Fatty Acid Synthesis and Eicosanoids Flashcards
1
Q
Lipogenesis
A
- synthesis of fats from glucose
- fatty acids are synthesized primarily in the liver
- dietary carbs are converted to glucose which serves as the substrate for glycolysis which produces pyruvate
- pyruvate enters the mitochondria where it is converted to citrate
- citrate, unlike acetyl CoA, can exit the mitochondria into the cytosol
- once in the cytosol the citrate is converted to acetyl CoA, which is then converted to malonyl CoA, the two carbon donor for fatty acid synthesis
- the product of fatty acid synthesis is palmitic acid, a sixteen carbon saturated fatty acid
2
Q
Conversion of Glucose to Cytosolic Acetyl-CoA
A
- pyruvate which enters the mitochondria is converted to Acetyl CoA by pyruvate dehydrogenase and to oxaloacetate (OAA) by pyruvate carboxylase
- these enzymes are regulated based on the level of acetyl CoA in the mitochondria
- when Acetyl CoA is at high concentration the pyruvate dehydrogenase is inhibited causing an increase pyruvate carboxylase resulting in higher levels of OAA
- when OAA increases it condense with acetyl CoA forming citrate
- the reduction in acetyl CoA leads to activation of the dehydrogenase and inhibition of carboxylase
- in the cytosol citrate is cleaved to acetyl CoA and OAA by citrate lyase
- the pyruvate dehydrogenase is only found in the mitochondria
- acetyl CoA cannot cross the mitochondria double membrane
3
Q
Citrate Lyase and Malic Enzyme
A
- NADPH is required for the synthesis of Fatty Acids. One source of this NADPH is the recycling of the OAA, which is formed as a product of citrate lyase, to pyruvate
- 2 steps:
- OAA is reduced by a NADH dependent cytosolic Malate dehydrogenase to malate
- malate is then converted by the NADP+-dependent Malic Enzyme which oxidized and decarboxylates Malate to Pyruvate
- pyruvate is reconverted to citrate and then NADPH, which is a product of the Malic Enzyme, is used in the production of Fatty Acids by Fatty Acid Synthase
4
Q
Source of NADPH for FA Synthesis
A
- recycling of the OAA
- a second source of NADPH for fatty acid synthesis is the Pentose-phosphate pathway
5
Q
Fatty Acid Synthesis
A
- two enzymes required for fatty acid synthesis from acetyl coA to palmitate
- the first is acetyl CoA carboxylase (ACC) and the second is fatty acid synthase
- ACC is the rate controlling step in fatty acid synthesis and is highly regulated
6
Q
Acetyl CoA Carboxylase (ACC)
A
- a regulatory enzyme which converts acetyl CoA to Malonyl CoA
- CO2 binds to the cofactor Biotin which is linked to the E-amino group of a lysine in ACC
- the attachment of CO2 to Biotin requires ATP hydrolysis
- the carboxyl group is then added to acetyl CoA converting it to Malonyl CoA
7
Q
Regulation of ACC
A
- citrate which causes it to polymerize
- insulin stimulated phosphatase
- caloric intake/ACC transcription levels
- glucagon/epinephrine stimulated A-Kinase
- palmitoyl CoA levels
- AMP (low energy levels)
- ACC is the rate limiting enzyme and key regulatory enzyme in Fatty acid synthesis
8
Q
Steps in Fatty Acid Synthesis
A
1) Activation: carboxylation of acetyl CoA to malonyl CoA( rate-controlling step) followed by its attachment to FAS (catalyzed by acetyl CoA carboxylase)
2) Condensation: formation of the B-keto group
3) Reduction of the B-keto group
4) Dehydration of the a,b carbons
5) Reduction of the a,b double b
Steps 2-5- catalyzed by fatty acid synthase- adds two carbon units from Malonyl COA to the growing fatty acyl chain with the final product being palmitate
-FAS is a homodimer of ~240,000 Da. Each subunit contains 7 catalytic activities and an acyl carrier protein (ACP) segment in a single polypeptide chain
9
Q
Phosphopantethenyl Residue of Fatty Acid Synthase (FAS)
A
- derived from the vitamin pantothenic acid
- phosphopantetheine is linked to a serine within the acyl carrier protein (ACP) portion of FAS
- SH group reacts with malonyl CoA to form a thioeser bond
- the ACP segment of FAS has a phospho-pantetheinyl residue covalently attached to the ACP segment
- the two subunits associate head to tail so that the phospho-pantetheinyl sulfhydryl group on one subunit is close to the cysteinyl sulfhydryl group on the other subunit
10
Q
Second step in Fatty Acid Synthesis
A
- the initial condensation step carried out by FAS
- an acetyl group from acetyl CoA first associates with the ACP phospho-pantetheinyl sulfhydryl groups and then is transferred to the cysteine sulfhydryl of the other subunit
- malonyl from malonyl CoA then associates with the ACP phospho-pantetheinyl sulfhydryl group
- the acetyl and malonyl groups condense with release of the malonyl carboxyl group as CO2
- this creates a four carbon B-keto acyl chain which is attached to the ACP phospho-pantetheinyl sulfhydryl group
- the carbon which will eventually become the omega methyl group of palmitate is labelled w
11
Q
Reduction of ketoacetyl group on FAS
A
- steps 3 to 5 in Fatty Acid synthesis include a reduction of the B-keto group to an alcohol (step 3)
- followed by a dehydration to form a double bond
