Fatty Acid Oxidation

Copyright © 1999-2007 by Joyce J. Diwan. All rights reserved.

Molecular Biochemistry II

A 16-C fatty acid with numbering conventions is shown.

Most naturally occurring fatty acids have an even number of carbon atoms.

The pathway for catabolism of fatty acids is referred to as the -oxidation pathway, because oxidation occurs at the -carbon (C-3).

C

O

O1

23

4

fatty acid with a cis-9 double bond

Triacylglycerols (triglycerides) are the most abundant dietary lipids. They are the form in which we store reduced C for energy.

Each triacylglycerol has a glycerol backbone to which are esterified 3 fatty acids

Most triacylglycerols are “mixed.” The 3 fatty acids differ in chain length & number of double bonds.

g ly c e ro l fa tty a c id tr ia c y lg ly c e ro l

H 2 C

HC

H 2 C

OH

OH

OH

H 2 C

HC

H 2 C

O

O

O

C R

O

C

C R

OR

O

HO C R

O

Lipid digestion, absorption, transport will be covered separately.

Lipases hydrolyze triacylglycerols, releasing 1 fatty acid at a time, yielding diacylglycerols, & eventually glycerol.

g ly c e ro l fa tty a c id tr ia c y lg ly c e ro l

H 2 C

HC

H 2 C

OH

OH

OH

H 2 C

HC

H 2 C

O

O

O

C R

O

C

C R

OR

O

HO C R

O

g l y c e r o l g l y c e r o l - 3 - P d i h y d r o x y a c e t o n e - P

C H 2

C H

C H 2

O H

H O

O PO 3

C H 2

C H

C H 2

O H

H O

O H

C H 2

C

C H 2

O H

O PO 3

O

A T P A D P H + +N A D + N A D H

1 2

Glycerol, arising from hydrolysis of triacylglycerols, is converted to the Glycolysis intermediate dihydroxyacetone phosphate, by reactions catalyzed by:

1 Glycerol Kinase

2 Glycerol Phosphate Dehydrogenase.

Free fatty acids, which in solution have detergent properties, are transported in the blood bound to albumin, a serum protein produced by the liver.

Several proteins have been identified that facilitate transport of long chain fatty acids into cells, including the plasma membrane protein CD36.

C

O

O1

23

4

fatty acid with a cis-9 double bond

Fatty acid activation:

Fatty acids must be esterified to Coenzyme A before they can undergo oxidative degradation, be utilized for synthesis of complex lipids, or be attached to proteins as lipid anchors.

Acyl-CoA Synthases (Thiokinases) of ER & outer mitochondrial membranes catalyze activation of long chain fatty acids, esterifying them to coenzyme A.

This process is ATP-dependent, & occurs in 2 steps.

There are different Acyl-CoA Synthases for fatty acids of different chain lengths. 

Acyl-CoA Synthases

Exergonic PPi (P~P) hydrolysis, catalyzed by Pyrophosphatase, makes the coupled reaction spontaneous.

2 ~P bonds of ATP are cleaved.

The acyl-CoA product includes one "~" thioester linkage.

Fatty acid activation

N

NN

N

NH2

O

OHOH

HH

H

CH2

H

OPOPOP O

O

O O

O O

O

N

NN

N

NH2

O

OHOH

HH

H

CH2

H

OPOC

O

O

R

O

SCR

O

CoA

R C

O

O

PPi

CoA SH

AMP

2 Pi

fatty acid

ATP

acyl- adenylate

acyl-CoA

Summary of fatty aid activation:

fatty acid + ATP acyladenylate + PPi

PPi 2 Pi

acyladenylate + HS-CoA acyl-CoA + AMP

Overall:

fatty acid + ATP + HS-CoA acyl-CoA + AMP + 2 Pi

For most steps of the pathway there are multiple enzymes specific for particular fatty acid chain lengths.

