Fatty Acid Synthesis Flashcards
(47 cards)
1
Q
What is the precursor of free fatty acids?
A
Acetyl CoA (carb or AA makes it)
2
Q
What is the precursor of the glycerol backbone?
A
Glycerol-3-P (carb/glycerol)
3
Q
Where are free fatty acids and TGs synthesized?
A
cytosol
4
Q
Why does eating excess carbs result in fat formation?
A
excess dietary glucose is converted to adipose tissue TGs
5
Q
What organs do fatty acid synthesis and TG synthesis?
A
- liver (major)
- lactating mammary gland
- adipose (minor)
6
Q
What form are fats transported from the liver?
A
- VLDL (endogenous synthesized)
- similar to chylomicrons (dietary)
7
Q
What protein plays a major role in storing fat in adipose tissue?
A
lipoprotein lipase
8
Q
Overview process of lipogenesis? (5)
A
- fatty acid synthesis occurs mainly in liver
1. glucose goes into the cytosol of the liver, undergoes glycolysis to pyruvate
2. pyruvate is transported to mitochondria
3. pyruvate either becomes OAA or Acetyl CoA depending on the concentration of Acetyl CoA
4. Citrate is formed by Citrate synthase (important in metabolism regulation-limits PFK1)
5. Citrate is transported to the cytosol where it splits into OAA and Acetyl CoA
6. Acetyl CoA is turned into Malonyl CoA (3C) by Acetyl CoA carboxylase
7. Multiple steps by fatty acid synthase, input of NADPH, to become palmitate
8. Palmitate becomes fatty acid CoA, combines with Glycerol-3-P to become TG
9. TG cannot move freely in blood stream, so apoproteins and other lipids combine with it to become VLDL
10. moves to adipose tissue for long term storage
9
Q
What is the major substrate for fatty acid synthesis?
A
- acetyl CoA
- can be from carbs or AA
- in mitochondria, move to cytosol
10
Q
Energy needed for FA synthesis?
A
ATP
11
Q
Reducing power of FA synthesis?
A
- NADPH
1. from pentose pathway
2. NADP linked malic enzyme
12
Q
Where does the glycerol backbone come from for TG?
A
-from glycolysis
13
Q
Purpose of FA synthesis?
A
- long term storage
- time you can survive starvation depends on fat storage
14
Q
What leads to the accumulation of citrate? (7)
A
- slowed down TCA cycle
- energy needs have been met
- high ATP/ADP ratio
- high NADH/NAD ratio
- inhibit isocitrate dehydrogenase, reversible, more citrate forms
15
Q
How does Acetyl CoA get from the mitochondria to the cytosol? (7)
A
-it cannot move through the membrane so it is converted to citrate which can be transported to the cytosol by a membrane transporter
16
Q
What happens to to citrate in the cytosol? (7)
A
- it is converted back to acetyl CoA and OAA by Citrate Lyase (also called citrate cleavage enzyme)
- ATP is used
- Citrate lyase is induced by insulin
17
Q
Where does Acetyl CoA go once it is in the cytosol? what does it depend on? (7)
A
- fatty acid synthesis
- cholesterol synthesis
- depends on insulin levels, higher insulin
18
Q
Where does OAA go once it is in the cytosol? (7)
A
- turns into malate by cytosolic malate dehydrogenase, NADH is used
- Malate is oxidized into pyruvate by malic enzyme and NADP, CO2 released
- malic enzyme is induced by insulin - NADPH will be used for long chain FA synthesis
19
Q
What are the sources for NADPH? (8)
A
- PPP pathway
- Malic enzyme
- one pathway is not enough
20
Q
What are the 5 inducible enzymes in FA synthesis? (8)
A
- Citrate lyase
- Malic enzyme
- G6P dehydrogenase
- FA synthase
- acetyl CoA carboxylase
- induced by insulin
- cytosolic malate dehydrogenase and malic enzyme provide a transhydrogenase mechanism in the cytosol to transfer H from NADH to NADPH
21
Q
What is the 1st stage in FA synthesis? Process? (9)
A
- Acetyl CoA (2C) to Malonyl CoA (3)
- acetyl CoA carboxylation
1. Acetyl CoA (2C)
2. Acetyl CoA carboxylase
3. CO2 and ATP used
4. Biotin needed as cofactor
5. Malonyl CoA formed (3C)
22
Q
What reactions use biotin as a cofactor?
