Fatty Acid Synthesis and Eicosanoids

Question 1

Lipogenesis

Answer 1

• 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

Question 2

Conversion of Glucose to Cytosolic Acetyl-CoA

Answer 2

• 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

Question 3

Citrate Lyase and Malic Enzyme

Answer 3

• 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

Question 4

Source of NADPH for FA Synthesis

Answer 4

• recycling of the OAA

- a second source of NADPH for fatty acid synthesis is the Pentose-phosphate pathway

Question 5

Fatty Acid Synthesis

Answer 5

• 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

Question 6

Acetyl CoA Carboxylase (ACC)

Answer 6

• 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

Question 7

Regulation of ACC

Answer 7

• 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

Question 8

Steps in Fatty Acid Synthesis

Answer 8

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

Question 9

Phosphopantethenyl Residue of Fatty Acid Synthase (FAS)

Answer 9

• 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

Question 10

Second step in Fatty Acid Synthesis

Answer 10

• 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

Question 11

Reduction of ketoacetyl group on FAS

Answer 11

• 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

Question 12

Synthesis of Palmitate on FAS

Answer 12

• 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

Question 13

Elongation of Long Chain Fatty Acids

Answer 13

• 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

Question 14

Fatty Acid Desaturation

Answer 14

• 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

Question 15

Monosaturated Fatty Acids (MUFAs)

Answer 15

• 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

Question 16

Polyunsaturated Fatty Acids: PUFAs

Answer 16

• 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

Question 17

Conversion of linoleic acid (18:2) to arachidonic acid (20:4)

Answer 17

• 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

Question 18

Eicosanoids

Answer 18

• 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

Question 19

Pathways for the synthesis of eicosanoids

Answer 19

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

Question 20

Prostaglandin and thromboxanes: COX-1 and COX-2

Answer 20

• 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

Question 21

Aspirin and other NSAIDs

Answer 21

• aspirin acetylates COX-1 and COX-2 and is an irreversible inhibitor
• acetaminophen and ibuprofen are reversible inhibitors of COX-1 and COX-2