Fatty Acid Metabolism

Question 1

How are the carbons in a fatty acid labelled?

Answer 1

• carboxylic acid group = group number 1
• first carbon after carboxylic acid carbon = alpha
• last carbon= omega

Question 2

How is this fatty acid named?

Answer 2

• 9,12-octadecadienoic acid
• 18:2 delta 9,12
• 18:2 (omega - 6) (double bond is 6 carbons from omega carbon)

Question 3

Describe naturally ocurring double bonds in fatty acids.

Answer 3

They are always cis, causing the fatty acid to kink.

Question 4

What are the structural consequences of fatty acid unsaturation?

Answer 4

-unsaturated chains bend and pack in less tight with greater potential for motion

Question 5

How do we come about trans unsaturated bonds in fatty acids?

Answer 5

Hydrogenation

Question 6

How do trans unsaturated fatty acids lead to cardiovascular disease?

Answer 6

They have higher melting temperature than cis unsaturated FAs, making the membrane more rigid (due to lack of kink, whereas cis unsat FAs have kinks)

Question 7

How are FAs stored?

Answer 7

On a triglycerol backbone.

Question 8

What are ketone bodies?

Answer 8

three related compounds (acetone, acetoacetic acid, beta-hydroxybutyric acid) produced during the metabolism of fats.

Question 9

During the fasted state, what is the energy source for gluconeogenesis once glycogen stores are gone?

Answer 9

beta oxidation of fatty acids

Question 10

Under what conditions are fatty acids synthesized?

Answer 10

When insulin is expressed (fed state). When energy is high (ATP and NADH) and acetyl coA from glycolysis is high.

Question 11

What is the overall reaction of palmitate fatty acid synthesis?

Answer 11

• 8 acetyl coA (2 carbons each contribute to overall 16 carbon palmitate)
• 7ATP
• 14 NADPH

Question 12

Where are FAs synthesized

Answer 12

cytosol of liver, lactating mammary gland, adipose

Question 13

What is the source of acetyl coA for FA synthesis?

Answer 13

acetyl coA ultimately comes from pyruvate in the mito, but we need acetyl coA to be in the cytosol. The citrate-malate-pyruvate shuttle allows for acetyl coA to be shuttled from mito to cytosol in the form of citrate (after condensing with OAA). In the energy rich state, the TCA cycle will become backed up, so citrate flows readily into the cytosol. Acetyl coA is then liberated by citrate lyase (activated by insulin) and OAA is reduced all the way to pyruvate, resulting in formation of NADPH for FA synthesis.

Question 14

What are the sources of NADPH for FA synthesis?

Answer 14

• pentose phosphate pathway
• citrate-malate-pyruvate shuttle (malate enzyme reduces malate to pyruvate)

Question 15

What is the rate-limiting and regulated step of FA synthesis?

Answer 15

acetyl coA carboxylase

Question 16

Describe the reaction carried out by acetyl coA carboxylase in FA synthesis.

Answer 16

1. ATP activates a bicarbonate atom by adding phosphate (biotin carboxylase domain)
2. activated carbonyl is added to biotin (biotin carboxylase domain)
3. carbonyl is moved from biotin to acetyl coA , forming malonyl coA (transcarboxylase). The biotin domain holds the long biotin arm to a lysine, allowing it to swing from the biotin domain to the transcarboxylase domain.

Question 17

Describe the regulation of acetyl coA carboxylase.

Answer 17

• ACC is activated when dephosphorylated (dephosphorylation from insulin, must be high energy state for FA synthesis)
• glucagon and epinephrine inhibit ACC (low energy, no FA synthesis)
• build-up of palmitate inhibits ACC (allosteric inhibitor)

Question 18

Describe the fatty acid synthase dimer protein and its role in FA synthesis.

