Fatty Acid Oxidation

Question 1

what are the key concepts of FA catabolism?

Answer 1

• FA are transported to tissues where they’re activated and enter the mitochondria
• FA are catabolized to 2-carbon units in mitochondria via B-oxidation pathway
• oxidation reactions occur in discreet steps coupled to FADH and NADH production and generation of ATP
• acetyl CoA units generated from fatty acids by B-oxidation ultimately enter TCA where they yield more ATP
• different enzymes catabolize short, medium and long chain FA

Question 2

under what conditions would you need to rely upon FA to generate ATP?

Answer 2

• prolonged fast
• running a marathon
• uncontrolled diabetes

Question 3

what does B-oxidation extract from FAs?

Answer 3

it extracts energy via ATP, CO2 and H2O from FAs

Question 4

what’s the overview of the stages in converting FA to CO2 and H2O?

Answer 4

1. activation by Coenzyme A
2. transport into mitochondria by carnitine shuttle
3. B-oxidation to Acetyl CoA (4 steps)
4. oxidation of acetyl CoA via TCA

Question 5

what is the first step in converting FA to CO2 and H2O?

Answer 5

to begin FA breakdown, FA enters the cell by crossing the cell membrane and entering the cytoplasm

FFA must be activated and be conjugated to Coenzyme A to begin the conversion process

Question 6

what is the second step in converting FA to CO2 and H2O?

Answer 6

now we need to get the FA from the cytoplasm into the mitochondria via the carnitine shuttle

Question 7

what is the third step in converting FA to CO2 and H2O?

Answer 7

once in the mitochondria, B-oxidation has 4 repeating steps that release a molecule of acetyl CoA and 2 carbons at a time until eventually we get all the way down to just acetyl CoA

Question 8

what is the fourth step in converting FA to CO2 and H2O?

Answer 8

acetyl CoA is oxidized via TCA cycle which releases energy (ATP), CO2 and H2O

Question 9

what’s the chemistry of the first step in converting FA to CO2 and H2O?

Answer 9

starts with FFA activation through conjugation to Coenzyme A - this enzyme reaction is a 2 step reaction

Step 1: FA + ATP –> Acyl AMP + PPi
Step 2: acyl AMP + HS-CoA –> acyl-CoA + AMP
Side rxn: pyrophosphatase: PPi + H2O –> 2Pi + 2H+

Overall: FA + ATP + HS-CoA –> Acyl-S-CoA + AMP + PPi

catalyst: acyl-CoA synthase = FA thiokinase

pyrophasphatase helps break the 2 phosphate bonds in ATP which drives the reaction forward

Question 10

does acyl-CoA synthetase work on everything?

Answer 10

works on all long chain FA including palmitate, oleate, etc.

the exception is for medium and short chain FA in which this activation step is skipped

Question 11

what’s the chemistry of the second step in converting FA to CO2 and H2O?

Answer 11

you want to move acyl-S-CoA from the cytosol to the mitochondrial matrix for B-oxidation

cytosolic acyl-S-CoA –> mitochondrial acyl-S-CoA

enzymes: carnitine acyltransferase I (CPTI), translocate, carnitine acyltransferase II (CPTII)

Question 12

what does CPTI do?

Answer 12

long chain acyl-CoA can’t cross the mitochondrial membrane so we need a carrier (carnitine) and an enzyme (CPTI)

CPTI transfers the acyl group to carnitine and at the end of the reaction RCO-carnitine is in the inter membrane mitochondrial space

Question 13

what does translocase do?

Answer 13

in the intermembrane space RCO-carnitine reacts with translocase enzyme to move the RCO-carnitine across the mitochondrial inner membrane into the mitochondrial matrix

Question 14

what does CPTII do?

Answer 14

once it’s in the mitochondrial matrix, RCO-carnitine is conjugated back onto S-CoA via CPTII enzyme

this gives us the final product of RCO-S-CoA inside the mitochondrial matrix and it’s ready for B-oxidation!

Question 15

do short and medium chain FA need the carnitine shuttle?

Answer 15

nope

short = 4-6 carbons

medium = 6-12 carbons

they don’t need the carnitine shuttle to move from the cytosol to the mitochondrial matrix, they can pass through via diffusion

Question 16

what’s the chemistry of the third step in converting FA to CO2 and H2O?

Answer 16

you want to break down acyl-S-CoA 2 carbons at a time and extract energy through B-oxidation

acyl-S-CoA + 7FAD + 7NAD+ + 7H2O –> 8acetyl-CoA + 7FADH2 + 7NADH + 7H+

enzymes: acyl CoA dehydrogenase**, enoyl-CoA hydrates, B-hydroxy acyl CoA dehydrogenase, B-ketothiolase

Question 17

what’s wrong with acyl CoA dehydrogenase?

Answer 17

it’s a genetic hot spot for mutation with clinical phenotype

Question 18

what are the 4 reoccurring steps in B-oxidation of fatty acyl-CoA?

Answer 18

1. oxidation by FAD
2. hydration
3. oxidation by NAD+
4. cleavage by CoA

acyl chain is shortened by 2 carbons

FADH2, NADH, and acetyl-CoA are generated

ATP is generated via ETC and we use the acetyl-CoA and put it through TCA which will also generate ATP!

Question 19

what is the first step in B-oxidation?

Answer 19

oxidation by FAD via acyl CoA dehydrogenase

dehydration that removes hydrogens and creates a double bond and also FADH2 is generated

FADH2 can generate energy in the ETC

enzyme is chain length specific!!!! aka there are long, medium and short chain acyl CoA dehydrogenase

Question 20

what’s the second step in B-oxidation?

Answer 20

enoyl-CoA hydratase is basically going to add water in the hydration step = you add a hydroxyl group to the double bond in the trans-enoyl CoA

trans-enoyl CoA + H2O –> B-hydroxyacyl CoA

enzyme: enoyl-CoA hydratase

Question 21

what’s the third step in B-oxidation?

Answer 21

oxidation by NAD

we remove two more hydrogens to yield NADH + H+ and convert the hydroxyl group to the keto group

Question 22

what’s the 4th step in B-oxidation?

Answer 22

extract energy via TCA

acetyl CoA goes into the TCA cycle
NADH & FADH2 go into ETC

Question 23

how do cells deal with odd chain FAs in B-oxidation?

Answer 23

they go through B-oxidation the same but once the chain has reached 3 carbons, it’s converted to succinylcholine CoA

succinylcholine CoA enters TCA cycle later so we lost out on 2 NADH!

odd numbered carbon chains are usually from plants and they’re very rare

Question 24

how do cells deal with mono and polyunsaturated FA?

Answer 24

these are very abundant in the diet!

they are also oxidized to yield ATP but the enzymes in the 4 normal steps of B-oxidation can’t react with the double bonds from these types of fats

two accessory enzymes are needed: cis-enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase

isomerase = “flips” double bonds around and repositions them so that the substrate is compatible fo r cleavage

reductase = important for polyunsaturated fats; will reduce some double bonds to change their positions