Fatty Acid Oxidation

Question 1

Are fatty acids a major source of energy for all tissue?

Answer 1

No. Many, but not all

Question 2

Dietary fatty acids are carried from the intestine to tissues to be metabolized (stored or oxidized) as part of what?

Answer 2

chylomicrons

Question 3

What results in increased fatty acid oxidation for ATP generation?

Answer 3

food restriction, fasting state

Question 4

What is Anorexia nervosa?

Answer 4

intentional starvation or bingeing/purging

Question 5

Stored fatty acids are released from storage (lipolysis), and transported bound to what to be oxidized?

Answer 5

serum albumin. As opposed to dietary FAs which are transported as part of a chylomicron

Question 6

Where does B-oxidation occur on a fatty acid?

Answer 6

b-oxidation refers to carbons after the carboxyl, at position 3 by organic chemical numbering (assuming C1 is the carboxyl carbon).

The a – b bond is cleaved in b–oxidation

Question 7

What are the common unsaturated FAs and how many carbons and double bonds does each have?

Answer 7

Palmitoleate- 16C; 1 double bond
Oleate- 18C; 1 double bond
Linoleate- 18C; 2 double bonds
Linolenate- 18C; 3 double bonds
Arachnidonate- 20C; 4 double bonds

longer chain that ~20C are handled by the peroxisome

Question 8

What are the common saturated FAs and how many carbons does each have?

Answer 8

Laurate- 12C
Myristrate- 14C
Palmitate- 16C
Stearate- 18C
Arachidate- 20C
Behenate- 22C

Question 9

What stimulates lipolysis in adipose tissue?

Answer 9

glucagon (fasting or starvation) or epinephrine (stress)

Question 10

What is the major antilipolytic hormone?

Answer 10

Insulin is the major antyilipolytic hormone.

Decreases adipocyte cAMP. Decreased serum free fatty acid (FFA).

Question 11

How does ketoacidosis arise from Insulin deficiency?

Answer 11

Insulin (when functioning normally) decreases lipolysis. So an increase of lipolysis in the absence of functioning insulin would result in increased ketone body production from the liver

Question 12

Discuss the steps of mobilization and transport of fatty acids from adipose tissue

Answer 12

When a hormone (glucagon or epinephrine) binds to a GPCR on the adipose tissue, this stimulates a PKA cascade (i.e. cAMP is made by adenylase cyclase and then PKA is activated) where TAG lipase (aka ‘hormone sensitive lipase’) is activated (phosphorylated by PKA). This enzyme converts TAG to diacylglycerol which is converted into FAs and glycerol (which goes to the liver to enter gluconeogenesis) by other lipases. The FAs then bind to albumin and are transported out of the adipose tissue and into the blood.

Question 13

Recall the production of glycerol from triacylglycerols and its role as gluconeogenic substrate.

Answer 13

Glycerol converted to Glyceraldehyde-3-Phosphate (through glycerol-3-p and DHAP intermediates) in liver to be used for gluconeogenesis.

Question 14

What happens to the FA-albumin complex once it enters it’s target tissue?

Answer 14

FAs come into muscle cells carried by albumin in the blood. These FAs must be “activated” to fatty acyl CoA (using ATP and CoA) in the cytosol (if they are short chain, they can be activated in the mitochondria). In the cytosol, fatty acyl CoA synthase (located on the outer membrane of the mitochondria with it’s functional unit facing the cytosol) converts fatty acid to fatty acyl CoA and pyrophosphate. Pyrophosphatase then converts the pyrophosphate into 2 inorganic phosphates.

Question 15

What are the 3 main metabolic routes of activated fatty acyl CoA?

Answer 15

• Energy (B-oxidation and ketogenesis)
• Membrane Lipid Formation (phospholipids and sphingolipids made)
• Storage (as TAGs)

Question 16

Describe the transport of activated acyl groups across the mitochondrial inner membrane into the matrix. Explain the role of carnitine.