-Oxidationpathway inmatrix

Fatty acyl-CoA formed in cytosol by enzymesof outer mitochondrial membrane & ER

Mitochondrion

-Oxidation pathway:

Fatty acids are degraded in the mitochondrial matrix via the -Oxidation Pathway.

Fatty acyl-CoA formed outside can pass through the outer mitochondrial membrane (which has large VDAC channels), but cannot penetrate the inner membrane.

-Oxidationpathway inmatrix

Fatty acyl-CoA formed in cytosol by enzymesof outer mitochondrial membrane & ER

MitochondrionMany of the constituent enzymes are soluble proteins located in the mitochondrial matrix.

But enzymes specific for very long chain fatty acids are associated with the inner membrane, facing the matrix.

Carnitine Palmitoyl Transferases catalyzes transfer of a fatty acid between the thiol of Coenzyme A and the hydroxyl on carnitine.

H3C N CH2 CH CH2

CH3

CH3

OH

COO+

R

C

SCoA

O+

H3C N CH2 CH CH2

CH3

CH3

O

COO+

C

R

O

+ HSCoA

carnitine

fatty acyl carnitine

Carnitine Palmitoyl Transferase

Transfer of the fatty acid moiety across the mitochondrial inner membrane involves carnitine. 

Carnitine-mediated transfer of the fatty acyl moiety into the mitochondrial matrix is a 3-step process:

1. Carnitine Palmitoyl Transferase I, an enzyme on the cytosolic surface of the outer mitochondrial membrane, transfers a fatty acid from CoA to the OH on carnitine.

2. An antiporter in the inner mitochondrial membrane mediates exchange of carnitine for acylcarnitine.

cytosol mitochondrial matrix O O

R-C-SCoA HO-carnitine HO-carnitine R-C-SCoA HSCoA R-C-O-carnitine R-C-O-carnitine HSCoA

O O

1 2

3

3. Carnitine Palmitoyl Transferase II, an enzyme within the matrix, transfers the fatty acid from carnitine to CoA. (Carnitine exits the matrix in step 2.)

The fatty acid is now esterified to CoA in the matrix.

cytosol mitochondrial matrix O O

R-C-SCoA HO-carnitine HO-carnitine R-C-SCoA HSCoA R-C-O-carnitine R-C-O-carnitine HSCoA

O O

1 2

3

Control of fatty acid oxidation is exerted mainly at the step of fatty acid entry into mitochondria.

Malonyl-CoA (which is also a precursor for fatty acid synthesis) inhibits Carnitine Palmitoyl Transferase I.

Malonyl-CoA is produced from acetyl-CoA by the enzyme Acetyl-CoA Carboxylase.

H3C C SCoA

O

CH2 C SCoA

O

OOC

acetyl-CoA

malonyl-CoA

Activated Kinase, leading to decreased malonyl-CoA.

The decrease in malonyl-CoA concentration leads to increased activity of Carnitine Palmitoyl Transferase I.

Increased fatty acid oxidation then generates acetyl-CoA, for entry into Krebs cycle with associated ATP production.

AMP-Activated Kinase, a sensor of cellular energy levels, is allosterically activated by AMP, which is high in concentration when [ATP] is low.

Acetyl-CoA Carboxylase is inhibited when phosphorylated by AMP-

H3C C SCoA

O

CH2 C SCoA

O

OOC

acetyl-CoA

malonyl-CoA

ATP + HCO3

ADP + Pi

Acetyl-CoA Carboxylase (inhibited by

AMP-Activated Kinase)

AMP-Activated Kinase functions under a variety of conditions that lead to depletion of cellular ATP (reflected as increased AMP), including: glucose deprivation, exercise, hypoxia & ischaemia.

AMP-Activated Kinase regulates various metabolic pathways to: promote catabolism leading to ATP synthesis

(e.g., stimulation of fatty acid oxidation) inhibit energy-utilizing anabolic pathways

(e.g., fatty acid synthesis).

