Fatty Acid Synthesis & Eicosanoids - RS

Question 1

What is lipogenesis? Where does it occur? What is the carbon source?

Answer 1

The synthesis of fats from glucose. The liver. Glucose is converted to pyruvate which enters the mitochondria where it is converted to acetyl CoA and oxaloacetate which then forms citrate which is transported to the cytosol. In the cytosol citrate is converted to acetyl CoA which is converted to malonyl CoA (the two carbon donor for FA synthesis).

Question 2

What happens to acetyl CoA when in the cytosol?

Answer 2

Acetyl CoA carboxylase (key regulatory enzyme) turns it into malonyl CoA (the two carbon donor for FA synthesis) and fatty acid syntase convert malonyl CoA to palmitic acid (16C saturated FA)

Question 3

What is the fate of palmitic acid?

Answer 3

Once activated to palmityl CoA the FA can be elongated or desaturated by enzymes on the ER.

Question 4

What are examples of eicosanoids? Where do they come from?

Answer 4

They include: Prostaglandins, thromboxanes, and leukotrienes. They are synthesized mainly from arachadonic acid. They are produced by every cell and have functions in inflammation, smooth muscle contraction, blood pressure regulation, bronchoconstriction and bronchodilation.

Question 5

Where is pyruvate dehydrogenase found? What does it catalyze?

Answer 5

Only in the mitochondria. It converts pyruvate to Acetyl CoA

Question 6

Can Acetyl CoA cross the mitochondrial double membrane?

Answer 6

NO!

Question 7

What does pyruvate carboxylase catalyze?

Answer 7

It converts pyruvate to Oxaloacetate (OAA).

Question 8

How are pyruvate carboxlase and pyruvate dehydrogenase regulated?

Answer 8

Based on the level of acetyl CoA in the mitochondria. When Acetyl CoA is high the pyruvate dehydrogenase is inhibited causing an increase in pyruvate carboxylase resulting in higher levels of OAA. When OAA increased it condenses with Acetyl CoA forming Citrate.

Question 9

What does a reduction in acetyl CoA in the mitochondria lead to?

Answer 9

Activation of pyruvate dehydrogenase and inhibition of pyruvate carboxylase

Question 10

What does citrate lyase do? Where is it found?

Answer 10

In the cytosol citrate is cleaved to acetyl CoA and OAA by Citrate lyase

Question 11

NADPH is required for FA synthesis, what is one source of NADPH (hint it’s a “recycling step”)? The other source is the Pentose-phosphate shunt.

Answer 11

OAA is reduced by an NADH dependent cytosolic malate dehydrogenase to malate. Malate is then converted by the NADP+-dependent Malic Enzyme which oxidizes and decarboxylates Malate to Pyruvate. Pyruvate is converted to citrate (thru the mitochondrial pathway again) and the NADPH is used to produce FA by FA synthase.

Question 12

What are the two enzymes required for FA synthesis from Acetyl CoA to palmitate? Which is the rate controlling step?

Answer 12

Acetyl CoA carboxylase (ACC) and the second is Fatty acid synthase. ACC is the rate controlling step (it is highly regulated)

Question 13

Acetyl CoA carboxylase converts _____ into ______. It’s prosthetic group is _______ and requires ________ and ______. It is active when _______.

Answer 13

Acetyl CoA carboxylase converts Acetyl CoA into Malonyl CoA. It’s prosthetic group is Biotin and requires CO2 and ATP. It is active when un-phosphorylated.

Question 14

What positively regulates Acetyl CoA carboxylase (ACC)? What negative regulates it?

Answer 14

Pos: Citrate which causes it to polymerize, Insulin stimulate phosphatase, caloric intake/ ACC transcription levels.
Neg: Glucagon/ epinephrine stimulate A-kinase, palmitoyl CoA levels, AMP (low energy).

Question 15

What are the 5 steps of FA synthesis? What enzyme does each step?

Answer 15

1 - Activation (ACC)
2 - Condensation of B-keto (FAS)
3 - Reduction of B-keto (FAS)
4 - Dehydration of that a,b carbons (FAS)
5 - Reduction of the a,b double bond (FAS)

Question 16

What does fatty acid synthase (FAS) catalyze? What is the only reducing agent it uses?

