Voet D., Voet Ju.G. Biochemistry

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Serine is converted to pyruvate through dehydration by

serine dehydratase. This PLP–enzyme, like the aminotrans-

ferases (Section 26-1), functions by forming a PLP–amino

acid Schiff base, which facilitates the removal of the amino

acid’s ␣-hydrogen atom. In the serine dehydratase reaction,

however, the C

␣

carbanion breaks down with the elimina-

tion of the amino acid’s C

␤

OH, rather than with tautomer-

ization (Fig.26-2,Step 2),so that the substrate undergoes ␣,␤

elimination of H

O rather than deamination (Fig. 26-13).

The product of the dehydration, the enamine aminoacrylate,

tautomerizes nonenzymatically to the corresponding imine,

which spontaneously hydrolyzes to pyruvate and ammonia.

Cysteine may be converted to pyruvate via several

routes in which the sulfhydryl group is released as H

or SCN

⫺

Glycine is converted to serine by the enzyme serine hy-

droxymethyltransferase, another PLP-containing enzyme

(Fig. 26-12, Reaction 4). This enzyme utilizes N

methylene-tetrahydrofolate (N

-methylene-THF) as a

cofactor to provide the C

unit necessary for this conver-

sion. We shall defer a detailed discussion of THF cofactors

until Section 26-4D.

a. The Glycine Cleavage System Is

a Multienzyme Complex

The methylene group of the N

-methylene-THF

utilized in the conversion of glycine to serine is obtained

from the methylene group of a second glycine via a reac-

tion in which this glycine’s remaining atoms are released

as CO

and (Fig. 26-12, Reaction 3). This reaction is

catalyzed by the glycine cleavage system (also called the

glycine decarboxylase multienzyme system in plants and

glycine synthase when acting in the reverse direction; Sec-

tion 26-5Ae), a complex resembling the pyruvate dehy-

drogenase complex (Section 21-2A) that consists of four

proteins (Fig. 26-14):

⫹

2⫺

1. A PLP-dependent glycine decarboxylase (P-protein).

2. A lipoamide-containing aminomethyl carrier

(H-protein), which carries the aminomethyl group remain-

ing after glycine decarboxylation.

Section 26-3. Metabolic Breakdown of Individual Amino Acids 1031

–

C COO

–

PLP

+ Serine

AminoacrylatePyruvate

2–

...

–

C COO

–

2–

...

–

ONH

–

C COO

–

+++

2–

PLP

...

C COO

–

C COO

–

C COO

–

Figure 26-13 The serine dehydratase

reaction. This PLP-dependent enzyme catalyzes

the elimination of water from serine. The steps

in the reaction are (1) formation of a serine–PLP

Schiff base, (2) removal of the ␣-H atom of

serine to form a resonance-stabilized carbanion,

(3) ␤ elimination of OH

⫺

, (4) hydrolysis of the

Schiff base to yield the PLP–enzyme and

aminoacrylate, (5) nonenzymatic tautomerization

to the imine, and (6) nonenzymatic hydrolysis to

form pyruvate and ammonia.

Figure 26-14 The reactions catalyzed by the glycine cleavage

system, a multienzyme complex. The enzymes involved are (1) a

PLP-dependent glycine decarboxylase (P-protein), (2) a

lipoamide-containing protein (H-protein), (3) a THF-requiring

enzyme (T-protein), and (4) an NAD

⫹

-dependent, FAD-requiring

dihydrolipoyl dehydrogenase (L-protein).

COO

–

PLPP

COO

–

Glycine

TTHF •

, N

-Methylene-THF •

NADH + H

NAD

JWCL281_c26_1019-1087.qxd 4/20/10 9:26 AM Page 1031

3. An N

-methylene-THF synthesizing enzyme

[T-protein; alternatively aminomethyltransferase (AMT)],

which accepts a methylene group from the aminomethyl car-

rier (H-protein; the amino group is released as ammonia).

4. An NAD

⫹

-dependent, FAD-requiring dihydrolipoyl

dehydrogenase (L-protein), a protein shared by and

known as E3 in the pyruvate dehydrogenase complex (Sec-

tion 21-2A).

