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Biochemistry I Fall Term, 2004 December 8, 2004

Lecture 33: Catabolism of Fatty Acids

Assigned reading in Campbell: Chapter 18.1-18.5

Key Terms:

Acyl-carnitine

Acyl-CoA

β-oxidation

Lipases

Phospholipases

Ketone bodies

Links:

(I) Review Quiz on Lecture 33 concepts

Formation of Acyl-CoA

The fatty acids in the cytosol are coupled to Coenzyme A to form acyl-CoA.

The activation reaction is catalyzed by acyl-CoA synthetase and involves the following steps

(Fig. 18.4):

1.  Nucleophilic attack of carboxyl group of fatty acid towards phosphate of ATP

2.  Formation of an acyl-AMP intermediate with release of PPi.

This reaction is slightly endergonic; however, it is driven by the subsequent hydrolysis of 

PPi. This is another example of:

The hydrolysis of a high energy compound to make a reaction irreversible; and

Irreversibility in the first step of a pathway.

3.  Nucleophilic attack of the sulfur group of CoA on the carbonyl carbon of the fatty acyl-

AMP intermediate.

The resulting high energy thioester bond (∆G = -31.5 kJ/mol for hydrolysis) preserves the high

energy of the ATP used to form the acyl-CoA.

Note that it is only necessary to utilize ATP in the formation of the first thioester bond.

Formation and Transport of Acyl-carnitine

The acyl-CoA is transported into the mitochondrial matrix for oxidation by transferring the acyl

group on CoA to carnitine. The acyl-carnitine is transported into the mitochondria.

1.  ∆G = 0, therefore the free energy of the initial ATP hydrolysis is still preserved.

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2.  Transport into mitochondria is a coordinated exchange: acyl-carnitine goes in, carnitine

comes out.

The acyl group is moved from carnitine to CoA in the matrix space.

This is an another example of a cytosol-mitochondria shuttle. There is no net transfer of CoA or

carnitine between these two cellular compartments. Many other types of shuttle exist, e.g. those

for ATP and NADH.

ββββ-Oxidation

In the mitochondrial matix space, the acyl-CoA is oxidized in 2-carbon units to acetyl-CoA.

The four steps required are illustrated below for a hexyl-CoA, where R1 is the CoA moiety.

1.  Formation of trans α-β double bond by dehyrogenation by acyl-CoA dehydrogenase, an

FAD enzyme.

The FADH2 product is used in oxidative phosphorylation. This results in the regeneration of 

two ATP molecules, and recovers the energy spent to form the first acyl-CoA intermediate.

2. Addition of water to the newly formed double bond to generate the alcohol by enoyl-CoA

hydratase

NH3

+

OH

O

O

NH3

+ O

O

O

R1

O

Carnitine Acyl-carnitine

S

O

R1

H

H H

H

S

O

R1

H

H

S

O

R1

H

HS

O

R1

OH

H

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3. Oxidation of the alcohol by NAD+ to give the ketone; catalyzed by ββββ-hydroxyacyl-CoA

dehydrogenase. The reduced NADH is reoxidized by molecular oxygen in oxidative

phosphorylation to yield 3 (or 2.5) ATP molecules.

4. Cleavage reaction by ββββ-ketoacyl-CoA thiolase (Thiolysis)

Summary of the thiolase steps:

A.  Nucleophilic attack of a Cys residue (Cys-S-) onto the ketone carbon (β-carbon) to form

an acyl-enzyme intermediate.

B.  Breakage of the α-β bond to produce the anionic acetyl-CoA group (carbanion).

C.  Anionic acetyl-CoA group abstracts a proton from a general base on the enzyme to formacetyl-CoA.

D.  Nucleophilic attack of CoASH onto the keto group of the enzyme-acyl intermediate.

E.  Breakage of the acyl-enzyme bond to release acyl (n-2) CoA.

Similarities of thiolase to serine proteases:

 Functionality Serine Protease Thiolase

Nucleophile Ser Cys

Acyl-enzyme formed yes yes

Proton provided by: His (base) unknown base

Acyl-enzyme hydrolyzed by: Water CoA

S

O

R1

OH

H

S

O

R1

O

H

S

O

R1

O

H

S

O

R1

O

SR2

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Energy Yield from Fatty Acid Oxidation

See Campbell, 18.3, for the detailed balance sheet (Table18.1)

ATP is derived from FADH2, NADH, and acetyl-CoA.

ATP's produced per six-carbon unit.

Glucose: 32 C6H12O6 FW = 180 g/mol

Fatty acids: 40 C6H12O2 FW = 116 g/molAcyl chains of fatty acids are hydrocarbon whereas carbohydrates are partially oxidized.

Additional Topics

•  Odd-numbered fatty acids and the catabolic products of some amino acids result in

propionyl-CoA after β-oxidation.

•  Ketone bodies (acetoacetate and acetone) result from the condensation of two acetyl-CoA

molecules.

Synthesis occurs in the liver. Oxidation occurs normally in other organs.

•  Mitochondrial metabolite transport systems

ADP

ATP

malate

malate

aspartate

acylcarnitine

carnitine

H+

Pi

pyruvate

H+

pyruvate

H+

ketoglutarateα-

citrate

glutamate

Cytosol Matrix

10.25.04