Lipid Metabolism 2: Energy from Lipids and Ketone Bodies

Why are trans fats dangerous?

Causes coronary heart diseases and contributes to type-2 diabetes.

What is elaidic acid?

synthetic; 9, 10-E- or trans-double bond; from partial hydrogenation of multi-saturation fatty acids; mp 46.5

What is oleic acid?

natural; 9, 10-Z- or cis-double bond; mp 13.5

Why are omega-3 fatty acids (PUFAs) beneficial?

Omega-3 can dissolve fats and cholesterol, acting like a sponge. Regular dietary intake of EPA and DHA can reduce blood cholesterol levels up to 10%.

Fatty acids have to be activated before they can be converted to energy by ___

an oxidative breakdown process (B-oxidation)

Fatty Acid + ATP -->

Fatty acyl-AMP + PPi

Fatty acyl-AMP + CoA -->

Fatty acyl CoA + AMP

What is the reaction of Fatty acyl-AMP + CoA catalyzed by?

acyl CoA synthetase

How much energy is consumed in the Fatty acyl-AMP + CoA reaction?

consumes 2 high energy anhydride bonds (thus, the calculations of net energy yield from fatty acids requires the subtraction of 2 ATP)

What is fatty acyl-AMP?

An intermediate of the activation process and contains one energy-rich anhydride bond.

Does a synthetase reaction require ATP?

Yes

Does a synthase reaction require ATP?

No

Where does fatty acid activation occur?

acyl-CoA synthetases in cytosol, ER, and peroxisomes

Activated fatty acids are transported into the mitochondrial matrix for B-oxidation by the ___.

carnitine carrier system

What is the purpose of the carnitine carrier system?

for the transportation of long chain fatty acids to the mitochondrial matrix

What is the carnitine carrier system composed of?

two membrane-bound enzymes and a carrier protein

What is CPT I?

carnitine: palmitoyltransferase I at the cytosolic site and inner membrane space

What is CPT II?

carnitine: parlmitoyltransferase II at the matrix site

Where is CPT I attached?

attached to the inside (intermembrane space side) of the outer mitochondrial membrane

Where is CPT II attached?

attached to the inside (matrix side) of the inner mitochondrial membrane

Where is the carnitine carrier protein attached?

(carnitine acylcarnitine translocase) spans the inner mitochondrial membrane

What does CPT I do?

catalyzes the transfer of a fatty acyl group from coenzyme A to the alcohol function of carnitine

What inhibits CPT I?

inhibited by malonyl CoA (intermediate in fatty acid synthesis)

The carnitine carrier protein (also called carnitine-acylcarnitine translocase) is an ___ system that is specified to carry the ___ into the mitochondrial matrix.

active transport; fatty acyl carnitine

What does the carrier protein recognize?

carnitine or carnitine-marked fatty acids

What does CPT II do?

transfers the fatty acyl group back to CoA from carnitine

After passing through the carnitine carrier protein, carnitine then travels back to the ___.

cytosol

Where does the principal degradation of fatty acids occur?

in the mitochondrial matrix

What are the four reactions that occur in the beta-oxidation of fatty acids?

-FAD-dependent dehydrogenation (= oxidation, formation of trans-a,B-double bond)

-Hydration (addition of water)

-NAD+-dependent dehydrogenation (= oxidation, formation of B-ketoacylCoA)

-Thiolysis (Ca-Cb cleavage into acetyl-CoA and a shorter fatty acyl-CoA)

Each set of reactions in the beta-oxidation of fatty acids shortens a fatty acid by ___.

two carbons

What is the final product of the beta-oxidation of fatty acids?

C2-compound acetyl CoA (e.g., a C18 fatty acid produces 9 acetyl CoA)

What is the process for a beta-oxidation reaction?

1. Two hydrogens with their electrons are transferred to FAD.

2. H2O adds across double bond

3. Oxidation (= dehydration) by NAD+

4. Bond between alpha and beta carbons is cleaved

5. Considered a spiral (not a cycle)

Which enzyme converts fatty acyl-CoA into trans-delta2-enoyl-CoA?

acyl-CoA dehydrogenase

Which enzyme converts trans-delta2-Enoyl-CoA into 3-L-Hydroxyacyl-CoA?

enoyl-CoA hydratase

Which enzyme converts 3-L-Hydroxyacyl-CoA into B-Ketoacyl-CoA?

3-L-hydroxyacyl-CoA dehydrogenase

Which enzyme converts B-Ketoacyl-CoA into Acetyl-CoA?

B-Ketoacyl-CoA thiolase

What are the 3 types of energy rich molecules produced in B-Oxidation?

• 1 FADH2 per cut : electron transport chain (1 FADH2 = 2 ATP)

• 1 NADH per cut : electron transport chain (1 NADH = 3 ATP)

• 1 Acetyl CoA per C2-unit : TCA cycle/electron transport chain (1 acetyl CoA = 12 ATP)

a saturated C18-fatty acid produces a net yield of ___ ATP.

