LIPID CATABOLISM

Carnitine

N+

CH2CH3

CH3

CH3

CH CH2

OH

CO

O

Formation of acyl carnitine

N+

CH2CH3

CH3

CH3

CH CH2

OH

CO

OCoAS C R

O

carnitine acyl CoA

N+

CH2CH3

CH3

CH3

CH CH2

O

CO

O

C

R

O

CoASH

acyl carnitine

+ +

• Can get transported across inner mitochondrial membrane

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. 

Fate of Acetyl CoA

1. Oxidation in Krebs Cycle

2. Synthesis of Acetylcholine

3. Synthesis of Cholesterol

4. Synthesis of FAs

1. Oxidation in Krebs Cycle– The major proportion of Acetyl-CoA may be

directed into TCA for oxidation and production of energy.

– Acetyl-CoA forms the central meeting point for oxidation of carbohydrates and fats through Krebs cycle.

2. Synthesis of Acetylcholine:– Acetyl-CoA may be utilized for acetylation

reactions. Mainly in brain tissues!!

3. Synthesis of Cholesterol– Acetyl-CoA is the starting material for the

biosynthesis of cholesterol. – It condenses with acetoacetyl-CoA

to form β-hydroxy-β-methylglutaryl-CoA (HMG CoA) which is an important intermediate in the synthesis of cholesterol.

4. Synthesis of Fatty Acids– Acetyl-CoA units are the building blocks for

the synthesis of fatty acids.

– Formation of ketone bodies– When the rate of acetyl-CoA formation

exceeds the capacity of Krebs cycle to deal with them, the following side reactions takes place…

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

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.

 

                                                                 

                                        

• Acetone may undergo 2 metabolic changes– It may be further cleaved to yield acetic acid and

formic acid.

– CH3COCH3 CH3COOH + HCOOH

– It can be transformed to propanediol which in turn gets oxidised to pyruvic acid.

– CH3COCH3 CH3CHOHCH2OH CH3COCOOH

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 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

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

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.

Overall Per Beta Oxidation cycle

• 1 FADH2…………………………1.5 ATP

• 1 NADH………………………….2.5 ATP

• 1 Acetyl CoA to Krebs– 3 NADH X 2.5 ATP / NADH………7.5 ATP

– 1 FADH2…………………………….1.5 ATP

– 1 GTP………………………………..1.0 ATP

Total = 14.0 ATP

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:

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. 

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

-Oxidation pathway:

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

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

Beta Oxidation on 16 C fatty Acid

CH2 CO

O

CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2

CH2CH3

Beta Oxidation on 16 C fatty Acid

CH2 CO

O

CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2

CH2CH3

12

34567

1 2 3 4 5 6 78

• 7 rounds of Beta oxidation (bottom numbers)• Form 8 acetyl Co A (top numbers)

Omega Oxidation

Alpha Oxidation

Alpha-oxi

CASTEELS MARIA REINHILDE• Knowledge of alpha-hydroxylation and alpha-

oxidation of 3-methyl-branched fatty acid as phytanic acid has progressed substantially in recent years. It is not known however what the role is of these enzyme in the synthesis and degradation of alpha-hydroxy-fatty acids in brain. Some findings seem to indicate that in brain these processes are catalysed by different enzymes. The role of alpha-hydroxylation in myelinisation will also be studied.

Odd-numbered carbons

Ketone Bodies

• Starvation causes accumulation of acetyl CoA– not enough carbohydrates to keep Kreb’s

Cycle going– acetyl CoA forms acetoacetate, b-

hydroxybutyrate, and acetone.

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.

Diabetes and Ketone Bodies

• When there is not enough insulin in the blood and it must break down fat for its energy.

• Ketones build up in the blood and then spill over into the urine so that the body can get rid of them. Acetone can be exhaled through the lungs. This gives the breath a fruity odor. Ketones that build up in the body for a long time lead to serious illness and coma. (Diabetic ketoacidosis)