Answer 16

If the fatty acyl CoA enters the “energy route,” it diffuses through the outer mitochondrial membrane. Once inside the mitochondrial membrane, long chain FAs cannot cross the inner mitochondrial membrane (short and medium chain FAs can- short by free diffusion and long by facilitate diffusion). It must be converted into fatty acylcarnitine (i.e. carnitine displaces the CoA group) to cross into the mitochondrial matrix (where it is reconverted to fatty acyl CoA).

Question 17

What enzyme catalyzes the conversion of fatty acyl CoA to fatty acyl carnitine?

Answer 17

CPT-I (Carnitine palmitoyl transferase I). This is located ON the outer membrane of the mitochondria with it’s functional unit facing the inner membrane space

Question 18

How does fatty acyl carnitine cross into the mitochonrial matrix?

Answer 18

fatty acylcarnitine crosses the inner mitochondrial membrane using a carnitine acylcarnitine translocase

Question 19

What enzyme catalyzes the conversion of fatty acyl carnitine back to fatty acyl CoA once inside the mitochondrial matrix?

Answer 19

CPT-II (Carnitine palmitoyl transferase II) located on the inner membrane

NOTE: The carnitine is then transported back across the mitochondrial membrane using the same translocase that brought fatty acylcarnitine into the matrix

Question 20

How do short chain (2-4C) and medium (4-12C) chain FAs pass through the inner mitochondrial membrane to enter the matrix?

Answer 20

they diffuse (short diffuses freely and medium by facilitated diffusion). But the main point is that they don’t have to undergo the carnitine cycle like long chain FAs

Question 21

Where are very long chain FAs oxidized?

Answer 21

peroxides. Recall that short, medium, and long chain FAs are all oxidized in the mitochondria, regardless of the path taken to enter into the matrix (i.e. diffusion vs. carnitine cycle)

Question 22

What is the first step in B-oxidation of FAs?

Enzymes?

Bi-products?

Answer 22

fatty acyl CoA is converted to trans fatty enoyl CoA using FAD and reducing it to FADH2 (equivalent to ~1.5 ATP)

enzyme: acyl CoA dehydrogenase

In the first step of beta-oxidation, a double bond is added between the alpha and beta carbon by a dehydrogenase (FADH2 is produced).
OXIDATION

Question 23

What kind of reaction is the conversion of fatty acyl CoA to trans fatty enoyl CoA considered as?

Answer 23

an oxidation

Question 24

What is the second step in B-oxidation of FAs?

Enzymes?

Bi-products?

Answer 24

trans fatty enoyl CoA is converted to L-B-hydroxyl acyl CoA by adding water

enzyme: enoyl CoA hydratase

In the second step, water is added across the a-B double bond, by a hydratase placing a hydroxyl (-OH) group on the beta carbon HYDRATION

Question 25

What is the third step in B-oxidation of FAs?

Enzymes?

Bi-products?

Answer 25

conversion of L-B-hydroxyl acyl CoA to B-keto acyl CoA ((using NAD+ and reducing it to NADH)- equivalent to ~2.5 ATP)

enzyme: B-hydroxyl acyl CoA dehydrogenase

In the third step, a dehydrogenase produces a carbonyl group where the hydroxyl group was on the beta carbon— releasing NADH. The product produced is a “beta keto acid”.
OXIDATION

Question 26

What is the fourth step in B-oxidation of FAs?

Enzymes?

Bi-products?

Answer 26

In the fourth step, B-keto thiolase adds a CoA-SH to the beta carbon—cleaving the a-B bond and releasing an Acetyl CoA (alpha carbon and acyl group). The fatty acyl CoA left over re-enters the cycle with 2 less carbons! CLEAVAGE

Question 27

How is FA transport regulated?

Answer 27

fatty acid synthesis intermediate malonyl CoA works to inhibit CPT I. This means that fatty acid oxidation cannot occur, because no fatty acids are being taken in.

