Fatty Acid Metabolism II Lec. 36 & Lec 37

Question 1

beta-oxidation or fatty acid oxidation

Answer 1

a mechanism through which cells utilize energy stored in fatty acids (carbons of fatty acyl chains)

• undergoes a series of enzymatic reactions within the mitochondrial matrix where fatty acids will be converted to acetyl-CoA
• each round of beta-oxidation will get rid of 2 carbons per cycle

Question 2

what do fatty acids need to be converted into?

Answer 2

fatty acids need to be converted into acyl-CoA which is when a molecule of coenzyme A will attach to the fatty acid molecule and an enzyme associated with this function is acyl-CoA synthetase (among other enzymes)
- these enzymes are within the cytoplasm

Question 3

what enzyme can convert fatty acids into acyl-CoA?

Answer 3

acyl-CoA synthetase which can convert fatty acids into acyl-CoA by attaching a coenzyme molecule to a fatty acid

Question 4

what is the general mechanism of acyl-CoA synthetase? (what happens between the fatty acid)

Answer 4

• fatty acid gets activated by the enzyme acyl-CoA synthetase which also requires a molecule of ATP (the whole reaction will be driven forward through the cleaving of the high energy bond between the beta and gamma phosphate of the ATP molecule)
• this will form acyladenylate intermediate which then gets attacked by a molecule of CoA (in specific, the attacking is done the sulfur of the CoA molecule)
• ultimately the fatty acid becomes transferred to a coenzyme A which produced acyl-CoA and a molecule of AMP as well
• during this process 2 ATP molecules are used because of the breaking of two high energy bonds

Question 5

how are long chain fatty acids transported into the mitochondria?

Answer 5

• the inner mitochondrial membrane is not permeable to long chain acyl-CoAs
• carnitine palmitoyltransferase is also know as carnitine acyltransferase (these are the enzymes that transport fatty acids into the mitochondria

Question 6

what are the two enzymes that are able to transport long fatty acid chains into the mitochondria?

Answer 6

• the enzymes carnitine acyltransferase I and II can transport fatty acid chains into the mitochondria

Question 7

carnitine acyltransferase I (CPTI) and

carnitine acyltransferase II (CPTII) enzymes (which one is on the faces cytosol and what one faces matrix?)

Answer 7

these enzymes that transport long fatty acid chains into the mitochondria –> will only occur when a long fatty acid chain is attached to carnitine acyltransferase enzyme (if fatty acid is attached to coenzyme A it cannot transport into the mitochondria)

• carnitine acyltransferase I found on the outer mitochondrial membrane and faces the cytosol
• carnitine acyltransferase II found on the inner mitochondrial membrane and faces the matrix

Question 8

what is the mechanism of both carnitine acyltransferase I and carnitine acyltransferase II?

Answer 8

• carnitine acyltransferase I transfers acyl group to carnitine which produces a molecule of acyl-carnitine that gets transported into the mitochondrial matrix
• carnitine acyltransferase II will transfers an acyl group back to the coenzyme A to generate acyl-CoA

Question 9

what is the mechanism of Carnitine acyltransferase? (there are two enzymes: carnitine acyltransferase I and II but what is the general mechanism?)

Answer 9

• generally speaking, the enzyme is going to take the long fatty acid chain and transfer it to a molecule of carnitine in the cytoplasm - by doing so it produces acyl-carnitine which is permeable to outer mitochondrial membrane and will transport into the matrix of the mitochondria
• ultimately liberates acyl-CoA from fatty acid chain
• this is the only way to transport fatty acids into the mitochondrial matrix

Question 10

what happens once the acyl-carnitine molecule is in the mitochondria?

Answer 10

once acyl-carnitine is within the mitochondria it will undergo a reverse reaction that will transfer the fatty acid from the acyl-carnitine back to a coenzyme A molecule in the matrix to regenerate an acyl-CoA within the matrix

Question 11

enzymes of the beta-oxidation pathway: 4 total

Answer 11

• AD: acyl-CoA dehydrogenase
• EH: enoyl- CoA hydrate
• HAD: hydroxyacyl- CoA dehydrogenase
• KT: ketoacyl-CoA thiolase

Question 12

AD: acyl-CoA dehydrogenase (involved in beta-oxidation) - what does it do

Answer 12

• creates an alpha-beta trans double bond through dehydration by acyl-CoA dehydrogenase (AD) and also generates a molecule of FADH2

Question 13

EH: enoyl- CoA hydrate (involved in beta-oxidation pathway) - what does it do

Answer 13

• enzyme involved in the second reaction of the beta-oxidation pathway that will hydrate the first double bond produced to reduce it

Question 14

• HAD: hydroxyacyl- CoA dehydrogenase (involved in beta-oxidation pathway) what does it do?

Answer 14

• creates a ketone on beta carbon using NAD+ and generates a ketone and NADH molecule
• have mutations founds in children

Question 15

• KT: ketoacyl-CoA thiolase (involved in beta-oxidation) - what does it do

Answer 15

• going to cleave the bond between alpha and beta carbon and generates acetyl-CoA and a fatty acyl-CoA (this is 2 carbons shorter than acetyl-CoA)

Question 16

palmitic acid (palmitate) - how many carbons

Answer 16

• 16 carbon fatty acid
• undergoes 7 rounds of beta-oxidation (2 carbons deplete per cycle of beta oxidation)
• produces 106 ATPs per fatty acid molecule (each cycle will produce 17.5 ATPs but 2 ATPs are also used for activation thus 106)