A
- Acetyl CoA carboxylase
- Propionyl carboxylase
- pyruvate carboxylase
- responsible for CO2 transfer in carboxylase enzymes
23
Q
What is the 2nd stage of FA synthesis? (10)
A
- Fatty acid synthase
- Dimer of 2 identical subunits
- each subunit has 7 enzymatic activities, multifunctional
- two arms:
1. acyl carrier protein (ACP or P site) containing phosphoantetheine (Vit B5)
2. cysteine residue (SH group)
24
Q
Process of FA synthesis? (11)
A
- this process is repeated 8 times to form a 16C palmitate (from 2C acetyl CoA)
1. stage 1: acetyl CoA provides 2 C on the P side of fatty acid synthase, covalently attached (priming step)
2. transfer acetyl CoA from P side to SH side of FA synthase
3. P side is open on FA synthase
4. Malonyl CoA (from carboxylation of acetyl CoA) adds to the P side of FA synthase, covalent attached (loading step-rxn 1)
5. methylene C of malonyl group attacks the acetyl group on the SH side, condensation, adds those carbons (4 C keto chain formed), CO2 is released, SH side of FA synthase is now free again
6. NADPH adds two H’s to the ketone group, H2O leaves
7. another NADPH adds two H’s to remove double bond
8. Carbon strand is transferred from P side to SH side, leaving P side open for another Malonyl CoA - reduction, dehydration, reduction
9. the process repeats until 16 C long
10. chain is released from enzyme complex - Notes:
- Acetyl CoA is used only once
- Malonyl CoA is used multiple times (from acetyl CoA carboxylase, biotin, CO2, ATP)
- two NADPH is used, reduction
25
Beta oxidation vs FA synthesis? (13)
pic
26
What is stage 3 of FA synthesis?
-elongation of palmitate to longer chain FA and desaturation of FA
27
Process of elongation? (14)
1. FA activation (becomes acyl CoA) by Fatty acyl CoA synthetase
- the enzyme attaches CoA to palmitate
- needs ATP
- FA 10-16 C is used as precursor for extension
2. Malonyl CoA is used to add 2 of its C to palmitoyl CoA
- FA elongase, a multi enzyme complex (similar to FA synthase except that fatty acyl CoA is the substrate which condenses with Malonyl CoA)
- occurs in smooth ER
3. carbons are added two at a time
28
Desaturation of fatty acids to create double bonds? (15)
1. Saturated fatty acyl CoA has 2 H's removed from consecutive carbons
- fatty acyl desaturase used
2. double bond is formed to create a monounsaturated fatty acyl CoA through oxidation
- occurs in ER
- uses molecular O2
- FA and NADH are oxidized
- cannot be C 10 or above
- cannot make omega 6 or 3 (essential-comes from diet)
29
What are human desaturases incapable of?
- cannot introduce double bonds between Carbon 9 and the methyl end
- when we need other double bonds, we use essential fatty acids as precursors (linolenic, linoleic)
30
Introducing double bonds to essential fatty acids? (16)
- we get essential FA from diet
- we can desaturate, add double bonds, on carbons less than 9, on carbon 6
- we cannot desaturate passed C 9
31
What are the essential FA?
1. linoleic (18:2, 9, 12)
- omega 6
- serves as precursor of arachidonic acid, which is the source of eicosanoids, prostaglandins, leukotrienes
2. Linolenic (18:3, 9, 12, 15)
- omega 3
- form eicosanoids
32
What are the conditions for FA synthesis?
- increased ATP
- increased acetyl CoA
- increased NADPH (lots of citrate in TCA cycle)
- when there are a lot of reactants
33
What is the rate limiting enzyme for FA synthesis?
acetyl CoA carboxylase
34
Short term regulation of FA synthesis? (17)
- Acetyl CoA carboxylase exists as inactive monomers that polymerize when active
- allosterically activated by citrate (allosterically inhibits PFK1)
- allosterically inhibited by palmitoyl CoA (end product, fatty acyl CoA)
- glucagon activates cAMP dependent protein kinase A to phosphorylate the enzyme and inactivate it (low energy levels, low AMP)
- insulin activates a phosphatase to dephosphorylate the enzyme to activate it
- inducible by insulin
35
Long term regulation FA synthesis?