AMP-Activated Kinase in the hypothalamus of the brain is involved also in regulation of food intake.

bond between carbon atoms 2 & 3.

There are different Acyl-CoA Dehydrogenases for short (4-6 C), medium (6-10 C), long and very long (12-18 C) chain fatty acids.

Very Long Chain Acyl-CoA Dehydrogenase is bound to the inner mitochondrial membrane. The others are soluble enzymes located in the mitochondrial matrix.

H3C(CH2)nCCCSCoA

H

H

H

HO

123

H3C(CH2)nCCCSCoA

H

HO

H3C(CH2)nCCH2CSCoA

OH

O

H2O

FADH2

FAD

H

H3C(CH2)nCCH2CSCoA

OO

H+ + NADH

NAD+

CH3CSCoA

O

H3C(CH2)nCSCoA +

O

HSCoA

fatty acyl-CoA

trans-2-enoyl-CoA

Acyl-CoA Dehydrogenase

-Oxidation Pathway:

Step 1. Acyl-CoA Dehydrogenase catalyzes oxidation of the fatty acid moiety of acyl-CoA to produce a double

FAD is the prosthetic group that functions as eacceptor for Acyl-CoA Dehydrogenase. Proposed mechanism:

A Glu side-chain carboxyl extracts a proton from the -carbon of the substrate, facilitating transfer of 2 e with H+ (a hydride) from the position to FAD.

The reduced FAD accepts a 2nd H+, yielding FADH2.

H3C(CH2)nCCCSCoA

H

H

H

HO

123

H3C(CH2)nCCCSCoA

H

HO

H3C(CH2)nCCH2CSCoA

OH

O

H2O

FADH2

FAD

H

H3C(CH2)nCCH2CSCoA

OO

H+ + NADH

NAD+

CH3CSCoA

O

H3C(CH2)nCSCoA +

O

HSCoA

fatty acyl-CoA

trans-2-enoyl-CoA

Acyl-CoA Dehydrogenase H3N+ C COO

CH2

CH2

C

H

OO

glutamate

The carbonyl O of the thioester substrate is hydrogen bonded to the 2'-OH of the ribityl moiety of FAD, giving this part of FAD a role in positioning the substrate and increasing acidity of the substrate -proton.

H3C(CH2)nCCCSCoA

H

H

H

HO

123

H3C(CH2)nCCCSCoA

H

HO

H3C(CH2)nCCH2CSCoA

OH

O

H2O

FADH2

FAD

H

H3C(CH2)nCCH2CSCoA

OO

H+ + NADH

NAD+

CH3CSCoA

O

H3C(CH2)nCSCoA +

O

HSCoA

fatty acyl-CoA

trans-2-enoyl-CoA

Acyl-CoA Dehydrogenase

The carbonyl O of the thioester substrate is hydrogen bonded to the 2'-OH of the ribitol moiety of FAD, giving the sugar alcohol a role in positioning the substrate and increasing acidity of the substrate -proton.

C

CCH

C

C

HC

NC

CN

NC

NHC

H3C

H3C

O

O

CH2

HC

HC

HC

H2C

OH

O P O P O

O

O-

O

O-

Ribose

OH

OH

Adenine

C

CCH

C

C

HC

NC

C

HN

NH

C

NHC

H3C

H3C

O

O

CH2

HC

HC

HC

H2C

OH

O P O P O

O

O-

O

O-

Ribose

OH

OH

AdenineFAD FADH2

2 e + 2 H+

dimethylisoalloxazine

The reactive Glu and FAD are on opposite sides of the substrate at the active site.

Thus the reaction is stereospecific, yielding a trans double bond in enoyl-CoA.

