Voet D., Voet Ju.G. Biochemistry

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891

OH H

H NAc

UDP

OH H

H NAc

OH OH

–

UDP–GlcNac Phosphatidylinositol (PI) GlcNac–PI

GlcNH

CHO

OH OH

–

CHO

1→4

CoA Acyl-CoA

4 3

1→4

1→41→61→2

6,7

HOAc

Phosphatidyl

ethanolamine

OCH

–

1→41→6

1→2

–

Phosphatidyl

ethanolamine

lipid

remodeling

(a)

OCH

C-TERMINAL PEPTIDE

1→41→6

1→2

OCH

PCH

+ H

NC-TERMINAL PEPTIDEHN

–

CTARGET PROTEIN

PCH

1→41→61→2

OCH

CTARGET PROTEIN

(b)

Dolichol phosphate

mannose

Glucosamine

Phosphalidylinositol (PI)

Acyl group

JWCL281_c23_871-900.qxd 6/8/10 9:43 AM Page 891

N-acetylglucosamine from UDP–N-acetylglucosamine

(UDP–GlcNAc) to the 6 hydroxyl of the inositol of phos-

phatidylinositol, followed by the removal of the acetyl

group. The mammalian pathway then continues with the

2-acylation of inositol, translocation to the luminal side of

the ER membrane, and the addition of mannose from

dolichol-P-mannose (Dol-P-Man; Fig. 23-16) and phos-

phoethanolamine from phosphatidylethanolamine (Table

12-2), as indicated in Fig. 23-25a. This core is modified with

a variety of additional sugar residues, depending on the

species and the protein to which it is attached.There is con-

siderable diversity in the fatty acid residues of GPI anchors

due to the extensive lipid remodeling that occurs during

anchor synthesis. Target proteins become anchored to the

membrane surface when the amino group of the GPI phos-

phoethanolamine nucleophilically attacks a specific amino

acyl group of the protein near its C-terminus, resulting in a

transamidation that releases a 20- to 30-residue hydropho-

bic C-terminal signal peptide (Fig. 23-25b). Since GPI

groups are appended to proteins on the luminal surface of

the RER, GPI-anchored proteins occur on the exterior

surface of the plasma membrane (Fig. 12-60). However,

they are distributed unevenly in the outer leaflet of the

plasma membrane because they prefer to associate with

sphingolipid–cholesterol rafts (Section 12-3Cb).

The core GPI structure is evolutionarily conserved among

all eukaryotes, although there are differences between

species in its synthesis. For example, the cell surface of the

trypanosomes that cause African sleeping sickness (a debili-

tating and often fatal disease that afflicts millions of people in

sub-Saharan Africa) has a dense coating of variant surface

glycoprotein (VSG) that is GPI-anchored to its plasma mem-

brane. The VSG coating conceals the trypanosome’s plasma

membrane from the host’s immune system although it recog-

nizes and attacks the VSG itself.The parasite is nevertheless

able to evade the host’s immunological defenses because it

has a genetic repertoire of about a thousand immunologically

distinct VSGs.An individual trypanosome expresses only one

of its VSG genes and hence the host can mount an effective

immunological attack against the prevailing population of

VSGs, a process that takes around 1 week (Section 35-2A).

However, by switching VSG genes, a new population of

trypanosomes arises that replicates unchecked until the host

can mount a new immune response, a cycle that repeats until

the death of the host. The comparison of the GPI biosyn-

thetic pathway in trypanosomes with that in mammalian sys-

tems has revealed several differences in the pathway order.

For example, Steps 3 and 4 of Fig. 23-25a are reversed in try-

panosomes. This and other differences in the substrate speci-

ficities of the enzymes catalyzing this pathway have brought

to light several promising drug targets for the treatment of

African sleeping sickness.

4 THE PENTOSE PHOSPHATE PATHWAY

ATP is the cell’s “energy currency”;its exergonic hydrolysis is

coupled to many otherwise endergonic cell functions. Cells

have a second currency, reducing power. Many endergonic

reactions, notably the reductive biosynthesis of fatty acids

(Section 25-4) and cholesterol (Section 25-6A), as well as

photosynthesis (Section 24-3A), require NADPH in addition

to ATP. Despite their close chemical resemblance, NADPH

and NADH are not metabolically interchangeable (recall that

these coenzymes differ only by a phosphate group at the 2¿-

OH group of NADPH’s adenosine moiety; Fig. 13-2).

