Voet D., Voet Ju.G. Biochemistry

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Section 25-2. Fatty Acid Oxidation 951

SCoA

2NAD

+ 2FAD + 2CoASH

2 rounds of β oxidation

enoyl-CoA isomerase

3,5–2,4-dienoyl-CoA

isomerase

Problem 1:

␤,␥ double bond

Problem 3:

Isomerization

SCoA

one round of ␤ oxidation

the first oxidation of

the next round

NAD

+ FAD + CoASH

NADH

+ FADH

+ Acetyl-CoA

3,2-enoyl-CoA

isomerase (mammalian)

Problem 2:

⌬

double bond

SCoA

Continuation of ␤ oxidation

2,4-dienoyl-CoA

reductase

(mammalian)

NADPH

+ H

NADP

2,4-dienoyl-

CoA reductase

(E. coli)

SCoA

2,4-dienoyl-CoA

reductase

NADPH

+ H

NADP

3,2-enoyl-CoA

isomerase

2NADH + 2FADH

+ 2acetyl-CoA

FAD

FADH

acyl-CoA dehydrogenase

Linoleic acid

2,5,8-Trienoyl-CoA

SCoA

12 9

SCoA

853

NAD

+ CoASH

2NADH

+ 2FADH

+ 2Acetyl-CoA

2NAD

+ 2FAD

+ 2CoA

NADH

+ acetyl-CoA

completion of

β-oxidation round

2 rounds

of ␤-oxidation

3,5,8-Trienoyl-CoA

⌬

, ⌬

-Trienoyl-CoA

SCoA

853

SCoA

3,2-enoyl-CoA

isomerase

JWCL281_c25_940-1018.qxd 6/18/10 7:27 AM Page 951

which is driven by the concomitant hydrolysis of ATP to

ADP and P

, activates the resulting carboxyl group for

transfer without further free energy input.

2. Stereospecific transfer of the activated carboxyl

group from carboxybiotin to propionyl-CoA to form (S)-

methylmalonyl-CoA. This step occurs via nucleophilic

attack on carboxybiotin by a carbanion at C2 of propionyl-

CoA (see below).

These two reaction steps occur at different catalytic sites

on propionyl-CoA carboxylase. It therefore appears that

the biotinyllysine linkage attaching the biotin ring to the

enzyme forms a flexible tether that permits the efficient

transfer of the biotin ring between these two active sites as

occurs in pyruvate carboxylase (Section 23-1Ae).

Formation of the C2 carbanion in the second stage of

the propionyl-CoA carboxylase reaction involves removal

of a proton  to a thioester. This proton is relatively acidic

since, as we have seen in Section 25-2Cd, the negative

charge on a carbanion  to a thioester can be delocalized

into the thioester’s carbonyl group. This explains the rela-

tively convoluted path taken in the conversion of propionyl-

CoA to succinyl-CoA (Fig. 25-18). It would seem simpler, at

least on paper, for this process to occur in one step, with car-

boxylation occurring on C3 of propionyl-CoA so as to form

succinyl-CoA directly. Yet, the C3 carbanion required for

952 Chapter 25. Lipid Metabolism

2,4-dienoyl-CoA reductase reduces the 

double bond.The

E. coli reductase produces trans-2-enoyl-CoA, a normal

substrate of  oxidation. The mammalian reductase, how-

ever, yields trans-3-enoyl-CoA, which, to proceed along

the -oxidation pathway, must first be isomerized to trans-

2-enoyl-CoA by 3,2-enoyl-CoA isomerase.

Problem 3: The Unanticipated Isomerization

of 2,5-Enoyl-CoA by 3,2-Enoyl-CoA Isomerase

Mammalian 3,2-enoyl-CoA isomerase catalyzes a re-

versible reaction that interconverts 

and 

double

bonds. A carbonyl group is stabilized by being conju-

gated to a 

double bond. However, the presence of a 

double bond (originating from an unsaturated fatty acid

with a double bond at an odd-numbered C atom such as

the 

double bond of linoleic acid) is likewise stabilized

by being conjugated with a 

double bond (right-hand

pathway of Fig. 25-17). If a 2,5-enoyl-CoA is converted by

3,2-enoyl-CoA isomerase to 3,5-enoyl-CoA, which occurs

up to 20% of the time, another enzyme is necessary to

continue the oxidation: 3,5–2,4-Dienoyl-CoA isomerase

isomerizes the 3,5-diene to a 2,4-diene, which is then re-

duced by 2,4-dienoyl-CoA reductase and isomerized by

3,2-enoyl-CoA isomerase as in Problem 2 above.After two

more rounds of  oxidation, the cis-

double bond origi-

nating from the cis-

double bond of linoleic acid is also

dealt with as in Problem 2.