Unlike the pyruvate dehydrogenase complex, the glycine

cleavage system components are only loosely associated

and hence are isolated as individual proteins. Nevertheless,

H-protein has the central role in this multienzyme system: Its

oxidized lipoyllysyl arm (Section 21-2Ad) is reduced as it ac-

cepts an aminomethyl group from P-protein (Fig.26-14, Step

2), it donates the methylene group to THF in complex with

T-protein as ammonia is released (Fig. 26-14, Step 3), and is

then reoxidized by L-protein (Fig. 26-14, Step 4). The X-ray

structure of pea leaf H-protein (Fig. 26-15), determined by

Roland Douce, reveals that it is largely composed of a sand-

wich of 3-stranded and 6-stranded antiparallel ␤ sheets that

structurally resembles the lipoyl domain of E2 in the pyru-

vate dehydrogenase complex (Fig. 21-10a).

The aminomethylthio group is unstable and ordinarily

is rapidly hydrolyzed to formaldehyde and. How-

ever, on its aminomethylation, the previously exposed

lipoyl group (Fig. 26-15a) inserts into a hydrophobic cleft

⫹

in the H-protein where its amino group hydrogen-bonds

to Glu 14 (Fig. 26-15b), thereby shielding the aminomethyl

group from hydrolysis. Indeed, the replacement of Glu 14

by Ala results in the rapid hydrolysis of the aminomethyl

group. It therefore appears that the T-protein in the

THF ⴢ T ⴢ H complex functions to release the lipoyl group

from the H-protein cleft and to orient the THF for ap-

proach to the methylene C of the aminomethyl group for

reaction.

Two observations indicate that the above pathway is the

major route of glycine degradation in mammalian tissues:

1. The serine isolated from an animal that has been fed

[2-

C]glycine is

C labeled at both C2 and C3.This obser-

vation indicates that the methylene group of the N

methylene-THF utilized by serine hydroxymethyltrans-

ferase is obtained from glycine C2.

2. The inherited human disease nonketotic hyper-

glycinemia, which is characterized by mental retardation

and accumulation of large amounts of glycine in body flu-

ids, results from the absence of one of the components of

the glycine cleavage system.

The glycine cleavage system and serine hydroxymethyl-

transferase occupy a vital role in green leaves, catalyzing

the rapid destruction of the huge amounts of glycine pro-

duced by photorespiration (Section 24-3Ca). In fact, these

1032 Chapter 26. Amino Acid Metabolism

Figure 26-15 X-ray structure of H-protein from the pea leaf

glycine cleavage system. (a) The oxidized lipoamide-containing

form in which the side chain of Glu 14 together with that of Lys

63 with its covalently linked lipoyl group are represented in

ball-and-stick form colored according to atom type (Glu 14 C

gold, Lys 63 C cyan, lipoyl C green, N blue, O red, and S yellow).

(b) The reduced aminomethyl-dihydrolipoamide form of

H-protein viewed and colored as in Part a. The dot surface

represents the protein’s solvent-accessible surface. Note how the

aminomethyl-dihydrolipoamide has changed conformation

relative to the lipoamide in Part a so as to bind in a hydrophobic

cleft in the protein where its amino group is hydrogen bonded to

Glu 14 (dashed black bond).This protects the aminomethyl

group from hydrolysis. [Based on X-ray structures by Roland

Douce, Centre National de la Recherche Scientifique et

Commisariat à l’Energy Atomique, Grenoble, France. PDBids

(a) 1HPC and (b) 1HTP.]

(a)

(b)

JWCL281_c26_1019-1087.qxd 6/8/10 9:38 AM Page 1032

enzymes comprise about half the proteins present in the

mitochondria from pea and spinach leaves.

Threonine is both glucogenic and ketogenic, since one

of its degradation routes produces both pyruvate and

acetyl-CoA (Fig. 26-12, Reactions 6 and 7). Its major route

of breakdown is through threonine dehydrogenase, pro-

ducing ␣-amino-␤-ketobutyrate, which is converted to

acetyl-CoA and glycine by ␣-amino-␤-ketobutyrate lyase.

The glycine may be converted, through serine, to pyruvate.

b. Serine Hydroxymethyltransferase Catalyzes

PLP-Dependent C

␣

¬C

␤

Bond Cleavage

Threonine may also be converted directly to glycine and

acetaldehyde (the latter being subsequently oxidized to

acetyl-CoA), at least in vitro, via Reaction 5 of Fig. 26-12.