H3C(CH2)nCCCSCoA

H

H

H

HO

123

H3C(CH2)nCCCSCoA

H

HO

H3C(CH2)nCCH2CSCoA

OH

O

H2O

FADH2

FAD

H

H3C(CH2)nCCH2CSCoA

OO

H+ + NADH

NAD+

CH3CSCoA

O

H3C(CH2)nCSCoA +

O

HSCoA

fatty acyl-CoA

trans-2-enoyl-CoA

Acyl-CoA Dehydrogenase

FADH2 is reoxidized by transfer of 2 electrons to

an electron transfer flavoprotein (ETF), which in turn passes the electrons to coenzyme Q of the respiratory chain.

Matrix

H+ + NADH NAD+

+ 2H+ 2H+ + ½ O2 H2O

2 e – – I Q III IV

+ +

4H+ 4H+ 2H+ Intermembrane Space

cyt c

Step 2.

Enoyl-CoA Hydratase catalyzes stereospecific hydration of the trans double bond produced in the 1st step, yielding L-hydroxyacyl-Coenzyme A.

H3C (CH2)n C C C SCoA

H

H

H

H O

123

H3C (CH2)n C C C SCoA

H

H O

H3C (CH2)n C CH2 C SCoA

OH

O

H2O

FADH2

FAD

H

H3C (CH2)n C CH2 C SCoA

OO

H+ + NADH

NAD+

CH3 C SCoA

O

H3C (CH2)n C SCoA +

O

HSCoA

fatty acyl-CoA

trans-2-enoyl-CoA

3-L-hydroxyacyl-CoA

Acyl-CoA Dehydrogenase

Enoyl-CoA Hydratase

H3C (CH2)n C C C SCoA

H

H

H

H O

123

H3C (CH2)n C C C SCoA

H

H O

H3C (CH2)n C CH2 C SCoA

OH

O

H2O

FADH2

FAD

H

H3C (CH2)n C CH2 C SCoA

OO

H+ + NADH

NAD+

CH3 C SCoA

O

H3C (CH2)n C SCoA +

O

HSCoA

3-L-hydroxyacyl-CoA

-ketoacyl-CoA

fatty acyl-CoA acetyl-CoA (2 C shorter)

Hydroxyacyl-CoA Dehydrogenase

-Ketothiolase

Step 3.

Hydroxyacyl-CoA Dehydrogenase catalyzes oxidation of the hydroxyl in the position (C3) to a ketone.

NAD+ is the electron acceptor.

A cysteine S attacks the -keto C.

Acetyl-CoA is released, leaving the fatty acyl moiety in thioester linkage to the cysteine thiol.

The thiol of HSCoA displaces the cysteine thiol, yielding fatty acyl-CoA (2 C less).

H3C (CH2)n C CH2 C SCoA

OO

CH3 C SCoA

O

H3C (CH2)n C SCoA +

O

HSCoA-ketoacyl-CoA

fatty acyl-CoA acetyl-CoA (2 C shorter)

-Ketothiolase

H3N+ C COO

CH2

SH

H

cysteine

Step 4.

-Ketothiolase catalyzes thiolytic cleavage.

A membrane-bound trifunctional protein complex with two subunit types expresses the enzyme activities for steps 2-4 of the -oxidation pathway for long chain fatty acids.

Equivalent enzymes for shorter chain fatty acids are soluble proteins of the mitochondrial matrix.

Summary of one round of the -oxidation pathway:

fatty acyl-CoA + FAD + NAD+ + HS-CoA

fatty acyl-CoA (2 C less) + FADH2 + NADH + H+

+ acetyl-CoA

The -oxidation pathway is cyclic.

The product, 2 carbons shorter, is the input to another round of the pathway.

If, as is usually the case, the fatty acid contains an even number of C atoms, in the final reaction cycle butyryl-CoA is converted to 2 copies of acetyl-CoA. 

NADH produced during fatty acid oxidation is reoxidized by transfer of 2e to respiratory chain complex I.

Transfer of 2e from complex I to oxygen causes sufficient proton ejection to yield approximately 2.5 ATP.

Recall that 4H+ enter the matrix per ATP synthesized, taking into account transmembrane flux of ADP, ATP & Pi.