E. Oxidation of Odd-Chain Fatty Acids

Most fatty acids have even numbers of carbon atoms and

are therefore completely converted to acetyl-CoA. Some

plants and marine organisms, however, synthesize fatty

acids with an odd number of carbon atoms. The final

round of ␤ oxidation of these fatty acids forms propionyl-

CoA, which, as we shall see, is converted to succinyl-CoA

for entry into the citric acid cycle. Propionate or propionyl-

CoA is also produced by oxidation of the amino acids

isoleucine, valine, and methionine (Section 26-3E). Fur-

thermore, ruminant animals such as cattle derive most of

their caloric intake from the acetate and propionate pro-

duced in their rumen (stomach) by bacterial fermentation

of carbohydrates. These products are absorbed by the ani-

mal and metabolized after conversion to the correspon-

ding acyl-CoA.

a. Propionyl-CoA Carboxylase Has

a Biotin Prosthetic Group

The conversion of propionyl-CoA to succinyl-CoA in-

volves three enzymes (Fig. 25-18). The first reaction is that

of propionyl-CoA carboxylase, a biotin-dependent enzyme

(Section 23-1Ab) with subunit composition 



.The reac-

tion, which resembles that catalyzed by the homologous

biotin-containing enzyme pyruvate decarboxylase (Section

23-1Ac), occurs in two steps (Fig. 25-19):

1. Carboxylation of biotin at N1¿ by bicarbonate ion as

in the pyruvate carboxylase reaction (Fig. 23-4). This step,

Figure 25-18 Conversion of propionyl-CoA to succinyl-CoA.

SCoA

propionyl-CoA carboxylase

methylmalonyl-CoA racemase

ATP

+ CO

ADP + P

SCoA

Propionyl-CoA

(S)-Methylmalonyl-CoA

–

methylmalonyl-CoA mutase

SCoA

–

(R)-Methylmalonyl-CoA

Succinyl-CoA

SCoA

JWCL281_c25_940-1018.qxd 4/20/10 1:58 PM Page 952

–

Resonance-stabilized carbanion

intermediate

(R)-Methylmalonyl-CoA

C SCoA

–

BEnz

C SCoA

–

BEnz

C CoA

–

BEnz

(S)-Methylmalonyl-CoA

C CoA

–

BEnz

such a carboxylation has a high free energy of formation.

Nature has instead chosen a more facile, albeit less direct

route, which carboxylates propionyl-CoA at a more reactive

position and then rearranges the C

skeleton to form the de-

sired product.

b. Methylmalonyl-CoA Mutase Contains

a Coenzyme B

Prosthetic Group

Methylmalonyl-CoA mutase, which catalyzes the third

reaction of the propionyl-CoA to succinyl-CoA conversion

(Fig. 25-18), is specific for (R)-methylmalonyl-CoA even

though propionyl-CoA carboxylase stereospecifically syn-

thesizes (S)-methylmalonyl-CoA. This diversion is recti-

fied by methylmalonyl-CoA racemase, which interconverts

the (R) and (S) configurations of methylmalonyl-CoA,

presumably by promoting the reversible dissociation of its

acidic -H via formation of a resonance-stabilized carban-

ion intermediate:

Section 25-2. Fatty Acid Oxidation 953

Figure 25-19 The propionyl-CoA carboxylase reaction. (1)

The carboxylation of biotin with the concomitant hydrolysis of

ATP is followed by (2) the carboxylation of a propionyl-CoA

denosine O

–

O HO C

–

ATP

denosine O

–

––

O OH++

ADP

Biotinyl–enzymeHCO

–

Carboxybiotinyl–enzyme

CoAS

–

CoAS

C C

(S)-Methylmalonyl-CoA

Biotinyl–enzyme

carbanion by its attack on carboxybiotin. Each reaction step

probably involves the intermediate formation of CO

as occurs

in the pyruvate carboxylase reaction (Fig. 23-4).