Surprisingly, this reaction is catalyzed by serine hydroxy-

methyltransferase. We have heretofore considered PLP-

catalyzed reactions that begin with the cleavage of an amino

acid’s C

␣

¬H bond (Fig. 26-2). Degradation of threonine to

glycine and acetaldehyde by serine hydroxymethyltrans-

ferase demonstrates that PLP also facilitates cleavage of an

amino acid’s C

␣

¬C

␤

bond by delocalizing the electrons of

the resulting carbanion into the conjugated PLP ring:

c. PLP Facilitates the Cleavage of Different

Bonds in Different Enzymes

How can the same amino acid–PLP Schiff base be in-

volved in the cleavage of the different bonds to an amino

acid C

␣

in different enzymes? The answer to this conun-

drum was suggested by Harmon Dunathan. For electrons

to be withdrawn into the conjugated ring system of PLP,

the ␲-orbital system of PLP must overlap with the bond-

ing orbital containing the electron pair being delocalized.

This is possible only if the bond being broken lies in the

plane perpendicular to the plane of the PLP ␲-orbital sys-

tem (Fig. 26-16a). Different bonds to C

␣

can be placed in

this plane by rotation about the C

␣

¬N bond. Indeed, the

X-ray structure of aspartate aminotransferase reveals

that the C

␣

¬H of its aspartate substrate assumes just this

...

–

2–

COO

–

HB:

Section 26-3. Metabolic Breakdown of Individual Amino Acids 1033

Figure 26-16 Bond orientation in a PLP–amino acid Schiff

base. (a) The ␲-orbital framework of a PLP–amino acid Schiff base.

The bond to C

␣

in the plane perpendicular to the PLP ␲-orbital

system (from X in the illustration) is labile as a consequence of its

overlap with the ␲ system, which permits the broken bond’s

electron pair to be delocalized over the conjugated molecule.

(b) The Schiff base complex of the inhibitor ␣-methylaspartate

with PLP in the X-ray structure of porcine aspartate

aminotransferase as viewed normal to the pyridoxal ring.This

inhibitor is drawn in ball-and-stick form with C green, N blue,

O red, and P gold, with the exception that the methyl C atom and

the bond linking it to the aspartate residue are magenta. Here

the methyl C occupies the position of the H atom that the

enzyme normally excises from aspartate. Note that the bond

linking the methyl C to aspartate is in the plane perpendicular

to the pyridoxal ring and is thus ideally oriented for bond

cleavage. [Part b based on an X-ray structure by David Metzler

and Arthur Arnone, University of Iowa. PDBid 1AJS.]

Delocalized carbanion

Amino acid–PLP Schiff base

–

(b)

(a)

JWCL281_c26_1019-1087.qxd 6/8/10 9:38 AM Page 1033

conformation (Fig. 26-16b). Evidently, each enzyme

specifically cleaves its corresponding bond because the

enzyme binds the amino acid–PLP Schiff base adduct with

this bond in the plane perpendicular to that of the PLP

ring. This is an example of stereoelectronic assistance

(Section 15-1Eb): The enzyme binds substrate in a confor-

mation that minimizes the electronic energy of the transi-

tion state.

C. Asparagine and Aspartate Are Degraded

to Oxaloacetate

Transamination of aspartate leads directly to oxaloacetate:

Asparagine is also converted to oxaloacetate in this man-

ner after its hydrolysis to aspartate by

L-asparaginase:

Interestingly,

L-asparaginase is an effective chemothera-

peutic agent in the treatment of cancers that must obtain

asparagine from the blood, particularly acute lymphoblas-

tic leukemia.The cancerous cells express particularly low lev-

els of the enzyme asparagine synthetase (Section 26-5Ab)

and hence die without an external source of asparagine.

However,

L-asparaginase treatment may select for cells

with increased levels of asparagine synthetase expression,

and hence, in these cases, the surviving cancer cells are re-

sistant to this treatment.

–

Asparagine

Aspartate

L-asparaginase

Aspartate

Oxaloacetate

––

CCH

–

CCH

aminotransferase

␣-Ketoglutarate

Glutamate

D. Arginine, Glutamate, Glutamine, Histidine, and

Proline Are Degraded to ␣-Ketoglutarate

Arginine, glutamine, histidine, and proline are all degraded

by conversion to glutamate (Fig. 26-17), which in turn is ox-

idized to ␣-ketoglutarate by glutamate dehydrogenase

(Section 26-1). Conversion of glutamine to glutamate in-

volves only one reaction: hydrolysis by glutaminase. Histi-

dine’s conversion to glutamate is more complicated: It is

nonoxidatively deaminated, then it is hydrated, and its imi-

dazole ring is cleaved to form N-formiminoglutamate. The

formimino group is then transferred to tetrahydrofolate

forming glutamate and N

-formimino-tetrahydrofolate

(Section 26-4D). Both arginine and proline are converted

to glutamate through the intermediate formation of

glutamate-5-semialdehyde.

