Release TAG to tissues
What does Lipoprotein lipase release?
- located on surface of tissues (adipose)
- releases fatty acids from TAG into tissues
Where do CM remnants go?
Return to liver
What happens to VLDL?
Becomes LDL to deliver cholesterol to tissues
Metabolism of LDL?
- carry cholesterol to the peripheral tissues
- contain apo B-100
How is LDL removed from blood?
- removed via cellular uptake
- LDL receptors recognize apo B-100 and LDL is endocytosed
What does the degradation of LDL release?
Releases cholesterol, a.a, FAs, phospholipids
What happens to LDL receptors?
Recycled to the cell surface
- to bring more cholesterol in
How does cholesterol uptake stop?
Once signalled enough cholesterol, will make a feedback loop, reducing number of receptors, reducing # of LDL’s
What happens when cholesterol is oversupplied?
- High cholesterol inhibit HMG CoA reductase (sterol dependent degradation), resulting in decreased cholesterol synthesis
- High cholesterol reduces the number of LDL receptors = less uptake
- Storage of cholesterol as cholesteryl esters
What is atherosclerosis?
- characterized by elevated levels of LDL in the plasma
- high cholesterol decreases the receptors for LDL
What happens to excess LDL?
Becomes oxidized
- instigates immune response
- macrophages take up oxLDL and become engorged forming foam cells
- foam cells get trapped in the blood vessel walls contributing to plaque formation