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 953

954 Chapter 25. Lipid Metabolism

Methylmalonyl-CoA mutase, which catalyzes an un-

usual carbon skeleton rearrangement (Fig. 25-20), utilizes a

5-deoxyadenosylcobalamin (AdoCbl) prosthetic group

(also called coenzyme B

). Dorothy Hodgkin determined

the structure of this complex molecule (Fig. 25-21) in 1956,

a landmark achievement, through X-ray crystallographic

analysis combined with chemical degradation studies.

AdoCbl contains a hemelike corrin ring whose four pyr-

role N atoms each ligand a 6-coordinate Co ion. The fifth

Co ligand in the free coenzyme is an N atom of a 5,6-

dimethylbenzimidazole (DMB) nucleotide that is covalently

linked to the corrin D ring. The sixth ligand is a 5¿-

deoxyadenosyl group in which the deoxyribose C5¿ atom

forms a covalent C¬Co bond, one of only two known

carbon–metal bonds in biology (the other being a C¬Ni bond

in the bacterial enzyme carbon monoxide dehydrogenase).

In some enzymes, the sixth ligand instead is a CH

group

that likewise forms a C¬Co bond.

Figure 25-20 The rearrangement catalyzed by methylmalonyl-

CoA mutase.

H H

CCC

–

CoAS

methylmalonyl-

CoA mutase

CCC

–

SCoA

Carbon

skeleton

Succinyl-CoA(R)-Methylmalonyl-CoA

OH OH

Co(III)

H H

5

2

1

4

3

–

5-Deoxyadenosylcobalamin (coenzyme B

)

HOH

Figure 25-21 Structure of 5-deoxyadenosylcobalamin

(coenzyme B

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 954

void of odd-chain fatty acids and low in the amino acid

residues that are degraded to propionyl-CoA (Ile, Val, and

Met; Section 26-3E).

c. The Methylmalonyl-CoA Mutase Reaction

Occurs via a Free Radical Mechanism

Methylmalonyl-CoA mutase from Propionibacterium

shermanii is an ␣␤ heterodimer whose catalytically active

728-residue ␣ subunit is 24% identical to its catalytically in-

active 638-residue ␤ subunit. In contrast, the human en-

zyme is a homodimer whose subunits are 60% identical in

sequence to P.shermanii’s ␣ subunit. Hence P. shermanii’s ␤

subunit is thought to be an evolutionary fossil.

The X-ray structure of methylmalonyl-CoA mutase

from P. shermanii in complex with the substrate analog 2-

carboxypropyl-CoA (which lacks methylmalonyl-CoA’s

thioester oxygen atom) was determined by Philip Evans.

Its AdoCbl cofactor is sandwiched between the ␣ subunit’s

two domains: a 559-residue N-terminal ␣/␤ barrel (TIM

barrel, the most common enzymatic motif; Section 8-3B)

and a 169-residue C-terminal ␣/␤ domain that resembles a

Rossmann fold (Section 8-3Bh). The structure of the ␣/␤

barrel contains several surprising features (Fig. 25-22):

1. The active sites of nearly all ␣/␤ barrel enzymes are

located at the C-terminal ends of the barrel’s ␤ strands.

Section 25-2. Fatty Acid Oxidation 955

AdoCbl’s reactive C¬Co bond participates in two types

of enzyme-catalyzed reactions:

1. Rearrangements in which a hydrogen atom is di-

rectly transferred between two adjacent carbon atoms with

concomitant exchange of the second substituent, X:

where X may be a carbon atom with substituents, an oxy-

gen atom of an alcohol, or an amine.

2. Methyl group transfers between two molecules.

There are about a dozen known cobalamin-dependent

enzymes. However, only two occur in mammalian systems:

(1) methylmalonyl-CoA mutase, which catalyzes a carbon

skeleton rearrangement (the X group in the rearrangement

is ¬COSCoA; Fig. 25-20) and is the only B

-containing

enzyme that occurs in both eukaryotes and prokaryotes;

and (2) methionine synthase, a methyl transfer enzyme that

participates in methionine biosynthesis (Sections 26-3Ec

and 26-5B). Defects in methylmalonyl-CoA mutase result

in methylmalonic aciduria, a condition that is often fatal in

infancy due to acidosis (low blood pH) without a diet de-

Figure 25-22 X-ray structure of P. shermanii methylmalonyl-

CoA mutase in complex with 2-carboxypropyl-CoA and AdoCbl.