E. Isoleucine, Methionine, and Valine

Are Degraded to Succinyl-CoA

Isoleucine, methionine, and valine have complex degrada-

tive pathways that all yield propionyl-CoA. Propionyl-

CoA, which is also a product of odd-chain fatty acid degra-

dation, is converted, as we have seen, to succinyl-CoA by a

series of reactions involving the participation of biotin and

coenzyme B

(Section 25-2E).

a. Methionine Breakdown Involves Synthesis

of S-Adenosylmethionine and Cysteine

Methionine degradation (Fig. 26-18) begins with its re-

action with ATP to form S-adenosylmethionine (SAM; al-

ternatively AdoMet). This sulfonium ion’s highly reactive

methyl group makes it an important biological methylating

agent. For instance, we have already seen that SAM is the

methyl donor in the synthesis of phosphatidylcholine from

phosphatidylethanolamine (Section 25-8Aa). It is also the

methyl donor in the conversion of norepinephrine to epi-

nephrine (Section 26-4B).

Methylation reactions involving SAM yield S-adenosyl-

homocysteine in addition to the methylated acceptor. The

former product is hydrolyzed to adenosine and homo-

cysteine in the next reaction of the methionine degra-

dation pathway. The homocysteine may be methylated

to form methionine via a B

-requiring reaction in which

-methyl-THF is the methyl donor. Alternatively, the ho-

mocysteine may combine with serine to yield cystathionine

in a PLP-requiring reaction, which subsequently forms cys-

teine (cysteine biosynthesis) and ␣-ketobutyrate. The ␣-

ketobutyrate continues along the degradative pathway to

propionyl-CoA and then succinyl-CoA.

b. Hyperhomocysteinemia Is Associated

with Disease

Imbalance between the rate of production of homocys-

teine through methylation reactions utilizing SAM

(Fig. 26-18, Reactions 2 and 3) and its rate of breakdown by

either remethylation to form methionine (Fig. 26-18, Reac-

tion 4) or reaction with serine to form cystathionine in the

1034 Chapter 26. Amino Acid Metabolism

JWCL281_c26_1019-1087.qxd 4/20/10 9:26 AM Page 1034

cysteine biosynthesis pathway (Fig. 26-18, Reaction 5) can

result in an increase in the release of homocysteine to the

extracellular medium and ultimately the plasma and urine.

Moderately elevated concentrations of homocysteine in

the plasma, hyperhomocysteinemia, for reasons that are

poorly understood, are closely associated with cardiovas-

cular disease, cognitive impairment, and neural tube de-

fects [the cause of a variety of severe birth defects includ-

ing spina bifida (defects in the spinal column that often

result in paralysis) and anencephaly (the invariably fatal

failure of the brain to develop, which is the leading cause of

infant death due to congenital anomalies)]. Hyperhomo-

cysteinemia is readily controlled by ingesting the vitamin

precursors of the coenzymes that participate in homocys-

teine breakdown, namely, B

(pyridoxine, the PLP precur-

sor; Fig. 26-1), B

(Fig. 25-21), and folate (Section 26-4D).

Folate, especially, appears to alleviate hyperhomocysteine-

mia; its administration to pregnant women dramatically re-

duces the incidence of neural tube defects in newborns.

This has led to the discovery that 10% of the population is

homozygous for the A222V mutation in N

-methylene-

tetrahydrofolate reductase (MTHFR; Fig. 26-18, Reaction

12; Section 26-4D), the enzyme that generates N

-methyl-

THF for the methionine synthase reaction (Fig. 26-18,

Section 26-3. Metabolic Breakdown of Individual Amino Acids 1035

Figure 26-17 Degradation pathways of arginine, glutamate, glutamine, histidine, and

proline to ␣-ketoglutarate. The enzymes catalyzing the reactions are (1) glutamate

dehydrogenase, (2) glutaminase, (3) arginase, (4) ornithine-␦-aminotransferase,

(5) glutamate-5-semialdehyde dehydrogenase, (6) proline oxidase, (7) spontaneous,

(8) histidine ammonia lyase, (9) urocanate hydratase, (10) imidazolone propionase, and

(11) glutamate formiminotransferase.