Answer 18

• multifunctional protein containing acetyl carrier protein (ACP)
• adds two carbonyls from malonyl coA to the carboxylate end of an acyl acceptor
• repetitive condensation and reduction

Question 19

Describe acetyl carrier protein

Answer 19

• holds acetyl coA
• has long flexible arm so that acetyl coA can move from one FAS domain to another
• contains prosthetic group 4’-phophopant

Question 20

List the repetitive sequence of reactions carried out by fatty acid synthase.

Answer 20

• condense
• reduce (NADPH)
• dehydrate
• reduce (NADPH)

each round of reactions adds a new malonyl to ACP of FAS, and each round a carbon is lost during condensation as CO2

Question 21

Describe the condensation reaction carried out by fatty acid synthase.

Answer 21

malonyl coA (3 carbons) and acetyl coA (2 carbons) are condensed together (Acetyl carrier protein esterifies to malonyl coA and brings acetyl coA into proximity). Condensation results in the loss of CO2, which is irreversible and drives the reaction forward. We know have a 4-carbon molecule. One coA group is lost as a leaving group.

Question 22

Once synthesized, how is palmitate released from FAS?

Answer 22

It is cleaved off by thioesterase enzyme.

Question 23

Where does elongation and desaturation of palmatic acid occur?

Answer 23

They occur in the ER, so the FA must be transported from cytosol to ER.

Question 24

Describe the process of FA elongation.

Answer 24

After palmitic acid is formed, elongation can take place in the ER by the same beta reduction that FAS synthesized in the cytosol, except

=fatty acyl coA is the substrate (not ACP) for condensation with malonyl coA

-elongation is catalyzed by ER-localized enzymes rather than by a single multifunctional enzyme

Question 25

Why is there a dietaty requirement for linoleic acid?

Answer 25

our cells do not have the enzymes needed to desaturate FAs after C10.

Question 26

Describe the process of FA desaturation.

Answer 26

O2 serves as an alectron acceptor at the end of an electron transport chain in the ER and leads to the oxidation of both the fatty acid (introduction of the double bond) and NADH.

Question 27

Describe fatty acid storage.

Answer 27

FAs esterify onto glycerol to form TAGs. TAGs are stored in a single large droplet, packed tightly together and coated with a monolayer of phospholipids, cholesterol, and proteins such as perillipin

Question 28

Why are TAGs a more efficient storage molecule than glycogen?

Answer 28

they are highly reduced and anhydrous

Question 29

Where are TAGs synthesized, and how are they transported?

Answer 29

TAGs are made in the liver, stored in adipose, and sent out into the blood in VLDLs.

Question 30

How are fatty acids retrieved from adipose?

Answer 30

1. binding of epinephrine to receptor on adipose cell is first step 2. kinase cascade activates TAG lipases, such as hormone sensitive lipase, making P-HSL 3. phosphorylation of perillipin by PKA allows P-HSL to bind P-perillipin on the lipid droplet to release FAs and glycerol 4. glycerol can be used for gluconeogenesis, and the FAs are carried in the blood on albumin where they are taken to liver and muslce for oxidation to acetyl coA

Question 31

How is beta oxidation of FAs useful for muscle?

Answer 31

muscle can extract energy from FAs to spare glucose which is needed by glucose-dependent tissues such as the brain

Question 32

How is beta oxidation of FAs in the liver useful for gluconeogenesis?

Answer 32

It produces the NADH and ATP needed to fuel gluconeogenesis.

Question 33

Why must fatty acids sometimes enter the mito matrix, and why is this a problem for some fatty acids?

Answer 33

Beta oxidation of FAs takes place in the mito matrix. Medium and short chain FAs can pass right through the membranes, but long chain FAs cannot so they require the help of a shuttle system.

Question 34

Describe long chain FA activation.