The other regulation step is high levels of ATP negatively affecting the ETC. This decreases the number of oxidative enzymes that can be used to run beta-oxidation.

Question 28

How many ATP would B-oxidation of Palmitate (16C) generate?

(Note: assume 2.5 ATP per NADH and 1.5 ATP for FADH2.). Acetyl CoA produces 10 ATP each

Answer 28

1 Palmitate would undergo 7 B-oxidations, generating 8 Acetyl-CoA, 7 NADH, and 7 FADH2.

So: (7x2.5)+ (7x1.5)+ (8x10)- 2 activating ATP= 106 ATP

Question 29

How does the body handle oxidation of unsaturated fatty acids?

Answer 29

For unsaturated fatty acids (those that have double bonds), the enzymes used for beta-oxidation do not recognize the unconjugated double bond, so it must be bypassed.

An isomerase enzyme moves the cis double bond between the beta and gamma carbons to a trans double bond between the alpha and beta carbons.

This produces a conformation that is an intermediate of normal beta oxidation, so one Acetyl CoA can be released (only difference energetically is that no FADH2 is produced). The remaining fatty acid has a conjugated double bond.

Conjugated double bonds are handled using a reductase enzyme and an isomerase enzyme that will remove the trans double bond and shift the cis double bond into the trans conformation between the alpha and beta carbon to give the normal beta oxidation intermediate!

Now the fatty acid can undergo normal beta oxidation.

Question 30

Explain the oxidation of “odd chain” fatty acids, including formation of propionyl CoA, which can be converted to succinyl CoA

Answer 30

Normal beta oxidation occurs on an “odd chain” until it reaches the final 3 carbons. The final thiolysis step in the oxidation of odd chain FAs forms propionyl CoA with these 3 carbons. The propionyl CoA then is metabolized using proponyl CoA carboxylase (with biotin as a cofactor) which adds another carbon. Intermediates are converted to succinyl-CoA (with vitamin B12 as a coenzyme) which can enter the TCA cycle.

Question 31

VLCFA Oxidation

Answer 31

Oxidation of very long chain fatty acids occurs in peroxisomes. It seems as though the main goal of very long chain fatty acids is as a detoxification step (the mitochondria cannot handle very long chain fatty acids building up, so the perixosome must help!).

The main difference is that the first step of oxidation (where a double bond is formed between the alpha and beta carbons) produces an FADH2 that cannot be oxidized by the ETC (because the ETC is only in the mitochondria). The FADH2 must be oxidized by oxygen—changing it to peroxide (H2O2).

Normal beta-oxidation continues until only short and medium chain fatty acyl-CoAs are left. These fatty acyl-CoAs are converted to carnitine derivatives by COT or CAT in the peroxisomes. In the mitochondria, MCFA-carnitine and SCFA-carnitine are converted back to acyl CoA derivatives by either CPT2 or CAT. These acyl CoA derivatives then can undergo normal beta-oxidation.

Question 32

Alpha-Oxiation of Branched-Chain FAs

Answer 32

Branches on fatty acids produce problems for the Beta-oxidation enzymes. Therefore, in the perixosome, alpha oxidation occurs in which the alpha carbon is oxidized, releasing CO2 (or what was formerly the carbonyl carbon). Then, perixosomal beta oxidation can occur where alternative acetyl CoAs and propionyl CoAs are released until the chain is around 8 carbons long. The chain is then sent to the mitochondria as fatty acyl-carnitine

Question 33

Omega-Oxidation

Answer 33

Omega oxidation does not commonly occur. It only happens when medium chain fatty acids build up in the mitochondria of individuals.
The omega carbon is oxidized, and reactions occur forming a dicarboxylic acid (this step occurs in the ER).

Dicarboxylic acids may be excreted or conjugated to glycine or carnitine (this makes them more soluble so that they can be excreted in the urine. (Sign looked for to see if beta- oxidation is compromised in an individual)

Question 34

What is Refsun’s Disease?

Answer 34

a rare neurological defect caused by defective a-oxidation