N NH

Histidine

COO

–

HC C

N NH

Urocanate

COO

–

N NH

4-Imidazolone-

5-propionate

COO

–

HN NH

-Formiminoglutamate

COO

–

OOC

–

OOC

Arginine

Urea

C NH

–

OOC

Ornithine

–

OOC

Proline

–

OOC

Pyrroline-

5-carboxylate

–

OOC

Glutamate-

5-semialdehyde

-Ketoglutarateα

Glutamate

NAD(P)

NAD(P)H

–

OOC

Glutamate

COO

–

OOC

Glutamine

-Formimino-THF

THF

NADP

NADPH

–

OOC

-Ketoglutarate

COO

–

JWCL281_c26_1019-1087.qxd 4/20/10 9:26 AM Page 1035

Reaction 4). This mutation does not affect this homo-

tetrameric enzyme’s reaction kinetics but instead increases

the rate at which it dissociates into dimers that readily

lose their essential flavin cofactor. Folate derivatives that

bind to the enzyme decrease its rate of dissociation and

flavin loss, thus increasing the mutant enzyme’s overall

activity and decreasing the homocysteine concentration.

The X-ray structure of E. coli MTHFR (which is 30%

identical with the catalytic domain of human MTHFR),

determined by Rowena Matthews and Martha Ludwig,

1036 Chapter 26. Amino Acid Metabolism

Methionine

-methyl-THF

methylene-

THF

ATP

-Ketobutyrate

COO

–

COO

–

HO OH

-Adenosylmethionine (SAM)

C COO

–

Adenine

HO OH

-Adenosylhomocysteine

C COO

–

NAD

+ glycine

NADP

NADPH + H

NADH

+ NH

+ CO

Adenine

Homocysteine

C COO

–

Cystathionine

C COO

–

C COO

–

C COO

–

Propionyl-CoA

SCoA

Succinyl-CoA

SCoA

–

OOC

methyl acceptor

methylated acceptor

Adenosine

NADH

CoASH

NAD

Serine

THF

Cysteine

cysteine

biosynthesis

biosynthetic

methylation

8910

Figure 26-18 The pathway of methionine degradation,

yielding cysteine and succinyl-CoA as products. The enzymes

involved are (1) methionine adenosyltransferase in a reaction

that yields the biological methylating agent S-adenosylmethionine

(SAM), (2) methyltransferase, (3) adenosylhomocysteinase,

(4) methionine synthase (a coenzyme B

–dependent enzyme),

(5) cystathionine ␤-synthase (a PLP-dependent enzyme),

(6) cystathionine ␥-lyase (a PLP-dependent enzyme), (7) ␣-keto

acid dehydrogenase, (8) propionyl-CoA carboxylase,

(9) methylmalonyl-CoA racemase, (10) methylmalonyl-CoA

mutase (a coenzyme B

–dependent enzyme; Reactions 8–10 are

discussed in Section 25-2E), (11) glycine cleavage system (Figs.

26-12 and 26-14) or serine hydroxymethyltransferase (Fig. 26-12),

(12) N

-methylene-tetrahydrofolate reductase (a coenzyme

– and FAD-dependent enzyme; Figs. 26-19 and 26-49).

JWCL281_c26_1019-1087.qxd 6/8/10 9:38 AM Page 1036

reveals that this 296-residue enzyme forms an ␣/␤ barrel.The

FAD cofactor binds at the C-terminal ends of barrel strands

␤3, ␤4, and ␤5 and along helix ␣5 (Fig. 26-19).Ala 177, which

corresponds to Ala 222 in the mammalian enzyme, does not

interact directly with the active site FAD. Instead it occupies

a position flush against helix ␣5 (which ends with residue

176). It is postulated that the replacement of Ala 177 by a

bulkier Val residue would force helix ␣5 to reorient. Since

this helix appears to be involved in the subunit interface as

well as in FAD binding, its reorientation is likely to decrease

the strength of subunit and FAD interactions.

Why should this mutation be so prevalent in the hu-

man population? What selective advantage, if any, might

it confer? We have seen that the gene for sickle-cell ane-

mia provides a selective advantage against malaria (Sec-

tion 7-3Ab). However, the selective advantage of the

A222V mutation in human MTHFR is as yet a matter of

speculation.

c. Methionine Synthase Is a Coenzyme

–Dependent Enzyme

Methionine synthase (alternatively homocysteine methyl-

transferase), the enzyme that catalyzes Reaction 4 in

Fig. 26-18, is the only coenzyme B

–associated enzyme in

mammals besides methylmalonyl-CoA mutase (Section

25-2Eb). However, in methionine synthase, the cobalamin

Co ion is axially liganded by a methyl group forming

methylcobalamin rather than by a 5¿-deoxyadenosyl group

as occurs in methylmalonyl-CoA mutase (Fig. 25-21). This

is because the cobalamin functions to accept the methyl

group from N

-methyl-THF to yield methylcobalamin (and

THF), which, in turn, donates the methyl group to homo-

cysteine to yield methionine.