(a) The catalytically active ␣ subunit in which the N-terminal

domain is cyan with the ␤ strands of its ␣/␤ barrel orange, and

the C-terminal domain is pink.The 2-carboxypropyl-CoA

(magenta) and AdoCbl (green) are drawn in space-filling form.

The 2-carboxypropyl-CoA passes through the center of the ␣/␤

barrel and is oriented such that the methylmalonyl group of

methylmalonyl-CoA would contact the corrin ring of the

AdoCbl, which is sandwiched between the enzyme’s N- and

C-terminal domains. (b) The arrangement of the AdoCbl and

2-carboxypropyl-CoA molecules which, together with the side

chain of His 610, are represented in stick form colored according

to atom type (AdoCbl and His C green, 2-carboxypropyl-CoA

C cyan, N blue, O red, P magenta, and S yellow). The corrin ring’s

Co atom is represented by a lavender sphere and the ␣/␤ barrel’s

␤ strands are represented by orange ribbons.The view is similar

to that in Part a. Note that the DMB group (bottom) has swung

away from the corrin ring (seen edgewise) to be replaced by the

side chain of His 610 from the C-terminal domain and that the

5¿-deoxyadenosyl group is unseen (due to disorder). [Based on

an X-ray structure by Philip Evans, MRC Laboratory of

Molecular Biology, Cambridge, U.K. PDBid 7REQ.]

See Interactive Exercise 24

(a)

(b)

JWCL281_c25_940-1018.qxd 10/19/10 9:42 AM Page 955

956 Chapter 25. Lipid Metabolism

However, in methylmalonyl-CoA mutase, the AdoCbl is

packed against the N-terminal ends of the barrel’s  strands.

2. In free AdoCbl, the Co atom is axially liganded by

an N atom of its DMB group and by the adenosyl residue’s

5¿-CH

group (Fig. 25-21). However, in the enzyme, the

DMB has swung aside to bind in a separate pocket and

has been replaced by the side chain of His 610 from the C-

terminal domain. The adenosyl group is not visible in the

structure due to disorder and hence has probably also

swung aside.

3. In nearly all other / barrel–containing enzymes,

the center of the barrel is occluded by large, often branched,

hydrophobic side chains. However, in methylmalonyl-CoA

mutase, the 2-carboxypropyl-CoA’s pantetheine group

binds in a narrow tunnel through the center of the / bar-

rel so as to put the methylmalonyl group of an intact sub-

strate in close proximity to the unliganded face of the

cobalamin ring.This tunnel provides the only direct access

to the active site cavity, thereby protecting the reactive free

radical intermediates that are produced in the catalytic re-

action from side reactions (see below). The tunnel is lined

by small hydrophilic residues (Ser and Thr).

Methylmalonyl-CoA mutase’s substrate binding mode re-

sembles those of several other AdoCbl-containing en-

zymes of known structure, which are collectively unique

among / barrel–containing proteins.

The proposed methylmalonyl-CoA mutase reaction

mechanism (Fig. 25-23) begins with homolytic cleavage of

the cobalamin C¬Co(III) bond (the C and Co atoms each

acquire one of the electrons that formed the cleaved elec-

tron pair bond). The Co ion therefore fluctuates between

its Co(III) and Co(II) oxidation states [the two states are

spectroscopically distinguishable: Co(III) is red and dia-

magnetic (no unpaired electrons), whereas Co(II) is yellow

and paramagnetic (unpaired electrons)]. Note that a ho-

molytic cleavage reaction is unusual in biology; most other

biological bond cleavage reactions occur via heterolytic

cleavage (in which the electron pair forming the cleaved

bond is fully acquired by one of the separating atoms).