146

Why are double bonds in unsaturated fatty acids a problem for B-oxidation?

They are usually in the wrong position (3,4-cis instead of 2,3-trans), preventing normal B-oxidation progression

What additional issue occurs during degradation of multi-unsaturated fatty acids?

a conjugated delta 4 system forms, which blocks B-oxidation

What are two enzymes required to fix double-bond problems during B-oxidation of unsaturated fatty acids?

an isomerase and a reductase

B-oxidation proceeds normally until the double bond reaches which carbon positions?

between carbons 3 and 4

What does the isomerase do during unsaturated fatty acid B-oxidation?

it moves the 3,4 double bond to between carbons 2 and 3, the correct position for B-oxidation

After isomerase action, what happens to B-oxidation?

it resumes, but skips reaction 1 of the cycle

What is the ATP consequence of skipping reaction 1 of B-oxidation?

One FADH2 is not produced --> 2 ATP less than saturated fatty acids

What special problem occurs with multiple double bonds in fatty acids?

a conjugated delta 4 system forms, inhibiting B-oxidation

What enzyme fixes the conjugated delta 4 system?

a reductase that removes the second double bond

How much ATP does reductase activity cost the cell?

3 ATPs (equivalent to 1 NADPH)

Why does oxidation of polyunsaturated fatty acids yield less energy than saturated fatty acids?

Because of skipped FADH2 production and the ATP cost of reductase activity

What is alpha-Linolenic acid (ALA)?

C18 omega-3 fatty acid with three isolated Z-double bonds in 9, 12, and 15 positions

How many ATPs are produced in the oxidation of alpha-linolenic acid (ALA)?

139

The B-oxidation of odd-numbered fatty acids ends with ___.

propionyl-CoA

What are peroxisomes?

Membrane enclosed microbodies found in all eukaryotic cells (animals, plants). Important for lipid metabolism and detoxification. Uses molecular oxygen, deal with H2O2

What are anaplerotic reactions?

(refilling reactions) are needed to make up for the loss of C4-intermediates

What are the 2 major anaplerotic reactions in the TCA cycle?

-pyruvate carboxylase

-propionyl-CoA carboxylase

Propionyl CoA --> ___ --->___.

methylmalonyl-CoA (B12 dependent); succinyl-CoA

Propionyl CoA --> Succinyl CoA yields ___ ATPs.

5

What type of fatty acids are primarily shortened by peroxisomal B-oxidation?

very long chain fatty acids (> 22 carbons)

What is the first enzyme in peroxisomal B-oxidation and what does it do?

Acyl-CoA oxidase transfers 2 H-atoms from FADH₂ to O₂, forming H₂O₂ and trans-Δ²-enoyl-CoA

What byproduct does acyl-CoA oxidase produce that mitochondria do not?

hydrogen peroxide (H2O2)

How do reactions 2 and 3 of peroxisomal B-oxidation compare to mitochondrial B-oxidation?

They use the same enzymes (enoyl-CoA hydratase and 3-L-hydroxyacyl-CoA dehydrogenase), but occur on a single polypeptide in peroxisomes

What enzyme catalyzes the final step (reaction 4) in peroxisomal B-oxidation?

peroxisomal thiolase

When does peroxisomal thiolase become almost inactive?

When the acyl-CoA chain is shortened to C₈ (octanoyl-CoA)

What happens to C₈-acyl-CoAs after peroxisomal β-oxidation stalls?

They are transported into the mitochondria by the carnitine carrier system for further oxidation

What enzyme converts H2O2 to water and oxygen in peroxisomes?

catalase

Why does peroxisomal β-oxidation produce less ATP than mitochondrial β-oxidation?

Because FADH₂ electrons reduce O₂ to H₂O₂ instead of entering the electron transport chain → 2 ATP less per cut.

What is the overall purpose of peroxisomal B-oxidation?

To shorten very long fatty acids to a manageable length for mitochondrial B-oxidation

What is the purpose of Clofibric acid and Clofibrate?

Drugs to treat hyperlipoproteinemia/hyperlipidemia (elevation of lipids in the blood) which are associated with stroke and heart attacks. They also stimulate peroxisomes to oxidize very long fatty acids.

What causes Zellweger's syndrome (inherited deficiency of peroxisomes)?

accumulate very long fatty acids, particularly in the brain

___ are also supposed to be useful to treat hyperlipoproteinemia/hyperlipidemia.

Omega-3 fatty acid (EPA, FHA) ethyl esters

Where does the gamma-oxidation of fatty acids occur?

at the far end (gamma end) of a fatty acid chain in the endoplasmic reticulum

When does gamma-oxidation of fatty acids occur?

frequently used when mitochondrial B-oxidation is compromised (carnitine deficiency)

The resulting dicarboxylic acid in gamma-oxidation of fatty acids undergoes B-oxidation until C6-C8-___ are reached which are then excreted in the urine.

dicarboxylic acids

What does alpha-oxidation of fatty acids do?