The X-ray structure of the 246-residue methylcobal-

amin-binding portion of the 1227-residue monomeric

E. coli methionine synthase, also determined by

Matthews and Ludwig, reveals that it consists of two do-

mains, an N-terminal helical domain and a C-terminal

Rossmann fold-like ␣/␤ domain, with the corrin ring

sandwiched between them (Fig. 26-20). The ␣/␤ domain

resembles the corrin-binding ␣/␤ domain in methyl-

malonyl-CoA mutase (Fig. 25-22) and, in fact, sequence

homologies suggest that this domain is a common binding

motif in B

-associated enzymes. The Co ion’s second ax-

ial ligand is a His side chain as is also the case in methyl-

malonyl-CoA mutase; the coenzyme’s 5,6-dimethylbenza-

midazole (DMB) moiety, which ligands the Co ion in free

methylcobalamin, has swung aside to become anchored to

the protein at some distance from the corrin ring.

Section 26-3. Metabolic Breakdown of Individual Amino Acids 1037

Figure 26-19 X-ray structure of E. coli N

-methylene-

tetrahydrofolate reductase (MTHFR). The structure is viewed

along the axis of its ␣/␤ barrel looking toward the C-terminal

ends of its ␤ strands.The protein is colored according to its

secondary structure with ␤ strands yellow and ␣ helices cyan

except for helix ␣5, which is red. The enzyme’s bound FAD is

drawn in ball-and-stick form with C yellow, N blue, O red, and P

green. Note that the AMP moiety of the FAD is in contact with

helix ␣5. [Courtesy of Rowena Matthews and Martha Ludwig,

University of Michigan. PDBid 1B5T.]

Figure 26-20 X-ray structure of the B

-binding domains of

E. coli methionine synthase. Its N-terminal helical domain

(residues 651–743) is cyan and its C-terminal ␣/␤ domain

(residues 744–896) is pink.The methylcobalamin cofactor and its

axially liganded His 759 side chain are drawn in stick form with

cobalamin C green, His C gold, N blue, O red, and the Co ion

and its axially liganded methyl group represented by light blue

and orange spheres, respectively. [Based on an X-ray structure by

Rowena Matthews and Martha Ludwig, University of Michigan.

PDBid 1BMT.]

JWCL281_c26_1019-1087.qxd 4/20/10 9:26 AM Page 1037

SCoA

COO

–

COO

–

SCoA

Methylacrylyl-CoA

SCoA

β-Hydroxybutyryl-CoA

–

β-Hydroxyisobutyrate

HC COO

–

Methylmalonate

semialdehyde

CoASH

NAD

NADH

NADH, CO

NAD

, CoASH

SCoA

Tiglyl-CoA

SCoA

α-Methyl-β-

hydroxybutyryl-CoA

α-Methylacetoacetyl-CoA

CoASH

SCoA

NAD

NADH

C C

SCoA

-Methylcrotonyl-CoA

SCoA

β-Methylglutaconyl-CoA

SCoA

β-Hydroxy-β-

methylglutaryl-CoA (HMG-CoA)

–

OOC

–

OOC

+ATP

ADP P

SCoA

Acetyl-CoA

–

OOC

Acetoacetate

SCoA

Acetyl-CoA

SCoA

CCH

Propionyl-CoA

Succinyl-CoA

FADH

FAD

NADH, CO

NAD

, CoASH

Glutamate

␣-Ketoglutarate

= CH

(A) Isoleucine

(B) Valine

H=,

(CH

)

= CH

(A) α-Keto-β-methylvalerate

(B) α-Ketoisovalerate

(A) α-Methylbutyryl-CoA

(B) Isobutyryl-CoA

(A) (B) (C)

1038 Chapter 26. Amino Acid Metabolism

Figure 26-21 The degradation of the branched-chain amino acids (A) isoleucine, (B) valine,

and (C) leucine. The first three reactions of each pathway utilize the common enzymes

(1) branched-chain amino acid aminotransferase, (2) branched-chain ␣-keto acid dehydrogenase