Whereas NADH participates in utilizing the free energy of

metabolite oxidation to synthesize ATP (oxidative phospho-

rylation), NADPH is involved in utilizing the free energy of

metabolite oxidation for otherwise endergonic reductive

biosynthesis. This differentiation is possible because the de-

hydrogenase enzymes involved in oxidative and reductive

metabolism exhibit a high degree of specificity toward their

respective coenzymes. Indeed, cells normally maintain their

[NAD



][NADH] ratio near 1000, which favors metabolite

oxidation, while keeping their [NADP



][NADPH] ratio

near 0.01, which favors metabolite reduction.

NADPH is generated by the oxidation of G6P via an al-

ternative pathway to glycolysis, the pentose phosphate path-

way [also called the hexose monophosphate (HMP) shunt

and the phosphogluconate pathway; Fig. 23-26]. The path-

way also produces ribose-5-phosphate (R5P), an essential

precursor in nucleotide biosynthesis (Sections 28-1, 28-2,

and 28-5). The first evidence of this pathway’s existence

was obtained in the 1930s by Otto Warburg, who discov-

ered NADP



through his studies on the oxidation of G6P

to 6-phosphogluconate. Further indications came from the

observation that tissues continue to respire in the presence

of high concentrations of fluoride ion, which, it will be re-

called, blocks glycolysis by inhibiting enolase (Section

17-2I). It was not until the 1950s, however, that the pentose

phosphate pathway was elucidated by Frank Dickens,

Bernard Horecker, Fritz Lipmann, and Efraim Racker.Tis-

sues most heavily involved in fatty acid and cholesterol

biosynthesis (liver, mammary gland, adipose tissue, and

adrenal cortex) are rich in pentose phosphate pathway

enzymes. Indeed, some 30% of the glucose oxidation in

liver occurs via the pentose phosphate pathway.

The overall reaction of the pentose phosphate pathway is

However, the pathway may be considered to have three

stages:

1. Oxidative reactions (Fig. 23-26, Reactions 1–3),

which yield NADPH and ribulose-5-phosphate (Ru5P).

2. Isomerization and epimerization reactions (Fig.

23-26, Reactions 4 and 5), which transform Ru5P either

to ribose-5-phosphate (R5P) or to xylulose-5-phosphate

(Xu5P).

3. A series of C¬C bond cleavage and formation reac-

tions (Fig. 23-26, Reactions 6–8) that convert two mole-

cules of Xu5P and one molecule of R5P to two molecules

3Ru5P Δ R5P  2Xu5P

6NADPH  6H



 3CO

 3Ru5P

3G6P  6NADP



 3H

6NADPH  6H



 3CO

 2F6P  GAP

3G6P  6NADP



 3H

O Δ

892 Chapter 23. Other Pathways of Carbohydrate Metabolism

JWCL281_c23_871-900.qxd 3/24/10 11:33 AM Page 892

of fructose-6-phosphate (F6P) and one of glyceraldehyde-

3-phosphate (GAP).

The reactions of Stages 2 and 3 are freely reversible so that

the products of the pathway vary with the needs of the cell.

R5P  2Xu5P Δ 2F6P  GAP

For example, when R5P is required for nucleotide biosyn-

thesis, Stage 3 works in reverse, producing R5P from F6P

and GAP nonoxidatively. In this section, we discuss the

three stages of the pentose phosphate pathway and how

this pathway is controlled.We close by considering the con-

sequences of one of its abnormalities.

Section 23-4. The Pentose Phosphate Pathway 893

Figure 23-26 The pentose phosphate pathway. The number of

lines in an arrow represents the number of molecules reacting in

one turn of the pathway so as to convert three G6P to three CO

two F6P, and one GAP. For the sake of clarity, sugars from

Reaction 3 onward are shown in their linear forms.The carbon

Glyceraldehyde-

3-phosphate

(GAP)

Ribulose-5-

phosphate (Ru5P)

NADP

6-phospho-

gluconate

dehydro-

genase

Ribose-5-

phosphate (R5P)

OPO

2–

Xylulose-5-

phosphate (Xu5P)

Fructose-6-

phosphate

(F6P)

OPO

2–

OPO

2–

Glyceraldehyde-

3-phosphate

(GAP)

Sedoheptulose-7-

phosphate (S7P)

6-Phospho-

gluconate

–

6-phospho-

glucono-

lactonase

OH H

HOH

OH H

HOH

NADP

H + CO

NADP

glucose-

6-phosphate

dehydrogenase

NADPH + H

OPO

2–

OPO

2–

OPO

2–

OPO

2–

6-Phosphoglucono-

-lactone

␦

Glucose-6-

phosphate (G6P)

O H

Erythrose-4-

phosphate (E4P)

Fructose-6-

phosphate (F6P)

O H

ribulose-5-phosphate

epimerase

ribulose-5-phosphate

isomerase

transketolase

transaldolase

transketolase

OPO

2–

OPO

2–

OPO

2–

OPO

2–

OPO

2–

skeleton of R5P and the atoms derived from it are drawn in red

and those from Xu5P are drawn in green.The C

units

transferred by transketolase are shaded in green and the C

units

transferred by transaldolase are shaded in blue. Double-headed

arrows indicate reversible reactions.