The role of AdoCbl in the catalytic process is that of a re-

versible free radical generator. The C¬Co(III) bond is well

suited to this function because it is inherently weak (disso-

ciation energy  109 kJ  mol

1

) and appears to be further

weakened through steric interactions with the enzyme. In-

deed, as Fig. 25-22 indicates, the Co atom in methylmalonyl-

CoA mutase has no sixth ligand and hence, as confirmed by

spectroscopic measurements, is in its Co(II) state. The His

N¬Co bond is extremely long (2.5 Å vs 1.9–2.0 Å in vari-

ous other B

-containing structures). It is proposed that

this strained and hence weakened bond stabilizes the

Co(II) state with respect to the Co(III) state, thus favoring

the formation of the adenosyl radical and facilitating the

homolytic cleavage through which the catalyzed reaction

occurs (Fig. 23-23). The adenosyl radical presumably ab-

stracts a hydrogen atom from the substrate, thereby facili-

tating the rearrangement reaction through the intermedi-

ate formation of a cyclopropyloxy radical.

d. Succinyl-CoA Cannot Be Directly Consumed

by the Citric Acid Cycle

Methylmalonyl-CoA mutase catalyzes the conversion

of a metabolite to a C

citric acid cycle intermediate, not

acetyl-CoA. The route of succinyl-CoA oxidation is there-

fore not as simple as it may first appear. The citric acid

cycle regenerates all of its C

intermediates so that these

compounds are really catalysts, not substrates. Conse-

quently, succinyl-CoA cannot undergo net degradation by

citric acid cycle enzymes alone. Rather, in order for a

metabolite to undergo net oxidation by the citric acid cycle,

it must first be converted either to pyruvate or directly to

acetyl-CoA. Net degradation of succinyl-CoA begins with

its conversion, via the citric acid cycle, to malate. At high

concentrations, malate is transported, by a specific trans-

port protein, to the cytosol, where it may be oxidatively de-

carboxylated to pyruvate and CO

by malic enzyme

(malate dehydrogenase, decarboxylating):

(We previously encountered this enzyme in the C

cycle of

photosynthesis; Fig. 24-41.) Pyruvate is then completely ox-

idized via pyruvate dehydrogenase and the citric acid cycle.

e. Pernicious Anemia Results from

Vitamin B

Deficiency

The existence of vitamin B

came to light in 1926 when

George Minot and William Murphy discovered that perni-

cious anemia, a rare but often fatal disease of the elderly

characterized by decreased numbers of red blood cells,

low hemoglobin levels (for reasons explained in Section

26-4D), and progressive neurological deterioration (caused

by the accumulation of odd-chain fatty acid residues in

neuronal membranes), can be treated by the daily con-

sumption of large amounts of raw liver (a treatment that

some patients considered worse than the disease). It was

not until 1948, however, after a bacterial assay for antiper-

nicious anemia factor had been developed, that vitamin B

was isolated.

Vitamin B

is synthesized by neither plants nor animals

but only by a few species of bacteria. Herbivores obtain

their vitamin B

from the bacteria that inhabit their gut (in

fact, some animals, such as rabbits, must periodically eat

some of their feces to obtain sufficient amounts of this es-

sential substance). Humans, however, obtain almost all

their vitamin B

directly from their diet, particularly from

meat. The vitamin is specifically bound in the intestine by

the glycoprotein intrinsic factor that is secreted by the

stomach.This complex is absorbed by a specific receptor in

the intestinal mucosa, where the complex is dissociated and

the liberated vitamin B

transported to the bloodstream.

–

PyruvateMalate

NADPHNADP

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 956

cells that produce it. The normal human requirement for

cobalamin is very small, ⬃3 ␮g ⴢ day

⫺1

, and the liver stores

a 3- to 5-year supply of this vitamin. This accounts for the

insidious onset of pernicious anemia and the fact that true

dietary deficiency of vitamin B

, even among strict vege-

tarians, is extremely rare.

Section 25-2. Fatty Acid Oxidation 957

There it is bound by at least three different plasma globu-

lins, called transcobalamins, which facilitate its uptake by

the tissues.