Answer 16

Ultimately, it adds two carbon units from malonyl CoA to the growing Fatty acyl chain with the final product being palmitate. NADPH (it is used in steps 3 and 5)

Question 17

There are 7 catalytic subunits on FAS each associated with an acyl carrier protein (ACP), what is covalently attached to the ACP?

Answer 17

A phospho-pantetheinyl residue derived from the vitamin patothenic acid. 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 (IDK IF THIS IS IMPORTANT. Fuck this lecture)

Question 18

After palmitate is released from FAS it is activated to palmitoyl-CoA and other long chain activated fatty acyl CoAs can be elongated in the ER by elongases. How is the elongation in the ER different?

Answer 18

The steps are the same as those previously described with malonyl CoA as the 2C donor, the exception is that the long chain FA is attached to CoA instead of a phosho-pantetheinyl residue attached to the acyl carrier protein (ACP)

Question 19

What is fatty acid desaturation? What is it used for?

Answer 19

It is the oxidation of FA resulting in cis double bonds. It creates lipids of increasing structural and functional complexity with distinct biological roles.

Question 20

What does FA desaturation require?

Answer 20

Desaturases located in the ER, O2, 2H+, NADH, and Cytochrome b5. Fatty acyl CoA desaturase(desaturases are highly regulated in response to diet. Decrease during starving and increase when fed only if there aren’t many unsaturated fats in the diet)

Question 21

Humans have 3 desaturases, what are they? Which is most common?

Answer 21

∆9 desaturase (most commonly used)
∆6 desaturase
∆5 desaturase

Question 22

What are required for eicosinoid synthesis?

Answer 22

polyunsaturated fatty acids with double bonds three carbonds or 6 carbons from the methyl end (ie. omega 3 or 6)

Question 23

How is lineoleic acid (18:2) converted to arachidonic acid (20:4)? Why is this important?

Answer 23

Arachidonic acid is an important component of certain membrane lipids, and precursor of several classes of the signaling molecules known as eicosanoids. ∆6 desaturase acts first, then elongase adds 2 carbons, then ∆5 desaturase puts in the last double bond.

Question 24

Arachidonic acid (an omega 6) often listed as essential FA: When is it not essential?

Answer 24

It is not essential in the diet if linoleic acid is present because it can be synthesized from dietary linoleic acid

Question 25

What are eicosinoids (brief and general)?

Answer 25

They are ubiquitous 20 carbon 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 situ and are local mediators

Question 26

What effect do glucocorticoids and Aspirin/ NSAIDs have on eicosinoids?

Answer 26

They inhibit their production. (at different steps) Glucocorticoids inhibit production of arachidonic acid (which inhibits all eicosinoids) while NSAIDs inhibit cyclo-oxygenase and formation of the prostaglandins and thromboxane (leukotrienes are produced)

Question 27

There are three pathways for the synthesis of eicosinoids: what are they? What are the products of each?

Answer 27

1 - The cyclooxygenase (cyclic) pathway which forms prostaglandins, thromboxanes and prostacyclins
2 - The lipoxygenase (linea) pathway, which forms leukotrienes, hydroxyeicosatetraenoic acids (HETEs), and lipoxins from a common intermediate HPETE
3 - The cytochrome P450 pathways, which forms epoxides

Question 28

How come all three pathways aren’t in competition?

Answer 28

A given cell usually possesses the enzymes for only one pathway and so makes only one kind of eicosanoid (however there are exceptions such as platelets that use both cyclic and linear pathways)

Question 29

The cyclooxygenase pathway produces prostaglandins and thromboxanes. Pathway inhibitors act on cyclooxygenase (COX) enzyme which converts arachidonic acid to PGH2. There are 2 COX isoforms: Where are they expressed? What inhibits each?

Answer 29

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. NSAIDs act on both COX 1 &2. COXIB’s act only on COX-2.

Question 30

How is aspirin different from acetaminophen and ibuprofen?

Answer 30

Aspirin acetylates COX-1 and 2 and is an irreversible inhibitor. Acetaminophen and Ibuprofen are reversible inhibitors of COX-1 and 2