JWCL281_c23_871-900.qxd 3/24/10 11:33 AM Page 893

A. Oxidative Reactions of NADPH Production

Only the first three reactions of the pentose phosphate

pathway are involved in NADPH production.

1. Glucose-6-phosphate dehydrogenase (G6PD) cat-

alyzes net transfer of a hydride ion to NADP



from C1 of

G6P to form 6-phosphoglucono-␦-lactone (Fig. 23-27). G6P,

a cyclic hemiacetal with C1 in the aldehyde oxidation state,

is thereby oxidized to a cyclic ester (lactone).The enzyme is

specific for NADP



and is strongly inhibited by NADPH.

2. 6-Phosphogluconolactonase increases the rate of hy-

drolysis of 6-phosphoglucono--lactone to 6-phosphoglu-

conate (the nonenzymatic reaction occurs at a significant

rate), the substrate of the next oxidative enzyme in the

pathway.

3. 6-Phosphogluconate dehydrogenase catalyzes the ox-

idative decarboxylation of 6-phosphogluconate, a -hydroxy

acid, to Ru5P and CO

(Fig. 23-28). The reaction is similar

to that catalyzed by the citric acid cycle enzyme isocitrate

dehydrogenase (Section 21-3C).

Formation of Ru5P completes the oxidative portion of

the pentose phosphate pathway. It generates two molecules

of NADPH for each molecule of G6P that enters the path-

way. The product Ru5P must subsequently be converted to

R5P or Xu5P for further use.

B. Isomerization and Epimerization of

Ribulose-5-Phosphate

Ru5P is converted to R5P by ribulose-5-phosphate iso-

merase (Fig. 23-26, Reaction 4) and to Xu5P by ribulose-

5-phosphate epimerase (Fig. 23-26, Reaction 5). These

isomerization and epimerization reactions, as discussed in

Section 16-2Db, are both thought to occur via enediolate

intermediates (Fig. 23-29).

R5P is an essential precursor in the biosynthesis of nu-

cleotides (Sections 28-1, 28-2, and 28-5). If, however, more

R5P is formed than the cell needs, the excess, along with

Xu5P, is converted to the glycolytic intermediates F6P and

GAP as described below.

C. Carbon–Carbon Bond Cleavage

and Formation Reactions

The conversion of three C

sugars to two C

sugars and one

sugar involves a remarkable “juggling act” catalyzed by

two enzymes, transaldolase and transketolase. As we dis-

cussed in Section 16-2E, enzymatic reactions that make or

break carbon–carbon bonds usually have mechanisms that

involve generation of a stabilized carbanion and its addi-

tion to an electrophilic center such as an aldehyde. This is

the dominant theme of both the transaldolase and the

transketolase reactions.

a. Transketolase Catalyzes the Transfer of C

Units

Transketolase, which has a thiamine pyrophosphate co-

factor (TPP; Section 17-3Ba), catalyzes the transfer of a C

unit from Xu5P to R5P, yielding GAP and sedoheptulose-

7-phosphate (S7P) (Fig. 23-26, Reaction 6). The reaction in-

volves the intermediate formation of a covalent adduct be-

tween Xu5P and TPP (Fig. 23-30).The X-ray structure of this

homodimeric enzyme shows that the TPP binds in a deep

cleft between the subunits such that residues from both

894 Chapter 23. Other Pathways of Carbohydrate Metabolism

Figure 23-27 The glucose-6-phosphate dehydrogenase reaction.

Figure 23-28 The phosphogluconate dehydrogenase reaction.

Oxidation of the OH group forms an easily decarboxylated

OH H

H OH

6-Phosphoglucono-

δ-lactone

OH H

H OH

G6P

OPO

–

OPO

–

glucose-6-phosphate

dehydrogenase

NADP

NADPH

6-phosphogluconate

dehydrogenase

NADPH

+ H

–

OPO

–

6-Phosphogluconate

C HHO

COH

NADP

Ru5P

β-Keto acid

intermediate

COH

–

OPO

–

C O

COH

COHH

C O

COH

COHH

OPO

–

-keto acid (although the proposed intermediate has not been

isolated).