Pernicious anemia is not usually a dietary deficiency dis-

ease but, rather, results from insufficient secretion of in-

trinsic factor, often due to an autoimmune attack against the

Figure 25-23 Proposed mechanism of methylmalonyl-CoA

mutase. (1) The homolytic cleavage of the C¬Co(III) bond

yielding a 5¿-deoxyadenosyl radical and cobalamin in its Co(II)

oxidation state. (2) Abstraction of a hydrogen atom from the

methylmalonyl-CoA by the 5¿-deoxyadenosyl radical, thereby

generating a methylmalonyl-CoA radical. (3) Carbon skeleton

HHC

–

SCoA

HHC

–

SCoA

Ado

+ HCC

–

(R)-Methylmalonyl-CoA

Co(II)

DMB DMBHis 610

His 610

•

Ado

HHC

–

SCoA

DMB

His 610

Co(II)

•

Ado

DMBHis 610

Co(II)

His 610

Co(II)

DMB

HHC

–

SCoA

•

Ado

–

SCoA

Ado

Cyclopropyloxy

radical

Hypothetical intermediates

Succinyl-CoA

rearrangement

Co(III)

DMB

Ado

His 610

rearrangement to form a succinyl-CoA radical via a proposed

cyclopropyloxy radical intermediate. (4) Abstraction of a

hydrogen atom from 5¿-deoxyadenosine by the succinyl-CoA

radical to form succinyl-CoA and regenerate the 5¿-deoxyadenosyl

radical. (5) Release of succinyl-CoA and reformation of the

coenzyme.

JWCL281_c25_940-1018.qxd 8/9/10 9:43 AM Page 957

958 Chapter 25. Lipid Metabolism

through the action of peroxisomal catalase (Section

1-2Ad).

2. Peroxisomal enoyl-CoA hydratase and 3-

L-hydroxy-

acyl-CoA dehydrogenase are activities that occur on a sin-

gle polypeptide and therefore join the growing list of mul-

tifunctional enzymes. The reactions catalyzed are identical

to those of the mitochondrial system (Fig. 25-12).

3. Peroxisomal thiolase has a different chain-length

specificity than its mitochondrial counterpart. It is almost

inactive with acyl-CoAs of length C

or less so that fatty

acids are incompletely oxidized by peroxisomes.

Although peroxisomal  oxidation is not dependent on

the transport of acyl groups into the peroxisome as their

carnitine esters, the peroxisome contains carnitine acyl-

transferases. Acyl-CoAs that have been chain-shortened

by peroxisomal  oxidation are thereby converted to their

carnitine esters. These substances, for the most part, pas-

sively diffuse out of the peroxisome to the mitochondrion,

where they are oxidized further.

G. Minor Pathways of Fatty Acid Oxidation

 Oxidation is blocked by an alkyl group at the C



of a fatty

acid, and thus at any odd-numbered carbon atom. One

such branched-chain fatty acid, a common dietary compo-

nent, is phytanic acid. This metabolic breakdown product

of chlorophyll’s phytyl side chain (Section 24-2A) is pres-

ent in dairy products, ruminant fats, and fish although, sur-

prisingly, chlorophyll itself is but a poor dietary source of

phytanic acid for humans.The oxidation of branched-chain

fatty acids such as phytanic acid is facilitated by  oxidation

(Fig. 25-25). In this process, the fatty acid is converted to

its CoA thioester and its C



is hydroxylated by the Fe

2

containing phytanoyl-CoA hydroxylase. The resulting

F. Peroxisomal ␤ Oxidation

In mammalian cells, the bulk of  oxidation occurs in the

mitochondria, but peroxisomes (Fig. 25-24) also oxidize

fatty acids, particularly those with very long chains or

branched chains. Peroxisomal ␤ oxidation in animals func-

tions to shorten very long chain fatty acids (22 C atoms) so

as to facilitate their degradation by the mitochondrial ␤-

oxidation system. In yeast and plants, fatty acid oxidation

occurs exclusively in the peroxisomes and glyoxysomes

(specialized peroxisomes, Sections 23-2 and 1-2Ad).

The peroxisomal pathway results in the same chemical

changes to fatty acids as does the mitochondrial pathway,

although the enzymes in these two organelles are different.

The protein that transports very long-chain fatty acids into

the peroxisome, ALD protein (see below), does not have a

carnitine requirement. The very long-chain fatty acids that

enter this compartment are activated by a peroxisomal

very long-chain acyl-CoA synthetase to form their CoA es-

ters, and are oxidized directly. The shorter chain acyl prod-

ucts of this -oxidation process are then linked to carnitine

for transport into mitochondria for further oxidation.

