Chapter 30 Recall (FA Transport and B-oxidation)

1. FA are released from adipocytes in the fasted state or are delivered to cells with lipoprotein lipase via chylomicrons and VLDLs
2. FA enter cells and are bond to FA-binding proteins (inc solubility)
3. The FA are then activated to Fatty acyl CoA
4. Fatty acyl CoA enters the intermembrane space of the mitochondria, where it's converted to Fatty acyl carnitine
5. Fatty acyl carnitine enters the matrix and is converted back to Fatty acyl CoA
6. Fatty acyl CoA enters B-oxidation, producing FADH2, NADH, and Acetyl CoA
7. Acetyl CoA is converted to KB in the liver during the fasted state, or it enters the TCA cycle to generate energy

Describe the general path that long-chain fatty acids take in order to provide energy for cells

serum albumin

also has non-specific binding capacity for several hormones and drugs

In the serum, fatty acids are transported by ________, which binds up to 6FA chains.

via both a saturable binding protein and free diffusion

How do FA enter cells?

fatty-acid binding proteins

Once inside the cell, FA are bound to ________, which facilitate their transport to the mitochondria, ER, or peroxisomes.

Before they can participate in metabolic pathways, fatty acids must be activated to Fatty acyl CoA. This is catalyzed by acyl CoA synthetase.

Before they can participate in metabolic pathways, fatty acids must be activated to ________. This is catalyzed by ________.

1. The FA reacts with ATP to produce Fatty acyl AMP, and Pyrophosphate (PPi) is the leaving group). This is catalyzed by Fatty acyl CoA synthetase
2. Fatty acyl AMP reacts with CoA to produce Fatty acyl CoA, and AMP is the leaving group. This is catalyzed by Fatty acyl CoA synthetase

because AMP is generated, the reaction uses the equivalent of 2 ATP because we need 2 ATP to convert the AMP back to ATP

Describe the 2 main steps in the activation of FA to FA CoA

4 isoforms
(VLC, LC, MC, SC)

There are at least ________ isoforms of FA CoA synthetase, each having different affinities for FA of different chain lengths and located in different membranes.

VLC = 14-26
LC = 12-20
MC = 6-12
SC = 4-6

Roughly how many carbons are in the chains of VLC, LC, MC, and SC FA?

the location of the acyl-CoA synthetase it happens to interact with

The metabolic fate of a FA CoA will depend largely on ....

1. Energy - B oxidation and ketogenesis
2. Storage - Triacylglycerols
3. Membrane lipids - Phospholipids and sphingolipids

What are the 3 fates of FA CoA?

VLCFA

Peroxisomal acyl-CoA synthetase only recognizes ....

LCFA

Mitochondrial acyl-CoA synthetases (on the outer membrane) only recognize ....

MCFA

________ acyl-CoA synthetase is found in the mitochondrial matrix

the cytosol

acyl-CoA synthetase for SCFA is found in ....

acyl-CoA synthetase found on the ER that recognizes LCFA

Fatty acyl CoA will be used for storage or to make membrane lipids when it interacts with ....

acyl-CoA synthetase in the Mitochondria and Peroxisomes

Fatty acyl CoA will be used for energy when it interacts with ....

carnitine (forming fatty acyl carnitine)

Prior to entry into the mitochondria, the fatty acid chain is transferred from the fatty acyl-CoA onto _______

1. LCFA interacts with acyl CoA synthetase on the outer mitochondrial membrane, producing fatty acyl CoA.

2. Fatty acyl CoA crosses the outer mitochondrial membrane into the intermembrane space. There, it interacts with Carnitine palmitoyl-transferase I (CPTI) to generate Fatty acylcarnitine, releasing its CoA.

3. Fatty acylcarnitine crosses the inner membrane into the matrix through Carnitine acylcarnitine transolcase, which moves fatty acylcarnitine into the matrix and carnitine out of the matrix (to the intermembrane space).

4. Fatty acyl carnitine interacts with Carnitine palmitoyl-transferase II (CPT II) to generate Fatty acyl CoA, releasing its carnitine.

5. Fatty acyl CoA then enters B-oxidation in the matrix

Describe the 5 main steps of FA transport into the mitochondria

it is found in many dietary sources and can be synthesized from lysine

Explain why deficiencies in carnitine are rare

D. Elevated blood acylcarnitine levels
- A deficiency in CPT II means we can make FA carnitine but we can't convert it to FA CoA, which will cause an accumulation of FA carnitine
- We would expect decreased ketone bodies because we are unable to do B-oxidation (since we can't make FA CoA)
- We would expect elevated blood FA levels because the system to transport FA into the mitochondria will be backed up
- We would expect decreased blood glucose. Since we're not getting energy from FA oxidation, we rely on glucose for energy which will decrease serum glucose

If your patient has classic carnitine:palmitoyltransferase II (CPTII) deficiency, which of the following laboratory test results would you expect to observe?
A. Elevated ketone bodies in the blood.
B. Reduced blood fatty acid levels.
C. Elevated blood glucose levels.
D. Elevated blood acylcarnitine levels.
E. Low creatine phosphokinase blood levels.

No
- We would still see a decrease in KB, increase in blood FA, and decrease in serum glucose
- However, we would see a decrease in FA carnitine since we are unable to synthesize it from FA CoA in the intermembrane space

Would you expect the same thing if your patient had CPT I Deficiency?

- Carnitine and choline are converted to trimethyl amine by gut microbiota, which is converted to trimethyl amine oxide by the liver
- Trimethyl amine oxide with decrease the activity of the reverse cholesterol transporter and decrease bile synthesis
- This will the lead to hypercholesterolemia due to an impaired ability to get rid of cholesterol in the body
- This will cause atherosclerosis (thickening or hardening of the arteries caused by a buildup of plaque)

How can carnitine and choline found in red meat be harmful to the body?