JWCL281_c23_871-900.qxd 3/24/10 11:33 AM Page 894

Section 23-4. The Pentose Phosphate Pathway 895

Figure 23-29 Ribulose-5-phosphate isomerase and

ribulose-5-phosphate epimerase. The reactions catalyzed

by both these enzymes involve enediolate intermediates.

In the isomerase reaction (right), a base on the enzyme

removes a proton from C1 of Ru5P to form a 1,2-enedio-

late and then adds a proton at C2 to form R5P. In the

epimerase reaction (left), a base on the enzyme removes

a C3 proton to form a 2,3-enediolate. A proton is then

added to the same carbon atom but with inversion of

configuration to yield Xu5P.

Figure 23-30 Mechanism of transketolase. Transketolase utilizes the coenzyme

thiamine pyrophosphate to stabilize the carbanion formed on cleavage of the

C2¬C3 bond of Xu5P. The reaction occurs as follows: (1) The TPP ylid attacks

the carbonyl group of Xu5P. (2) C2¬C3 bond cleavage yields GAP and enzyme-

bound 2-(1,2-dihydroxyethyl)-TPP, a resonance-stabilized carbanion. (3) The C2

carbanion attacks the aldehyde carbon of R5P forming an S7P–TPP adduct.

(4) TPP is eliminated yielding S7P and the regenerated TPP–enzyme.

OPO

–

2,3-Enediolate

intermediate

1,2-Enediolate

intermediate

ribulose-5-phosphate

isomerase

ribulose-5-phosphate

epimerase

Ru5P

–

OHH

OPO

–

OHH

OPO

–

Xu5P

OHH

OPO

–

OHH

OPO

–

OHH

R5P

C CH

•

–

Thiamine pyrophosphate (TPP)

ylid form

OPO

2–

Xu5P

CR

•

2-(1,2-Dihydroxyethyl)-

TPP

CR

•

–

R5P

CR

•

S7P

TPP

CR

•

GAP

–

OPO

2–

OPO

2–

OPO

2–

•

ylid

attack

bond

cleavage

ylid

elimination

unit

transfer

JWCL281_c23_871-900.qxd 3/24/10 11:33 AM Page 895

subunits participate in its binding, just as in pyruvate decar-

boxylase (another TPP-requiring enzyme; Figure 17-28). In

fact the structures are so similar that it is likely that they di-

verged from a common ancestor.

b. Transaldolase Catalyzes the Transfer of C

Units

Transaldolase catalyzes the transfer of a C

unit from

S7P to GAP, yielding erythrose-4-phosphate (E4P) and

F6P (Fig. 23-26, Reaction 7). The reaction occurs by aldol

cleavage, which begins with the formation of a Schiff base

between an ε-amino group of an essential enzyme Lys

residue and the carbonyl group of S7P (Fig. 23-31).Transal-

dolase and Class I aldolase (Section 17-2Da) share a com-

mon reaction mechanism and may also share a common

ancestor, despite their lack of significant sequence identity.

Both are  barrel proteins (Section 8-3Bh), but while the

Schiff base–forming Lys is on 4 (the fourth  strand from

the N-terminus) of transaldolase, it is on 6 of Class I al-

dolase. Superimposing the barrel structures of these two

enzymes while maintaining the alignment of the  strands

bearing the Schiff base–forming Lys residues results in a

significantly better fit than doing so while maintaining the

alignment of their entire  barrels. Moreover, five of the

pairs of matched active site residues in the former superpo-

sition are identical.This suggests that, during evolution, the

DNA sequence for two  units was transferred from the

N-terminus to the C-terminus of the evolving Class I al-

dolase, moving the active site Lys from 6 to 4. Such a cir-

cular permutation of an / barrel’s structural elements

does not greatly change its structure.

c. A Second Transketolase Reaction Yields

GAP and a Second F6P Molecule

In a second transketolase reaction, a C

unit is trans-

ferred from a second molecule of Xu5P to E4P to form

GAP and another molecule of F6P (Fig. 23-26, Reaction 8).

The third phase of the pentose phosphate pathway thus

transforms two molecules of Xu5P and one of R5P to two

molecules of F6P and one molecule of GAP. These carbon

skeleton transformations (Fig. 23-26, Reactions 6–8) are

summarized in Fig. 23-32.