a. Adrenoleukodystrophy Is Caused by

a Defect in ALD Protein

Adrenoleukodystrophy (ALD) is a rare X-linked inher-

ited disease that results in progressive brain damage and

adrenal gland failure. It causes very long-chain saturated

fatty acids to accumulate in the blood and destroy the insu-

lating myelin sheath surrounding the axons of many neu-

rons (Section 20-5Bc). Its varied neurological symptoms

present (become evident) between the ages of 4 and 10

years and are usually fatal within 1 to 10 years (except after

a successful bone marrow transplant). ALD is caused by

a defective ALD protein, an ABC transporter (Section

20-3E). Thus in ALD patients, lignoceric acid (24:0; recall

that the symbol n:m indicates a C

fatty acid with m double

bonds) is converted to lignoceroyl-CoA at only 13% of the

normal rate, although once formed, it undergoes  oxida-

tion at the normal rate.

b. Peroxisomal  Oxidation Differs in Detail

from Mitochondrial  Oxidation

The -oxidation pathway in peroxisomes differs from

that in mitochondria as follows:

1. The first enzyme in the peroxisomal pathway, acyl-

CoA oxidase, catalyzes the reaction

This reaction involves participation of an FAD cofactor

but differs from its mitochondrial counterpart in that the

abstracted electrons are transferred directly to O

rather

than passing through the electron-transport chain with its

concomitant oxidative phosphorylation (Fig. 25-12). Perox-

isomal fatty acid oxidation is therefore less efficient than

the mitochondrial process by two ATPs for each C

cycle.

The H

produced is disproportionated to H

O and O

Fatty acyl-CoA  O

trans-¢

-enoyl-CoA  H

Figure 25-24 Peroxisomes. These membrane-bounded

organelles perform a variety of metabolic functions, including

Visuals Unlimited.]

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 958

CoA thioester is, in effect, oxidatively decarboxylated to

yield a new fatty acid with an unsubstituted C



. Further

degradation of the molecule can then continue via six

cycles of normal  oxidation to yield three propionyl-

CoAs, three acetyl-CoAs, and one 2-methylpropionyl-CoA

(which is converted to succinyl-CoA).

A rare genetic defect, Refsum’s disease or phytanic acid

storage syndrome, results from the accumulation of this

metabolite throughout the body.The disease,which is char-

acterized by progressive neurological difficulties such as

tremors, unsteady gait, and poor night vision, results from a

defective phytanoyl-CoA hydroxylase. Its symptoms can

therefore be attenuated by a diet that restricts the intake of

phytanic acid–containing foods.

Medium- and long-chain fatty acids are converted to di-

carboxylic acids through  oxidation (oxidation of the last

carbon atom). This process, which is catalyzed by enzymes

of the ER, involves hydroxylation of a fatty acid’s C



atom

by a cytochrome P450, a monooxygenase that utilizes

NADPH and O

(Section 15-4Bc).This CH

¬OH group is

then oxidized to a carboxyl group, converted to a CoA de-

rivative at either end, and oxidized via the -oxidation

pathway.  Oxidation is probably of only minor signifi-

cance in fatty acid oxidation.

3 KETONE BODIES

Acetyl-CoA produced by oxidation of fatty acids in liver

mitochondria can be further oxidized via the citric acid

cycle as is discussed in Chapter 21.A significant fraction of

this acetyl-CoA has another fate, however. By a process

known as ketogenesis, which occurs primarily in liver mito-

chondria, acetyl-CoA is converted to acetoacetate or

D-␤-

hydroxybutyrate. These compounds, which together with

acetone are somewhat inaccurately referred to as ketone

bodies,

serve as important metabolic fuels for many peripheral tis-

sues, particularly heart and skeletal muscle.The brain, under

normal circumstances, uses only glucose as its energy

source (fatty acids are unable to pass the blood–brain bar-

rier), but during starvation, ketone bodies become the

brain’s major fuel source (Section 27-4A). Ketone bodies

are water-soluble equivalents of fatty acids.

CCH

–

C C

Acetone

D--Hydroxybutyrate

Acetoacetate

–

Section 25-3. Ketone Bodies 959

Figure 25-25 Pathway of  oxidation of fatty acids. Phytanic

acid, a degradation product of the phytyl side chain of

chlorophyll, is metabolized through  oxidation to pristanic

acid followed by  oxidation.