- induction of key enzymes:
1. citrate lyase
2. acetyl CoA carboxylase
3. G6P dehydrogenase
4. Malic enzyme
5. FA synthase
- enzyme synthesis is increased if:
- person has a good diet over time
- person has high carb or fat free diet
- due to insulin/glucagon levels
- enzyme synthesis is decreased if:
- person is fasting or on high fat diet
36
Regulation to prevent futile cycle between FA synthesis and breakdown?
- futile cycle between FA synthesis and FA oxidation is avoided
- Malonyl CoA (product of 1st stage of FA synthesis) inhibits fatty acids from being transferred to carnitine and entering the mitochondria
- beta oxidation occurs in mitochondria
- example of regulation by compartmentalization
37
In the fed state, how does Malonyl CoA prevent futile cycles? (20)
- malonyl CoA inhibits CPTI (carnitine acyl transferase I or carnitine palmitoyltransferase I)
- this prevents beta oxidation
38
TG synthesis? (21)
1. in liver, glycerol turns into glycerol-3-phosphate by glycerol kinase (only liver has this enzyme), ATP to ADP
- in liver and adipose, glucose goes through glycolysis until it reaches DHAP, it is reduced to glycerol-3-phosphate by glycerol-p-dehydrogenase, NADH to NAD
2. two Fatty acyl CoA is added to G3P by acyltransferase
3. phosphatase removes phosphate groups
4. acyltransferase adds a third fatty acyl coa to the diacylglycerol to make TG
5. from liver to blood VLDL
- or adipose stores
39
Sources of glycerol phosphate in TG synthesis? (22)
pic
40
How is futile cycle avoided in TG synthesis?
- futile cycle between TG synthesis and breakdown is avoided in adipose because:
1. adipose must obtain G3P from glycolysis
2. there is no glycerol kinase in adipose
3. when HSL (activated by glucagon) cleaves TG to FA + glycerol, the reaction is not reversible without glycerol kinase, so FA and glycerol leave adipose to be used for energy
4. TG are only synthesized in times of plenty, when glycolysis is increased
41
TG synthesis in different tissues? (24)
pic
42
How are TGs made by the liver carried through blood?
on Very Low Density Lipoproteins (VLDL)
43
Synthesis of VLDL? (25)
1. proteins (Apo B100) are synthesized on the rough ER
- TG are synthesized on smooth ER and/or on FA synthase in cytosol
2. TG and proteins are packaged in Golgi complex to form VLDL
3. VLDL are transported to the cell membrane in secretory vesicles and secreted by exocytosis
- dots represent VLDL particles
- enlarged VLDL particle is depicted at bottom of figure
- graph shows composition of VLDL particle
44
Apo B100 vs Apo B48? (26)
- made on the same gene
- RNA editing (deamination) turns cytosine into uracil, encodes stop codon
- 48 is in intestine
- 100 is in liver
45
Transport of TG in blood? (28)
1. glucose enters liver and is converted to fatty acids and G3P
2. TG are formed from fatty acyl CoAs and G3P
3. TG are incorporated into VLDL
4. VLDL goes into blood and carried to adipose tissue
5. TG in VLDL are hydrolyzed by lipoprotein lipase (LPL) and the released fatty acids are taken into adipose
6. in adipocytes, fatty acids are resynthesizes into TG for storage
46
Conversion of excess dietary fat to adipose tissue TG? (27)
1. dietary lipids are digested and absorbed into intestinal cells
2. absorbed lipids are incorporated into chylomicrons in intestinal cells
3. chylomicrons enter the blood and carry the dietary lipids to adipose tissue
4. TG in the chylomicrons are hydrolyzed by LPL and the released fatty acids are taken into adipose tissue
5. fatty acids are incorporated into storage TG
47
Accumulation of TG in adipose tissue? (29)
1. insulin stimulates the transport of glucose into adipose cells
2. glucose provides the G3P for TG synthesis
3. insulin stimulates the synthesis and secretion of lipoprotein lipase (LPL)
4. ApoC (obtained from HDL in blood) activates LPL
- ApoC is present in both VLDL and chylomicrons