Answer 34

This is required to shuttle LCFAs into the mito matrix for beta oxidation. - free FA is esterified to coA for activation. this molecule cannot pass the inner membrane either, - to get through the inner membrane, carnitine palmitoyltransferase (CPT) I exchanges carnitine for the coA on the FA-coA. This complex passes through the inner membrane - CPTII then transfers coA back in the place of carnitine, regenerating fatty acyl-coA

Question 35

List the repetitive steps of FA beta oxidation in the mito matrix.

Answer 35

opposite of synthesis 1. oxidize (create a double bond) 2. hydrate 3. oxidize hydroxyl 4. clip (thiolytic cleavage to release two carbon unit)

Question 36

What is the starting molecule of FA beta oxidation?

Answer 36

fatty acyl coA

Question 37

What happens to the acetyl coA that is cleaved from fatty acyl coA during beta oxidation?

Answer 37

it is fed into the TCA where it is oxidized (ketogenesis) using FADH2, NADH, CO2 and GTP.

Question 38

What happens to NADH and FADH2 that are produced from FA beta oxidation?

Answer 38

They are fed into the ETC to sythesize ATP. ATP made and extra NADH are used in gluconeogenesis.

Question 39

Describe the regulation of FA beta oxidation in the mito.

Answer 39

- substrate availability icreases beta oxidation - malonyl-coA slows beta oxidation by inhibiting CPT (can't make and break FAs at the same time) - thiolytic cleavage step that releases carbon from fatty acyl coA is inhibited as the acetyl-coA/coA ratio increases (product inhibition)

Question 40

How many acetyl-coA molecules result from the complete oxidation of palmitate?

Answer 40

8

Question 41

How much energy can be extracted from palmitate oxidation and converted to ATP in the TCA cycle for use in gluconeogenesis?

Answer 41

12 ATP per acetyl coA molecule

Question 42

Describe the beta oxidation of odd-numbered FAs.

Answer 42

Once the fatty acyl coA is oxidized down to a 3-carbon molecule, CO2 is added to make it 4-carbon (biotin coenzyme). This is then rearranged to create succinyl coA (vitamin B12 coenzyme). Succinyl coA can then enter the TCA cycle for further oxidation.

Question 43

How is vitamin B12 deficiency diagnosed?

Answer 43

By a buildup of methyl-malonyl coA (product before rearrangement to succinyl coA in the oxidation of odd-numbered carbon fatty acyl chains)

Question 44

How do fatty acyl chains with cis double bonds in an odd carbon position achieve full beta oxidation?

Answer 44

The cis double bond must first be converted to a trans double bond and then oxidized.

Question 45

How does beta oxidition begin for very long chain FAs?

Answer 45

Only peroxisomes have the acyl coA synthetase required to activate these FAs with coA, so beta oxidation begins in the peroxisome. Once they become medium chain FAs, beta oxidation is completed in the mito.

Question 46

Why are ketone bodies made after beta oxidation?

Answer 46

There is a buildup of acetyl coA pools, which will cause ATP buildup and subsequent stalling of TCA. Therefore, to allow for gluconeogenesis to continue, acetyl coA molecules are condensed together to form ketone bodies

Question 47

How can ketone bodies be used?

Answer 47

They can be broken down during fasting to avoid breaking down proteins and amino acids. Though the liver cannot use them, they are soluble and can be used by the brian. Beta-hydroxybuterate is the main molecule used by the brain during fasting. Beta-hydroxybuterate is converted to acetoacetate, and coA is transferred from succinyl coA to acetoacetate by beta-ketoacyl coA transferase, creating succinate which can enter the TCA cycle. Acetoacetyl coA is further broken down in acetyl coA to be fed into TCA.

Question 48

Which is the predominate ketone body?

Answer 48

beta hydroxybuterate

Question 49

Where are ketone bodies made?

Answer 49

Mito matrix of liver cells.

Question 50

Describe ketoacidosis in type I diabetes.

Answer 50

Insulin is not produced, so glucagon runs free and results in the high, unhindered production of ketones, resulting in ketoacidosis. Therefore, even if the body is fed, it thinks it is in a starved state.