Matrix

H+ + NADH NAD+

+ 2H+ 2H+ + ½ O2 H2O

2 e – –

I Q III IV

+ +

4H+ 4H+ 2H+ Intermembrane Space

cyt c 3H+

F1

Fo

ADP + Pi ATP

FADH2 of Acyl-CoA Dehydrogenase is reoxidized by transfer of 2e via ETF to CoQ of the respiratory chain.

H+ ejection from the matrix that accompanies transfer of 2e from coenzyme Q to oxygen, leads to production of approximately 1.5 ATP.

Matrix

H+ + NADH NAD+

+ 2H+ 2H+ + ½ O2 H2O

2 e – –

I Q III IV

+ +

4H+ 4H+ 2H+ Intermembrane Space

cyt c 3H+

F1

Fo

ADP + Pi ATP

Acetyl-CoA can enter Krebs cycle, yielding additional NADH, FADH2, and ATP.

Fatty acid oxidation is a major source of cell ATP.

Catabolism of two 6-C glucose through Glycolysis, Krebs, & ox phos yields about 60 ~P bonds of ATP (30/glucose).

Compare energy yield oxidizing a 12-C fatty acid. Assume: 1.5 ATP produced per FADH2 reoxidized in the

respiratory chain (via coenzyme Q). 2.5 ATP produced per NADH reoxidized in the

respiratory chain.

Matrix

H+ + NADH NAD+

+ 2H+ 2H+ + ½ O2 H2O

2 e – –

I Q III IV

+ +

4H+ 4H+ 2H+ Intermembrane Space

cyt c 3H+

F1

Fo

ADP + Pi ATP

Problem(See web handout, tutorial)

How many "high energy" (~) bonds are utilized in activating the fatty acid, by esterifying it to coenzyme A? ()________

How many times is the -oxidation pathway repeated during oxidation of a 12-C fatty acid? _________ 

How many each of NADH______, FADH2______, and Acetyl CoA______ are produced, per 12-carbon fatty acid, in the -oxidation pathway?

Oxidation of each acetyl CoA in Krebs cycle yields 3 NADH and one FADH2 (from succinate), resulting in additional production of _______NADH and _______FADH2.

Thus the yield is a total of _______NADH and _______FADH2.

In the respiratory chain, approx. 2.5 ~ bonds of ATP are produced per NADH and 1.5 ~ bonds of ATP per FADH2 (electrons entering the respiratory chain via coenzyme Q). Thus from reoxidation of NADH and FADH2 a total of _______ ~ bonds of ATP are produced per 12-C fatty acid. 

Add to this the ~P bonds of GTP produced in Krebs Cycle (one GTP per acetyl-CoA) for a total of _______ ~P bonds produced.

Summing input and output yields a total of _______ ~P  bonds per 12-C fatty acid oxidized. Does fat yield more energy than carbohydrate? _______ 

2

5

5 5 6

186

23 11

74

80

78YES

Human genetic diseases have been identified that involve mutations in:

the plasma membrane fatty acid transporter CD36

Carnitine Palmitoyltransferases I & II (required for transfer of fatty acids into mitochondria)

Acyl-CoA Dehydrogenases for various chain lengths of fatty acids

Hydroxyacyl-CoA Dehydrogenases for medium & short chain length fatty acids

Medium Chain -Ketothiolase

the trifunctional protein complex

Electron Transfer Flavoprotein (ETF).

Human genetic diseases:

Symptoms vary depending on the specific genetic defect but may include: hypoglycemia and failure to increase ketone body

production during fasting fatty degeneration of the liver heart and/or skeletal muscle defects maternal complications of pregnancy sudden infant death (SIDS).

Hereditary deficiency of Medium Chain Acyl-CoA Dehydrogenase (MCAD), the most common genetic disease relating to fatty acid catabolism, has been linked to SIDS. 