D. Control of the Pentose Phosphate Pathway

The principal products of the pentose phosphate pathway are

R5P and NADPH.The transaldolase and transketolase reac-

tions serve to convert excess R5P to glycolytic intermediates

896 Chapter 23. Other Pathways of Carbohydrate Metabolism

Figure 23-31 Mechanism of transaldolase. Transaldolase contains an

essential Lys residue that forms a Schiff base with S7P to facilitate an

aldol cleavage reaction.The reaction occurs as follows: (1) The ε-amino

group of an essential Lys residue forms a Schiff base with the carbonyl group

of S7P. (2) A Schiff base–stabilized C3 carbanion is formed in an aldol cleavage

reaction between C3 and C4 that eliminates E4P. (3) The enzyme-bound

resonance-stabilized carbanion adds to the carbonyl C atom of GAP, forming

F6P linked to the enzyme via a Schiff base. (4) The Schiff base hydrolyzes,

regenerating active enzyme and releasing F6P.

–

OHC

Lys(CH

)

OHC

OPO

–

NHLys(CH

)

OHC

OPO

–

NHLys(CH

)

NHLys(CH

)

S7P

E4P

–

OPO

–

OHC

Lys(CH

)

OHC

OPO

–

NHLys(CH

)

F6P

OPO

–

GAP

–

Schiff base

formation

aldol

cleavage

carbanion addition

to carbonyl

Schiff base

hydrolysis

JWCL281_c23_871-900.qxd 3/24/10 11:33 AM Page 896

when the metabolic need for NADPH exceeds that of

R5P in nucleotide biosynthesis. The resulting GAP and

F6P can be consumed through glycolysis and oxidative

phosphorylation or recycled by gluconeogenesis to form

G6P. In the latter case, 1 molecule of G6P can be converted,

via six cycles of the pentose phosphate pathway and gluco-

neogenesis, to 6 CO

molecules with the concomitant gener-

ation of 12 NADPH molecules. When the need for R5P

outstrips that for NADPH, F6P and GAP can be diverted

from the glycolytic pathway for use in the synthesis of R5P

by reversal of the transaldolase and transketolase reac-

tions. In fact, mass spectral analysis of the

C-labeled

carbons from [1,2-

C]glucose incorporated into RNA in

rapidly proliferating cancer cells has shown that more than

⬃70% of the de novo ribose synthesis arises through this

nonoxidative reversal of the pentose phosphate pathway

(rather than its forward direction).

Flux through the oxidative pentose phosphate pathway

and thus the rate of NADPH production is controlled by the

rate of the glucose-6-phosphate dehydrogenase reaction

(Fig. 23-26, Reaction 1). The activity of this enzyme, which

catalyzes the pentose phosphate pathway’s first committed

step (G 17.6 kJ ⴢ mol

1

in liver), is regulated by the

NADP



concentration (substrate availability). When the

cell consumes NADPH, the NADP



concentration rises,

increasing the rate of the glucose-6-phosphate dehydroge-

nase reaction, thereby stimulating NADPH regeneration.

E. Glucose-6-Phosphate Dehydrogenase Deficiency

NADPH is required for several reductive processes in ad-

dition to biosynthesis. For example, erythrocyte mem-

brane integrity requires a plentiful supply of reduced glu-

tathione (GSH), a Cys-containing tripeptide (Sections

21-2Ba and 26-4C). A major function of GSH in the ery-

throcyte is to eliminate H

and organic hydroperoxides.

, a toxic product of various oxidative processes

(Section 22-4Cg), reacts with double bonds in the fatty

acid residues of the erythrocyte cell membrane to form or-

ganic hydroperoxides. These, in turn, react to cleave fatty

acid C¬C bonds, thereby damaging the membrane. In

erythrocytes, the unchecked buildup of peroxides results

in premature cell lysis. Peroxides are eliminated through

the action of glutathione peroxidase, one of the handful of

enzymes with a selenium cofactor, yielding glutathione

disulfide (GSSG).

Section 23-4. The Pentose Phosphate Pathway 897

2GSH R O O H+ GSSG ROH

glutathione

peroxidase

+ H

O+

Organic

hydroperoxide

GSH is subsequently regenerated by the NADPH reduc-

tion of GSSG catalyzed by glutathione reductase (Section

21-2Ba).

A steady supply of NADPH is therefore vital for erythro-

cyte integrity.

a. Primaquine Causes Hemolytic Anemia in

Glucose-6-Phosphate Dehydrogenase Mutants

A genetic defect, common in African, Asian, and

Mediterranean populations, results in severe hemolytic

anemia on infection or on the administration of certain

drugs including the antimalarial agent primaquine.