- and finally another reduction step reduces the double bond
- NADPH provides the reducing equivalents for steps 3 and 5
12
Q
Synthesis of Palmitate on FAS
A
- the four carbon fatty acyl chain is transferred from the ACP phospho-pantetheinyl sulfhydryl group
- this allows binding of the next malonyl group (2 carbon donor) to the ACP phospho-pantetheinyl sulfhydryl group
- condensation (2), reduction (3), dehydration (4), and reduction (5) are repeated with the result being a 6 carbon fatty acyl chain
- this series of reactions is repeated until the fatty acid chain is 16 carbons long and then hydrolysis occurs and palmitate is released
13
Q
Elongation of Long Chain Fatty Acids
A
- after palmitate is released from FAS it is activated to plamitoyl-CoA
- Palmityl CoA and other long chain activated fatty acyl CoAs (saturated and unsaturated) can be elongated two carbons at a time
- the fatty acid chains are elongated in the ER by enzymes called elongases
- the steps are the same as those previosuly described with Malonyl CoA as the two carbon donor, except that the long chain fatty acid is attached to Coenzyme A instead of a phospho-pantetheinyl residue attached to the ACP
- the major elongation that occurs in our body is palmyityl CoA to stearyl CoA (18 carbon) but longer fatty acids (20-24 carbon) are also formed especially in the brain
14
Q
Fatty Acid Desaturation
A
- desaturation is the oxidation of fatty acids resulting in cis double bonds
- desaturation creates lipids of increasing structural and functional complexity with distinct biological roles
- the process requires desaturases, located in the ER, that require O2, NADH, and cytochrome b5. The electrons are passed through an ER Cyt b5 electron transport chain
- desaturases are highly regulated in response to diet. During starvation, desaturase activities decrease sharply and they increase upon re-feeding carbohydrate. When large amounts of unsaturated fats are eaten desaturase activities decrease
15
Q
Monosaturated Fatty Acids (MUFAs)
A
- humans have three distinct desaturases distinguished by the position of the double bond insertion with the fatty acid chain
- Delta9,6,5 desaturase
- the most common desaturation reactions involve an oxidation leading to a double bond between C9 and C10
- palmitic acid to plamitoleic acid (16:1, delta9)
- stearic acid to oleic acid (18:1, delta9)
- both are MUFAs
16
Q
Polyunsaturated Fatty Acids: PUFAs
A
- PUFAs with double bonds three carbons (w3) or 6 carbons (w6) from the methyl end are required for eicosinoid synthesis
- we do not synthesize the omega3 and omega6 fatty acids de novo so they must come from our diet. We obtain linoleic (18:2, delta9, 12) and linolenic (18:3, delta9,12,15) fatty acids primarily from plant oils. They are considered essential FAs as they must come from our diet.
- fish oil contains different omega3 and omeg6 PUFAs that they obtain from eating plankton
17
Q
Conversion of linoleic acid (18:2) to arachidonic acid (20:4)
A
- linoleic acid can be converted by elongation and desaturation reactions to arachidonic acid (20:4, delta5, 8,11,14) which is used for the synthesis of the major class of human prostaglandins and other eicosanoids
- arachidonic acid is listed in some texts as an essential FA
- although it is an omega 6 FA, it is not essential in the diet if linoleic acid is present because arachidonic acid can be synthesized from dietary linoleic acid
18
Q
Eicosanoids
A
- eicosanoids include the prostaglands, thromboxanes, and leukotrienes and are derived from arachidonic acid
- they are ubiquitous C20 compounds that have hormone-like effects on cell physiology
- they tend to be unstable and have a very short biological half-life
- they are generated in situu and are local mediators
- they are involved in the inflammatory response, smooth muscle contraction, and bronchoconstriction or bronchodilation
19
Q
Pathways for the synthesis of eicosanoids
A
1-the cyclic (or cyclooxygenase) pathway, which forms the prostaglandins, thromboxanes and prostacyclins
2- the linear (or lipoxygenase) pathway, which forms leukotrienes, hydroxyeicosatetraenoic acids (HETEs), and lipoxins from a common intermediate HPETE
3- the cytochrome P450 pathway, which forms epoxides
-a given type of cell usually possesses the enzyme for only one pathway, and so makes only one kind of eicosanoid. However, there are exceptions- such as platelets- that use both the cyclic and linear pathways
20
Q
Prostaglandin and thromboxanes: COX-1 and COX-2
A
- the cyclooxygenase pathway produces prostaglandins and thromboxanes
- pathways inhibitors (aspirin and other NSAIDs) act on the cyclooxygenase (COX) enzyme which converts arachidonic acid to PGH2
- there are two COX isoforms: COX-1 and COX-2
- COX-1 is the constitutive form expressed in all tissues
- COX-2 is the inducible form regulated by a variety of cytokines and growth factors and increased in response to inflammation
21
Q
Aspirin and other NSAIDs
A
- aspirin acetylates COX-1 and COX-2 and is an irreversible inhibitor
- acetaminophen and ibuprofen are reversible inhibitors of COX-1 and COX-2