CH CCH

SCoACH

2 3

CH C SCoA

)

CCH

SCoACH

(

Phytanic acid

6 cycles of  oxidation

2-hydroxyphytanoyl-CoA

lyase

phytanoly-CoA

hydroxylase

ATP

+ CoASH

AMP

NAD(P)

Phytanoyl-CoA

CCH

SCoACH

OH O

HC SCoA

(

2-Hydroxyphytanoyl-CoA

Formyl-CoA

(

Pristanal

2

-Ketoglutarate + O

succinate + CO

acyl-CoA synthase

ATP

+ CoA

AMP + PP

aldehyde dehydrogenase

NAD(P)H

CH CH

()

–

CH CH

()

–

)

Pristanic acid

(

SCoA

Acetyl-CoA2-Methyl-

propionyl-CoA

3CH

SCo

Propionyl-CoA

JWCL281_c25_940-1018.qxd 6/8/10 8:59 AM Page 959

960 Chapter 25. Lipid Metabolism

The overall reaction catalyzed by HMG-CoA synthase

and HMG-CoA lyase is

One may well ask why this apparently simple hydrolysis re-

action occurs in such an indirect manner.The answer is un-

clear but may lie in the regulation of the process.

Acetoacetate may be reduced to

D--hydroxybutyrate

by -hydroxybutyrate dehydrogenase:

Note that this product is the stereoisomer of the

L--

hydroxyacyl-CoA that occurs in the -oxidation pathway.

Acetoacetate, being a -keto acid, also undergoes rela-

tively facile nonenzymatic decarboxylation to acetone and

NADH NAD

C O

–

C HHO

D--Hydroxybutyrate

-hydroxybutyrate

dehydrogenase

Acetoacetate

Acetoacetyl-CoA  H

acetoacetate  CoA

Acetoacetate formation occurs in three reactions (Fig.

25-26):

1. Two molecules of acetyl-CoA are condensed to

acetoacetyl-CoA by thiolase (also called acetyl-CoA

acetyltransferase) working in the reverse direction from the

way it does in the final step of  oxidation (Section 25-2Cd).

2. Condensation of the acetoacetyl-CoA with a third

acetyl-CoA by HMG-CoA synthase forms -hydroxy--

methylglutaryl-CoA (HMG-CoA). The mechanism of this

reaction resembles the reverse of the thiolase reaction (Fig.

25-15) in that an active site thiol group forms an

acyl–thioester intermediate.

3. Degradation of HMG-CoA to acetoacetate and

acetyl-CoA in a mixed aldol–Claisen ester cleavage is cat-

alyzed by HMG-CoA lyase. The mechanism of this reac-

tion is analogous to the reverse of the citrate synthase reac-

tion (Section 21-3A). (HMG-CoA is also a precursor in

cholesterol biosynthesis and hence may be diverted to this

purpose as is discussed in Section 25-6A.)

Figure 25-26 Ketogenesis: the enzymatic reactions forming

acetoacetate from acetyl-CoA. (1) Two molecules of acetyl-CoA

condense to form acetoacetyl-CoA in a thiolase-catalyzed

reaction. (2) A Claisen ester condensation of the acetoacetyl-CoA

with a third acetyl-CoA to form -hydroxy--methylglutaryl-CoA

(HMG-CoA) as catalyzed by HMG-CoA synthase. (3) The

degradation of HMG-CoA to acetoacetate and acetyl-CoA in a

mixed aldol–Claisen ester cleavage catalyzed by HMG-CoA

lyase.

Figure 25-27 The metabolic conversion of ketone bodies to

acetyl-CoA.

C SCoA

SCoA

Acetyl-CoA Acetyl-CoA

thiolase

(acetyl-CoA acetyltransferase)

SCoA

Acetoacetyl-CoA

O CH

SCoA

C CCH

–

CCH

–

SCoA

CCH

SCoA

hydroxymethylglutaryl-CoA synthase

(HMG-CoA synthase)

-Hydroxy--methylglutaryl-CoA (HMG-CoA)

hydroxymethylglutaryl-CoA lyase

(HMG-CoA lyase)

Acetoacetate Acetyl-CoA

–

D-

-Hydroxybutyrate

NAD

NADH

+ H

β-hydroxybutyrate dehydrogenase

–

Acetoacetate

SCo

–

Succinyl-CoA

3-ketoacyl-CoA transferase

–

Succinate

–

SCoA

Acetoacetyl-CoA

SCoAH

thiolase

SCoA

Acetyl-CoA

2 CH

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