(BCKDH), and (3) acyl-CoA dehydrogenase. Isoleucine degradation then continues (left) with

(4) enoyl-CoA hydratase, (5) ␤-hydroxyacyl-CoA dehydrogenase, and (6) acetyl-CoA

acetyltransferase to yield acetyl-CoA and the succinyl-CoA precursor propionyl-CoA.Valine

degradation (center) continues with (7) enoyl-CoA hydratase, (8) ␤-hydroxyisobutyryl-CoA

hydrolase, (9) ␤-hydroxyisobutyrate dehydrogenase, and (10) methylmalonate semialdehyde

dehydrogenase to also yield propionyl-CoA. Leucine degradation (right) continues with

(11) ␤-methylcrotonyl-CoA carboxylase (a biotin-dependent enzyme), (12) ␤-methylglutaconyl-

CoA hydratase, and (13) HMG-CoA lyase to yield acetyl-CoA and acetoacetate.

JWCL281_c26_1019-1087.qxd 4/20/10 9:26 AM Page 1038

d. Branched-Chain Amino Acid Degradation

Pathways Contain Themes Common to All

Acyl-CoA Oxidations

Degradation of the branched-chain amino acids isoleucine,

leucine, and valine begins with three reactions that employ

common enzymes (Fig. 26-21, top): (1) transamination to the

corresponding ␣-keto acid, (2) oxidative decarboxylation

to the corresponding acyl-CoA, and (3) dehydrogenation

by FAD to form a double bond.

The remainder of the isoleucine degradation pathway (Fig.

26-21, left) is identical to that of fatty acid oxidation (Section

25-2C): (4) double-bond hydration, (5) dehydrogenation by

NAD

⫹

, and (6) thiolytic cleavage yielding acetyl-CoA and

propionyl-CoA, which is subsequently converted to succinyl-

CoA. Valine degradation is a variation on this theme (Fig.

26-21, center): Following (7) double-bond hydration, (8) the

CoA thioester bond is hydrolyzed before (9) the second dehy-

drogenation reaction.The thioester bond is then regenerated

as propionyl-CoA in the sequence’s last reaction (10), an ox-

idative decarboxylation rather than a thiolytic cleavage.

e. Maple Syrup Urine Disease Results from a Defect

in Branched-Chain Amino Acid Degradation

Branched-chain ␣-keto acid dehydrogenase (BCKDH;

also known as ␣-ketoisovalerate dehydrogenase), which

catalyzes Reaction 2 of branched-chain amino acid degra-

dation (Fig. 26-21), is a multienzyme complex containing

three enzymatic components, E1, E2, and E3, together

with BCKDH kinase (phosphorylation inactivates) and

BCKDH phosphatase (dephosphorylation activates),

which impart control by covalent modification. This com-

plex closely resembles the pyruvate dehydrogenase and

␣-ketoglutarate dehydrogenase multienzyme complexes

(Sections 21-2A and 21-3D). Indeed, all three of these mul-

tienzyme complexes share a common protein component,

E3 (dihydrolipoyl dehydrogenase), and employ the coen-

zymes thiamine pyrophosphate (TPP), lipoamide, and

FAD in addition to their terminal oxidizing agent, NAD

⫹

A genetic deficiency in BCKDH causes maple syrup

urine disease (MSUD), so named because the consequent

buildup of branched-chain ␣-keto acids imparts the urine

with the characteristic odor of maple syrup. Unless

promptly treated by a diet low in branched-chain amino

acids (but not too low because they are essential amino

acids; Section 26-5), MSUD is rapidly fatal.

MSUD is an autosomal recessive disorder that is caused

by defects in any of four of the complex’s six subunits, E1␣,

E1␤, E2, or E3 (E1 is an ␣

␤

heterotetramer). The determi-

nation of the X-ray structure of human BCKDH E1 by Wim

Hol (Fig. 26-22) has enabled the interpretation of several of

the mutations causing MSUD.The most common mutation is

Y393N-␣, the so-called Mennonite mutation, which occurs

once in every 176 live births in the Old Order Mennonite

population (versus 1 in 185,000 worldwide). This mutation is

so common among Old Order Mennonites that it has been

attributed to a founder effect, that is, a mutation that origi-

nated in one of the handful of founders of this isolated com-

munity. The E1 tetramer can be considered to be a dimer of

␣␤ heterodimers with a TPP cofactor at the interface be-

tween an ␣ subunit and a ␤ subunit and with each ␣ subunit

contacting both the ␤ and ␤¿ subunits (Fig. 26-22a). The

amino acid change in the Mennonite mutation occurs at the

␣–␤¿ interface:Tyr 393␣ is hydrogen bonded to both His 385␣

and Asp 328␤¿ (Fig. 26-22b). Its mutation to Asn disrupts

these interactions and thereby impedes tetramerization.