Question 17

genetic disorders of beta oxidation: acyl-CoA dehydrogenases

Answer 17

• VLCAD: cardiomyopathy and muscle weakness
• LCAD: pulmonary surfactant dysfunction
• MCAD: most common defect; hypoketotic hypoglycemia with lethargy that develops into coma
• SCAD: relatively mild; leads to elevated levels of butyrate

Question 18

genetic disorders of beta oxidation: HAD enzyme

Answer 18

• HAD is lethargy cardiomyopathy, infant-onset hepatic form (lethargy), or peripheral neuropathy
• hearts in children stop beating - accounts for about 10% of SIDS cases –> due to feeding on mothers rich, fatty acid milk and if child does not have effective HAD enzyme then the heart essentially dies

Question 19

beta- oxidation of UNsaturated fatty acids (what are two unsaturated fatty acids)

Answer 19

• oleic acid

- linoleic acid

Question 20

ketone bodies (what are the two ketone bodies)

Answer 20

• ketone bodies are released by the liver into the bloodstream (ketone bodies are only produced in liver)
• used for energy during fasting/low blood glucose
• can diffuse into the brain
• the two ketone bodies are D-beta-hydroxybutyrate and acetoacetate
• result from condensation of two acetyl-CoA molecules

Question 21

where do the majority of fatty acids come from for biosynthesis?

Answer 21

glucose

- the more glucose you eat the more fats we are going to make and store

Question 22

fatty acid biosynthesis occurs where in the cell? where does beta oxidation occur

Answer 22

• fatty acid metabolism occurs in the cytoplasm (occurs when insulin levels are high because we have a lot of energy we’re going to store from the glucose - and beta oxidation will in turn be reduced)
• beta oxidation occurs in the cytoplasm

Question 23

pool of acetyl CoA in the cytosol will be a precursor, for what? what big, overall function?

Answer 23

a pool of acetyl CoA in the cytosol will be a precursor for biosynthesis, it is required to be in cytosol NOT in the matrix ( acetyl CoA is not permeable through the mitochondrial membranes, like the matrix for example)

Question 24

major organ where fatty acid biosynthesis occurs?

Answer 24

liver

- major precursor for acetyl-CoA is in the liver that are from glucose (sugars)

Question 25

key enzymes in fatty acid metabolism (two enzymes both in the cytoplasm)

Answer 25

• acetyl-CoA carboxylase (ACC) –> converts acetyl-CoA into a compound called malonyl-CoA and this conversion will commit the acetyl-CoA to biosynthesis
• fatty acid synthase (FAS/FASN)

Question 26

acetyl-CoA carboxylase (ACC) enzyme involved in fatty acid synthesis is involved in what step? what does it do?

Answer 26

this is involved in converting an acetyl-CoA into a molecule of malonyl-CoA –> this is the committing step to biosynthesis

Question 27

acetyl-CoA carboxylase (ACC) activation - how is it activated?

Answer 27

ACC is activated by cytosolic citrate (remember a large pool of citrate is indicative of a high energy state) which tells the acetyl-CoA carboxylase there’s going to be a lot of acetyl-CoA to start converting it to use towards fatty acid biosynthesis

Question 28

regulation of malonyl-CoA –> ensures that fatty acid biosynthesis and beta-oxidation do not occur simultaneously (how’s it work, what’s it inhibiting)

Answer 28

malonyl-CoA is an inhibitor of the CPT1 enzyme (a cytosolic enzyme - faces the cytosol) –> so once the cell is committed to fatty acid biosynthesis the malonyl-CoA feeds back into the CPT1 and prevents further oxidation of fatty acids

Question 29

citrate stimulates the activity of what?

Answer 29

citrate stimulates the activity of ACC (acetyl-CoA carboxylase - which converts acetyl-CoA into malonyl-CoA and is the committed step for fatty acid biosynthesis)

Question 30

substrates for fatty acid synthase

Answer 30

acetyl-CoA and malonyl-CoA

Question 31

malonyl-CoA directly inhibits what enzyme? what happens?

Answer 31

malonyl-CoA inhibits the CPT1 enzyme (cytosolic enzyme that converts acyl-CoA into acyl carnitines - then that allows fatty acids to be transported to the matrix for beta oxidation)

Question 32

fatty acid synthase (enzyme) reaction - how does it work? - its involved in the conversion of malonyl-CoA to palmitate

Answer 32

this is an enzyme that catalyzes fatty acid biosynthesis

• acetyl-CoA converts to malonyl-CoA by the acetyl COA carboxylase and then malonyl-CoA will convert to palmitate by adding 2 carbons to growing fatty acid chain per cycle
• there is an ACP (acyl carrier protein) that transports fatty acid chain to each domain of the fatty acid synthase enzyme as the chain is growing

Question 33

ACP (acyl carrier protein)

Answer 33

ACP ensures that intermediates of fatty acid biosynthesis remain attached to the enzyme until palmitate forms
- ACP needs to/and will move the fatty acid chains to each of their domains as the chain grows

Question 34

lipid biosynthesis in the liver (general mechanism)

Answer 34

biosynthesis rxn occurs largely within the liver and in the cytosol of the liver (acetyl-CoA is not permeable to the membrane)
- after eating the glucose from food will provide the major source for the precursor of fatty acid biosynthesis –> the liver takes up the glucose and converts it to pyruvate through the TCA cycle and acetyl-CoA gets converted to citrate and back to acetyl-CoA to be exported to the cytosol to generate of pool of cytosolic acetyl-CoA then ACC acyl-CoA carboxylase) and FAS (fatty acid synthase - converts malonyl-CoA to palmitate) will convert the acetyl-CoA into palmitate