- The B carbon is oxidized to a carbonyl, after which the a-B bond is cleaved, producing acetyl-CoA

this is repeated several times producing several molecules of ACoA

Explain the general pattern of B-oxidation in even-chain FA

1. Oxidation: FA CoA is oxidized by acyl CoA dehydrogenase, forming a double bond (alkene) between the a and B carbons. This produces 1 FADH2.

2. Hydration: Enoyl CoA hydratase adds an OH group to the B carbon, forming a single bond between the a and B carbons.

3. Oxidation: B-hydroxy acyl CoA dehydrogenase oxidizes the B alcohol group to a carbonyl (C=O). This produces 1 NADH.

4. Thiolysis: B-Keto thiolase catalyzes a reaction in which the a-B bond is broken. CoA is added to the B carbon, forming fatty acyl CoA, and the a carbon leaves with its CoA group as acetyl CoA

5. The acetyl CoA can enter the TCA cycle or is used for KB synthesis. The fatty acyl CoA repeats this cycle.

Describe the 5 main steps of the B-oxidation spiral

2

Each round of B-oxidation results in the loss of ________ carbons from the chain in the form of acetyl CoA

- FADH2 transfers its electrons to the ETC without dissociating from acyl CoA dehydrogenase
- NADH enters the ETC

What happens to the FADH2 and NADH generated by B-oxidation?

- VLCAD
- LCAD
- MCAD
- SCAD

so LCFA go through oxidation using LCAD until they become MCFA, at which point they will use MCAD until they become SCFA, etc

There are 4 isozymes of acyl CoA dehydrogenase, each with a different substrate specificity:

acetoacetyl CoA

The four carbon B-keto fatty acyl CoA seen in the last round of the B-oxidation spiral with even-chain FA is called ....

- Butyryl-CoA undergoes the first three steps of B-oxidation (oxidation, hydration, oxidation) producing acetoacetyl CoA
- Acetoacetyl CoA can be cleaved by thiolase, producing 2 acetyl CoA molecules (this reaction is reversible)
- When [acetyl CoA] is high, the reaction will run in reverse and produce acetoacetyl CoA, which can be used to synthesize acetoacetate (a ketone body)

How is Acetoacetyl CoA formed? What is its significance?

proprionyl CoA (3C)

Fatty acids with an odd number of carbons undergo multiple groups of B-oxidation, resulting in the formation of several molecules of acetyl CoA and one molecule of ....

1. Proprionyl CoA is converted to D-methylmalonyl CoA by proprionyl CoA carboxylase. This requires an ATP and biotin

2. D-methylmalonyl CoA is epimerized to L-methylmalonyl CoA

3. L-methylmalonyl CoA is converted to Succinyl CoA by Methylmalonyl CoA mutase. This requires coenzyme B12

Describe the 3 steps in which Proprionyl CoA is converted to Succinyl COA

biotin

Propionyl CoA carboxylase requires ....

B12

Methylmalonyl CoA mutase is one of 2 enzymes in the body that requires ....

TCA cycle

this is an important anaplerotic reaction

Proprionyl CoA is converted to Succinyl CoA, which enters the ....

1. Enoyl CoA isomerase
2. 2,4 dienoyl CoA reductase

B-oxidation of unsaturated FA requires the assistance of 2 enzymes:

1. SCFA are produced by fermentation of dietary fiber by gut microbes
2. They are more soluble than LCFA, so they are not stored in adipose tissue, they are transported directly to the liver or other tissues for oxidation
3. SCFA are the primary fuel source for colonocytes and are oxidized by the liver using SCFA CoA synthetase and SC acyl-CoA dehydrogenase

Describe how SCFA are oxidized in the body

monocarboxylate transporter

MCFA enter the mitochondria via a ________, and are then activated to fatty acyl COAs and undergo B-oxidation.

1. Acetate
2. Proprionate
3. Butyrate

What are the 3 main SCFA?

1. Dairy
2. Coconut oil
3. Palm oil

What are 3 foods rich in MCFA?

- It also recognizes several other compounds, including pharmaceutical drugs

What is unique about the acyl-CoA synthetase that recognizes MCFA?

1. The compound reacts with MC acyl-CoA synthetase, and CoA is added to the compound
2. Then, the compound reacts with glycine N-acyltransferase -- which adds a glycine onto the compound and the CoA leaves -- producing acyl glycine
3. Acylglycine is then excreted

Describe the reaction that occurs between pharmaceutical drugs and MC acyl-CoA synthetase

Disorders in B-oxidation will often cause the accumulation of acylglycines and acylcarnitines in the serum/urine.

Disorders in B-oxidation will often cause the accumulation of ________ and ________ in the serum/urine.

- Fatty acyl-CoA reacts with glycine to form fatty acylglycine and CoA (catalyzed by glycine N-acyltransferase and is reversible)
- Disorders in B-oxidation will cause an accumulation of fatty acyl-CoA
- Increased [fatty acyl CoA] will drive the reaction towards the formation of acylglycine and frees up CoA for other uses in the cell (primarily TCA)

How can disorders in B-oxidation cause an accumulation of acylglycine?

- Fatty acyl carnitine reacts with CoA to form Fatty acyl CoA and carnitine (catalyzed by CPT II and is reversible)
- Disorders in B-oxidation will cause an accumulation of fatty acyl-CoA
- Increased [fatty acyl CoA} will drive the reaction towards the formation of acylcarnitine and frees up CoA for other uses in the cell (primarily TCA)

How can disorders in B-oxidation cause an accumulation of acylcarnitines?