Section 26-3. Metabolic Breakdown of Individual Amino Acids 1039

Figure 26-22 X-ray structure of the E1 component of the

human branched-chain ␣-keto acid dehydrogenase multienzyme

complex. (a) The ␣

␤

heterotetramer.The ␣ subunits are colored

cyan and orange, and the ␤ subunits are blue and pink. The

thiamine pyrophosphate (TPP) cofactor and Tyr 393␣ (which is

mutated to Asn in the Mennonite mutation, causing maple syrup

urine disease) are shown in space-filling form with TPP C green,

Tyr 393␣ C gold, N blue, O red, S yellow, and P magenta. Note

the similarity of this structure to that of the E1 component of the

pyruvate dehydrogenase multienzyme complex (Fig. 21-12a).

(b) The ␣–␤¿ interface colored as in Part a and showing the

interactions of Tyr 393␣ with His 385␣ and Asp 328␤¿. The side

chains of these residues are drawn in ball-and-stick form with C

green, N blue, and O red and with the hydrogen bonds between

them represented by dashed lines. [Based on an X-ray structure

by Wim Hol, University of Washington. PDBid 1DTW.]

(b)

(a)

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COO

–

Lysine

CNH CH

COO

–

Saccharopine

COO

–

COO

–

α-Ketoglutarate

NADPH

NADP

COO

–

-Aminoadipate-6-

semialdehyde

–

OOC

CCH

COO

–

-Aminoadipate

–

OOC

COO

–

-Ketoadipate

–

OOC

CCH

SCoA

Glutaryl-CoA

–

OOC

CH CCH

SCoA

Glutaconyl-CoA

CH CH

SCoA

Crotonyl-CoA

SCoA

-Hydroxybutyryl-CoA

SCoA

Acetoacetyl-CoA

SCoA

HMG-CoA

–

OOC

COO

–

Acetoacetate

CCH

NADH

NAD

Glutamate

NAD(P)H

NAD(P)

α-Ketoglutarate

Glutamate

NADH

NAD

CoA

FADH

FAD

NADH

NAD

CoA

Acetyl-CoA

O +

F. Leucine and Lysine Are Degraded to

Acetoacetate and/or Acetyl-CoA

Leucine is oxidized by a combination of reactions used in ␤

oxidation and ketone body synthesis (Fig. 26-21, right).The

first dehydrogenation and the hydration reactions are in-

terspersed by (11) a carboxylation reaction catalyzed by

the biotin-containing enzyme ␤-methylcrotonyl-CoA car-

boxylase. The hydration reaction (12) then produces

␤-hydroxy-␤-methylglutaryl-CoA (HMG-CoA), which is

cleaved by HMG-CoA lyase to form acetyl-CoA and the

ketone body acetoacetate (13) (which, in turn, may be con-

verted to 2 acetyl-CoA; Section 25-3).

Although there are several pathways for lysine degrada-

tion, the one that proceeds via formation of the ␣-ketoglu-

tarate–lysine adduct saccharopine predominates in mam-

malian liver (Fig. 26-23).This pathway is of interest because

we have encountered 7 of its 11 reactions in other pathways.

Reaction 4 is a PLP-dependent transamination. Reaction 5

is the oxidative decarboxylation of an ␣-keto acid by a

multienzyme complex similar to pyruvate dehydrogenase

1040 Chapter 26. Amino Acid Metabolism

Figure 26-23 The pathway

of lysine degradation in

mammalian liver. The

enzymes involved are

(1) saccharopine dehydrogenase

(NADP

⫹

, lysine forming),

(2) saccharopine

dehydrogenase (NAD

⫹

glutamate forming),

(3) aminoadipate

semialdehyde dehydrogenase,

(4) aminoadipate

aminotransferase (a PLP

enzyme), (5) ␣-keto acid

dehydrogenase, (6) glutaryl-

CoA dehydrogenase,

(7) decarboxylase, (8) enoyl-CoA

hydratase, (9) ␤-hydroxyacyl-

CoA dehydrogenase,

(10) HMG-CoA synthase,

and (11) HMG-CoA lyase.

Reactions 10 and 11 are

discussed in Section 25-3.

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