Occurs with very long chain fatty acids (> 20 carbons). The alpha-oxidation facilitates the release of the carboxyl group as CO2. Removes one carbon at a time. Used mainly for the breakdown of dietary branched fatty acids (from plants)

Where does alpha-oxidation of fatty acids occur?

in the peroxisomes of mainly the brain and nervous tissue

What are some examples of ketone bodies?

-Acetoacetate (3-oxo-butanoic acid)

-D-B-Hydroxybutyrate ((R)-3-hydroxybutanoic acid)

-Acetone (propan-2-one)

Where does the metabolism of ketone bodies occur?

Occurs in the fasting state (when fatty acid levels are elevated). Occurs mainly in the mitochondria of the liver (synthesis) and muscles and brain (degradation).

What are the 3 steps that form acetoacetate from acetyl-CoA?

-Claisen condensation of two acetyl CoA form acetoacetyl CoA (acetyl-CoA acetyltransferase or thiolase)

-Claisen condensation of acetoacetyl CoA + acetyl CoA produces 3-hydroxy-3-methylglutaryl CoA (HMG CoA) (HMA-CoA synthase)

-HMG CoA is degraded to acetyl CoA + acetoacetate in a mixed aldol-Claisen cleavage (HMG CoA lyase)

What is the fate of acetoacetate?

-Enters the blood

-May be reduced to D-B-hydroxybutyrate by B-hydroxybutyrate dehydrogenase (liver enzyme)

-Spontaneously decarboxylates to acetone which is expired by the lungs.

What is the oxidation of B-Hydroxybutyrate?

Occurs in the target tissue. This way the target tissue gets more energy (3 ATPs more than from acetoacetate). The liver donates this extra energy.

How does acetoacetate enter cells?

Either directly or is oxidized from B-hydroxybutyrate in target cells before it can be utilized for energy

Oxidation of B-hydroxybutyrate produces ___; thus, more energy is produced from oxidation of B-hydroxybutyrate.

NADH/H+

Where does the activation of acetoacetate occur?

the mitochondria

How many ATP equivalents does the activation of acetoacetate cost?

1

What enzyme is responsible for the activation of acetotacetate?

3-Ketoacyl-CoA transferase (not found in the liver!)

Succinyl CoA + acetoacetate →

succinate + acetoacetyl CoA

Which reactions occur in the metabolism of ketone bodies?

-Oxidation (dehydrogenation)

-Transesterification

-Oxidative Cleavage

How many acetyl CoA are produced from acetoacetate?

2

How many ATP are formed from 1 acetyl CoA?

12

Why are fatty acids oxidized during the fasting state?

because malonyl-CoA levels are low, which activates CPT I, allowing fatty acids to enter the mitochondria for B-oxidation

When are ketone bodies biosynthesized?

during gluconeogenesis (low oxaloacetate levels lead to acetyl CoA accumulation)

What are the advantages of ketone bodies?

-Liver gets some energy from partial oxidation of fatty acids in preparation for gluconeogenesis.

-Other tissues use remainder of energy contained in ketone bodies for fuel.

-Ketone bodies are particularly important for the brain under extreme starvation.

-Ketone bodies are water soluble and can travel freely through the blood.

What is diabetic ketoacidosis?

due to increased rate of fatty acid release from adipose tissue because of the missing insulin

What is alcohol related acidosis?

alcoholics often suffer from lack of appetite.

-low levels of glycogen in the liver

In addition, gluconeogenesis is decreased due to ethanol oxidation (NADH excess blocks lactate dehydrogenase).

-increased rate of fatty acid release from adipose tissue

A lack of the enzyme ETF:CoQ oxidoreductase may lead to death. This is due to which one of the following reasons?

The energy yield from fatty acid utilization is dramatically reduced.

3 multiple choice options

The ATP yield from the complete oxidation of 1 mol of a C18:0 fatty acid to carbon dioxide and water would be closest to which one of the following?

120

3 multiple choice options

Which one of the following sets of reactions best describes the oxidation of fatty acids?

Oxidation, hydration, oxidation, carbon-carbon bond breaking

3 multiple choice options

Elevated levels of ketone bodies can be found in the blood of untreated type I diabetics and individuals on severe diets. A major difference in the laboratory findings of metabolites in the blood of each type of individual (type I diabetic versus the dieter) would be which of the following?

glucose levels

3 multiple choice options

In which one of the following time spans will fatty acids be the major source of fuel for the tissues of the body?

While running the last mile of a marathon

3 multiple choice options

If your patient has classic carnitine palmitoyl transferase II deficiency, which one of the following laboratory test results would you expect to observe?

Elevated blood acylcarnitine levels

3 multiple choice options