The reactions presented accomplish catabolism of a fatty acid with an even number of C atoms & no double bonds.

Additional enzymes deal with catabolism of fatty acids with an odd number of C atoms or with double bonds.

The final round of -oxidation of a fatty acid with an odd number of C atoms yields acetyl-CoA & propionyl-CoA.

Propionyl-CoA is converted to the Krebs cycle intermediate succinyl-CoA, by a pathway involving vitamin B12 (to be presented later).

Most double bonds of naturally occurring fatty acids have the cis configuration.

As C atoms are removed two at a time, a double bond may end up in the wrong position or wrong configuration to be the correct substrate for Enoyl-CoA Hydratase.

The reactions that allow unsaturated fatty acids to be fully catabolized by the -oxidation pathway are summarized in the textbook.

-Oxidation of very long-chain fatty acids also occurs within peroxisomes.

FAD is e acceptor for peroxisomal Acyl-CoA Oxidase, which catalyzes the 1st oxidative step of the pathway.

Single membrane

Enzymes, some of which produce H2O2 , &always including Catalase, that degrades H2O2.

Crystalline inclusionoften present

Peroxisome

Within the peroxisome, FADH2 generated by fatty acid oxidation is reoxidized producing hydrogen peroxide:

FADH2 + O2 FAD + H2O2

The peroxisomal enzyme Catalase degrades H2O2:

2 H2O2 2 H2O + O2

These reactions produce no ATP.

Once fatty acids are reduced in length within the peroxisomes they may shift to the mitochondria to be catabolized all the way to CO2.

Carnitine is involved in transfer of fatty acids into and out of peroxisomes.

Serious genetic diseases are associated with defects in or deficiency of enzymes of the peroxisomal -oxidation system.

Peroxisomes also contain enzymes for an essential -oxidation pathway that degrades fatty acids having methyl branches, such as phytanic acid, a breakdown product of chlorophyll.

This impedes entry of acetyl-CoA into Krebs cycle.

Acetyl-CoA in liver mitochondria is converted then to ketone bodies, acetoacetate & -hydroxybutyrate.

Glucose-6-phosphatase glucose-6-P glucose

Gluconeogenesis Glycolysis

pyruvate fatty acids

acetyl CoA ketone bodies cholesterol oxaloacetate citrate

Krebs Cycle

During fasting or carbohydrate starvation, oxaloacetate is depleted in liver due to gluconeogenesis.

H3C CH2C C

O O

SCoA

H3C C

O

SCoA

HSCoA

H2C C

H2C C

OH O

SCoA

CH3

C

O

O

H3C C

O

SCoA + H3C C

O

SCoA

HSCoA

O CH2C C

O O

CH3 H3C C

O

SCoA+

acetyl-CoA acetyl-CoA

acetoacetyl-CoA

acetyl-CoA

HMG-CoA

acetoacetate acetyl-CoA

Thiolase

HMG-CoA Synthase

HMG-CoA Lyase

Ketone body synthesis:

-Ketothiolase. The final step of the -oxidation pathway runs backward.

HMG-CoA Synthase catalyzes condensation with a 3rd acetate moiety (from acetyl-CoA).

HMG-CoA Lyase cleaves HMG-CoA to yield acetoacetate & acetyl-CoA.

Ketone bodies are transported in the blood to other cells, where they are converted back to acetyl-CoA for catabolism in Krebs cycle, to generate ATP.

While ketone bodies thus function as an alternative fuel, amino acids must be degraded to supply input to gluconeogenesis when hypoglycemia occurs, since acetate cannot be converted to glucose.

-H ydroxybutyrate D ehydrogenase

C H 3

C

C H 2

C O O

O

C H 3

C H

C H 2

C O O

H O

acetoacetate D - -hydroxybutyrate

H + N A D H N A D +

-Hydroxybutyrate Dehydrogenase catalyzes reversible interconversion of the ketone bodies acetoacetate & -hydroxybutyrate.