Similar effects, which go by the name of favism, occur when

individuals bearing this trait eat fava beans (broad beans,

Vicia faba), a staple Middle Eastern vegetable that con-

tains small quantities of toxic glycosides (the Greek

philosopher and mathematician Pythagoras, who lived in

the sixth century

BCE, forbade his followers from eating

fava beans, possibly because of their deleterious effects).

This trait has been traced to an altered gene for glucose-6-

phosphate dehydrogenase (G6PD). Under most condi-

tions, mutant erythrocytes have sufficient enzyme activity

for normal function. Agents such as primaquine and fava

beans, however, stimulate peroxide formation, thereby in-

creasing the demand for NADPH to a level that mutant

cells cannot meet.

The major reason for low enzymatic activity in affected

cells appears to be an accelerated rate of breakdown of the

mutant enzyme (protein degradation is discussed in Sec-

tion 32-6). This explains why patients with G6PD defi-

ciency react to primaquine with hemolytic anemia but re-

cover within a week despite continued primaquine

treatment. Mature erythrocytes lack a nucleus and protein

synthesizing machinery and therefore cannot synthesize

new enzyme molecules to replace degraded ones (they

likewise cannot synthesize new membrane components,

which is why they are so sensitive to membrane damage in

the first place). The initial primaquine treatments result in

NH CH CH

Primaquine

GSSG NADPH H

2GSH NADP

glutathione

reductase

++ +

Figure 23-32 Summary of carbon skeleton rearrangements in

the pentose phosphate pathway. A series of carbon–carbon bond

formations and cleavages convert three C

sugars to two C

and

one C

sugar.The number to the left of each reaction is keyed to

the corresponding reaction in Fig. 23-26.

(6)

(7)

(8)

(Sum)

3 C

2 C

JWCL281_c23_871-900.qxd 3/24/10 11:33 AM Page 897

the lysis of old red blood cells whose defective G6PD has

been largely degraded. Lysis products stimulate the release

of young cells that contain more enzyme and are therefore

better able to cope with primaquine stress.

It is estimated that over 400 million people are deficient

in G6PD, which makes this condition the most common hu-

man enzymopathy. Indeed, ⬃400 G6PD variants have been

reported and at least 140 of them have been characterized

at the molecular level. G6PD is active in a dimer–tetramer

equilibrium. Many of the mutation sites in individuals with

the most severe G6PD deficiency are at the dimer inter-

face, shifting the equilibrium toward the inactive and un-

stable monomer.

Several G6PD variants occur with high incidence. For

example, the so-called type A



deficiency, which exhibits

⬃10% of the normal G6PD activity, has an incidence of

11% among African Americans. This variant is also the

most common form of G6PD deficiency in sub-Saharan

Africa. The variant “Mediterranean” is found throughout

the Mediterranean and Middle East regions, and occurs in

65% of Kurdish Jews, the population with the highest

known incidence of this trait.The high prevalence of defec-

tive G6PD in malarial areas of the world suggests that such

mutations confer resistance to the malarial parasite, Plas-

modium falciparum (as we likewise saw to be the case for

the sickle-cell trait; Section 7-3Ab). Indeed, two epidemio-

logical studies involving over 2000 African children with



G6PD deficiency indicate that this form is associated

with an ⬃50% reduction in the risk of severe malaria for

both female heterozygotes and male hemizygotes (G6PD

deficiency is an X-linked trait).

In vitro studies indicate that erythrocytes with G6PD

deficiency are less suitable hosts for plasmodia than are

normal cells. This is presumably because the parasite re-

quires the products of the pentose phosphate pathway

and/or because the erythrocyte is lysed before the parasite

has had a chance to mature.Thus, like the sickle-cell trait, a

defective G6PD confers a selective advantage on individuals

living where malaria is endemic.

898 Chapter 23. Other Pathways of Carbohydrate Metabolism

1 Gluconeogenesis Lactate, pyruvate, citric acid cycle in-

termediates, and many amino acids may be converted, by gluco-

neogenesis, to glucose via the formation of oxaloacetate. For

this to occur, the three irreversible steps of glycolysis must be

bypassed.The pyruvate kinase reaction is bypassed by convert-

ing pyruvate to oxaloacetate in an ATP-driven reaction cat-

alyzed by the biotinyl-containing enzyme pyruvate carboxylase.

The two phases of the pyruvate decarboxylase reaction are cat-

alyzed on different active sites of the homotetrameric enzyme,

which translocates its covalently linked carboxybiotinyl group

from its BC domain to the CT domain of a neighboring subunit.

The oxaloacetate is subsequently decarboxylated and phospho-

rylated by GTP to form PEP in a reaction catalyzed by PEPCK.

For this to happen in species in which PEPCK is a cytosolic en-

zyme, the oxaloacetate must be transported from the mitochon-

drion to the cytosol via its interim conversion to either malate

or aspartate. Conversion to malate concomitantly transports re-

ducing equivalents to the cytosol in the form of NADH.The two

other irreversible steps of glycolysis, the PFK reaction and the

hexokinase reaction, are bypassed by simply hydrolyzing their

products, FBP and G6P, by FBPase and glucose-6-phosphatase,

respectively. A glucose molecule may therefore be synthesized

from pyruvate at the expense of four ATPs more than are gen-

erated by the reverse process.

Glycolysis and gluconeogenesis are reciprocally regulated so

as to consume glucose when the demand for ATP is high and

synthesize it when the demand is low.The control points in these

processes are at pyruvate kinase/pyruvate carboxylase–PEPCK,

PFK/FBPase, and hexokinase/glucose-6-phosphatase. Regula-

tion of these enzymes is exerted largely through allosteric inter-

actions, cAMP-dependent enzyme modifications, and, for

PEPCK, gene expression. Muscle, which is incapable of gluco-

neogenesis, transfers much of the lactate it produces to the liver

via the blood for conversion to glucose and return to the muscle.

This Cori cycle shifts the metabolic burden of oxidative ATP

generation for gluconeogenesis from muscle to liver.

2 The Glyoxylate Cycle Animals cannot convert fatty

acids to glucose because they lack the enzymes necessary to

synthesize oxaloacetate from acetyl-CoA.Plants, however,can

do so via the glyoxylate cycle, a glyoxysomal process that con-

verts two molecules of acetyl-CoA to one molecule of succi-

nate via the intermediate formation of glyoxylate. Succinate is

converted to oxaloacetate for use in gluconeogensis or the cit-

ric acid cycle.

3 Biosynthesis of Oligosaccharides and Glycoproteins

Glycosidic bonds are formed by transfer of the monosaccharide

unit of a sugar nucleotide to a second sugar unit. Such reactions

occur in the synthesis of disaccharides such as lactose and in the

synthesis of the carbohydrate components of glycoproteins. In

N-linked glycoproteins, the carbohydrate component is attached

to the protein via an N-glycosidic bond to an Asn residue in the

sequence Asn-X-Ser/Thr. In O-linked glycoproteins, the carbo-

hydrate attachment is an O-glycosidic bond to Ser or Thr or, in

collagens, to 5-hydroxylysine. In GPI-anchored proteins a glyco-

sylphosphatidylinositol group is linked to the protein through an

intermediary phosphoethanolamine bridge, which forms an

amide bond to the protein’s C-terminal amino acid residue.

Synthesis of N-linked oligosaccharides begins in the endo-

plasmic reticulum with the multistep formation of a lipid-linked

precursor consisting of dolichol pyrophosphate bonded to a

common 14-residue core oligosaccharide. The carbohydrate is

then transferred to an Asn residue of a growing polypeptide

chain.The correct folding of the immature N-linked glycoprotein

is assisted via the calnexin/calreticulin cycle and it is subse-

quently transferred, via a membranous vesicle, to the cis Golgi

network of the Golgi apparatus. Processing is completed by the

trimming of mannose residues followed by attachment of a vari-

ety of other monosaccharides as catalyzed by specific enzymes in

the cis, medial, and trans Golgi cisternae. Completed N-linked

glycoproteins are sorted in the trans Golgi network according to

the identities of their carbohydrate components for transport,via

membranous vesicles, to their final cellular destinations. Three

CHAPTER SUMMARY

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major types of N-linked oligosaccharides have been identified,

high mannose, complex,and hybrid oligosaccharides,all of which

contain a common pentasaccharide core. Studies of glycoprotein

formation have been facilitated by the use of antibiotics, such as

tunicamycin and bacitracin, which inhibit specific enzymes in-

volved in the synthesis of these oligosaccharides.

O-Linked oligosaccharides are synthesized in the Golgi ap-

paratus by sequential attachments of specific monosaccharide

units to certain Ser or Thr residues. Carbohydrate components

of glycoproteins are thought to act as recognition markers for

the transport of glycoproteins to their proper cellular destina-

tions and for cell–cell and antibody recognition. The GPI mem-

brane anchor is appended to proteins on the luminal surface of

the endoplasmic reticulum, thereby targeting GPI-anchored

proteins to the external surface of the plasma membrane.

4 The Pentose Phosphate Pathway The cell uses NAD

⫹

in oxidative reactions and employs NADPH in reductive

biosynthesis. NADPH is synthesized by the pentose phos-

phate pathway, an alternate mode of glucose oxidation. This

pathway also synthesizes R5P for use in nucleotide biosynthe-

sis.The first three reactions of the pentose phosphate pathway

involve oxidation of G6P to Ru5P with release of CO

and for-

mation of two NADPH molecules. This is followed by reac-

tions that either isomerize Ru5P to R5P or epimerize it to

Xu5P. Each molecule of R5P not required for nucleotide

biosynthesis, together with two Xu5P, is converted to two

molecules of F6P and one molecule of GAP via the sequential

actions of transketolase, transaldolase, and, again, transketo-

lase. The products of the pentose phosphate pathway depend

on the needs of the cell. The F6P and GAP may be metabo-

lized through glycolysis and the citric acid cycle or recycled via

gluconeogenesis. If NADPH is in excess, the latter portion of

the pentose phosphate pathway may be reversed to synthesize

R5P from glycolytic intermediates. The pentose phosphate

pathway is controlled at its first committed step, the glucose-6-

phosphate dehydrogenase reaction, by the NADP

⫹

concentra-

tion. A genetic deficiency in glucose-6-phosphate dehydroge-

nase leads to hemolytic anemia on administration of the

antimalarial drug primaquine. This X-linked deficiency, which

results from the accelerated degradation of the mutant en-

zyme, provides resistance against severe malaria to female

heterozygotes and male hemizygotes for this sex-linked trait.

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900 Chapter 23. Other Pathways of Carbohydrate Metabolism

1. Compare the relative energetic efficiencies, in ATPs per

mole of glucose oxidized, of glucose oxidation via glycolysis ⫹ the

citric acid cycle versus glucose oxidation via the pentose phos-

phate pathway ⫹ gluconeogenesis. Assume that NADH and

NADPH are each energetically equivalent to 2.5 ATP.

2. Although animals cannot synthesize glucose from acetyl-

CoA, if a rat is fed

C-labeled acetate, some of the label will ap-

pear in the glycogen extracted from its muscles. Explain.

3. Substances that inhibit specific trimming steps in the pro-

cessing of N-linked glycoproteins have been useful tools in eluci-

dating the pathway of this process. Explain.

4. Through clever genetic engineering you have developed an

unregulatable enzyme that can interchangeably use NAD

⫹

NADP

⫹

in a redox reaction.What would be the physiological con-

sequence(s) on an organism of having such an enzyme?

5. What is the free energy change of the reaction

under physiological conditions? Assume that ⌬G°¿ ⫽ 0 for this re-

action and that T ⫽ 37°C.

6. If G6P is

C-labeled at its C2 position, what is the distribu-

tion of the radioactive label in the products of the pentose phos-

phate pathway after one turnover of the pathway? What is the dis-

tribution of the label after passage of these products through

gluconeogenesis followed by a second round of the pentose phos-

phate pathway?

NADH ⫹ NADP

⫹

Δ NAD

⫹

⫹ NADPH

7. After feeding rapidly growing and proliferating cells [1,2-

C]glucose and isolating the RNA, you find that both the C1 and

C2 atoms of the ribosyl units are labeled. Show, using chemical

structures and the appropriate enzymes, how the pentose phos-

phate pathway can yield this distribution of the label.

8. The relative metabolic activities in an organism of glycoly-

sis ⫹ the citric acid cycle versus the pentose phosphate path-

way ⫹ gluconeogenesis can be measured by comparing the rates

generation on administration of glucose labeled with

C at C1 with that of glucose labeled at C6. Explain.

9. (a) Describe the lengths of the products of the transketo-

lase reaction when the two substrates are both five-carbon sugars.

(b) Describe the products of the reaction when the substrates are

a five-carbon aldose and a six-carbon ketose. Does it matter which

of the substrates binds to the enzyme first?

10. In light of the finding that an otherwise benign or even ad-

vantageous mutation leads to abnormal primaquine sensitivity

combined with the fact that human beings have enormous genetic

complexity, comment on the possibility of developing drugs that

exhibit no atypical side effects in any individual.

11. Glucose-6-phosphatase is located inside the endoplasmic

reticulum. Describe the probable symptoms of a defect in G6P

transport across the endoplasmic reticulum membrane.

PROBLEMS

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