Voet D., Voet Ju.G. Biochemistry

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and is symmetrically linked to Cys 97 and Cys 132 from

both subunits such that an Fe-protein resembles an “iron

butterfly” with the [4Fe–4S] cluster at its head. Its two

identical nucleotide binding sites are located at the inter-

face between its two subunits. The [4Fe–4S] cluster cycles

between its ⫹1 and ⫹2 oxidation states.

The MoFe-protein’s ␣ and ␤ subunits assume similar

folds and extensively associate to form a pseudo-2-fold

symmetric ␣␤ dimer, two of which more loosely associate

to form the 2-fold symmetric ␣

␤

tetramer (Fig. 26-67).

Each ␣␤ dimer has two bound redox centers:

1. The P-cluster (Fig. 26-68a,b), which consists of two

[4Fe–3S] clusters linked through an additional sulfide ion

forming the eighth corner of each of the clusters to make

cubane-like structures, and bridged by two Cys thiol lig-

ands, each coordinating one Fe from each cluster. Four ad-

ditional Cys thiols coordinate the remaining four Fe atoms.

The positions of two of the Fe atoms in one of the [4Fe–3S]

clusters change on oxidation, rupturing the bonds from

these Fe atoms to the linking sulfide ion. These bonds are

replaced in the oxidized state by a Ser oxygen ligand to one

of the Fe atoms, and by a bond to the amide N of a Cys

from the other Fe atom.

2. The FeMo-cofactor (Fig. 26-68c), which consists of a

[4Fe–3S] cluster and a [1Mo–3Fe–3S] cluster bridged by

three sulfide ions.The FeMo-cofactor’s Mo atom is approx-

imately octahedrally coordinated by three cofactor sulfide

ions, a His imidazole nitrogen, and two oxygens from a

bound homocitrate ion:

(an essential component of the FeMo-cofactor). The

FeMo-cofactor contains a central cavity that a high resolu-

tion (1.16 Å) X-ray structure of A. vinelandii MoFe-

protein, also determined by Rees, reveals contains what

most probably is a nitrogen atom (although a C or an O atom

cannot be ruled out). This putative N atom is liganded to

the FeMo-cofactor’s central six Fe atoms such that it com-

pletes the approximate tetrahedral coordination environ-

ment of each of these Fe atoms.

The FeMo-cofactor is located ⬃10 Å below the ␣ subunit

surface, and hence the N

is thought to gain access to its

binding site through conformational fluctuations of the

protein (recall that myoglobin and hemoglobin likewise

have no clear path for O

to approach its heme binding

–

COO COO

–

COO

–

Homocitrate

Section 26-6. Nitrogen Fixation 1081

Figure 26-68 The prosthetic groups of the nitrogenase

MoFe-protein. The molecules are drawn in ball-and-stick form

with C green, N blue, O red, S yellow, Fe orange, and Mo pink.

(a) The reduced Klebsiella pneumoniae P-cluster. It consists of

two [4Fe–3S] complexes linked by an additional sulfide ion

forming the eighth corner of each cubane-like structure, and

bridged by two Cys thiol ligands, each coordinating one Fe from

each cluster. Four additional Cys thiols coordinate the remaining

4 Fe atoms. (b) The 2-electron-oxidized K. pneumoniae P-cluster.

In comparison with the reduced complex in Part a, two of the

Fe¬S bonds from the centrally located sulfide ion that bridges

the two [4Fe–3S] clusters have been replaced by ligands from the

Cys 87␣ amide N and the Ser 186␤ side chain O yielding a

[4Fe–3S] cluster (left) and a [4Fe–4S] cluster (right) that remain

linked by a direct Fe¬S bond and two bridging Cys thiols.

and a [1Mo–3Fe–3S] cluster that are bridged by three sulfide

ions.The FeMo-cofactor is linked to the protein by only two

ligands at its opposite ends, one from His 442␣ to the Mo atom

and the other from Cys 275␣ to an Fe atom. The Mo atom is

additionally doubly liganded by homocitrate. What is most likely

an N atom (blue sphere) is liganded to the FeMo-cluster’s six

central Fe atoms (dashed black lines). [Parts a and b based on

X-ray structures by David Lawson, John Innes Centre, Norwich,

U.K. Part c based on an X-ray structure by Douglas Rees,

California Institute of Technology. PDBids (a) 1QGU, (b) 1QH1,

and (c) 1M1N.]

(a) (b)

(c)

JWCL281_c26_1019-1087.qxd 4/20/10 9:26 AM Page 1081

sites in these proteins; Section 10-2).The P-cluster, which is

also ⬃10 Å below the protein surface,is at the interface be-

tween the ␣ and ␤ subunits on the pseudo-2-fold axis that

roughly relates these two subunits. The 2-fold axis of the

Fe-protein and the pseudo-2-fold axis of the MoFe-pro-

teins coincide in their complex.

The Fe-protein hydrolyzes two ATP molecules for each

electron it transfers from its [4Fe–4S]

⫹1

cluster to the

P-cluster. Since the nucleotide binding sites and the [4Fe–4S]

cluster on the Fe-protein are separated by ⬃20 Å, a distance

too large for direct coupling between electron transfer and

ATP hydrolysis, it appears that these processes are alloster-

ically coupled through conformational changes at the sub-

unit interface. Indeed, portions of the Fe-protein resemble

those of G-proteins, in which nucleotide hydrolysis is cou-

pled to conformational changes controlling the protein’s ac-

tions (Sections 19-2Cb and 19-3Cf).Specifically,two regions

of the Fe-protein, designated Switch I and Switch II (Fig.26-

67),are homologous with those of Ras (Section 19-3Cf).The

binding of ADP ⴢ AIF

⫺

to Fe-protein induces conforma-

tional changes in Switch I that affect the interactions be-

tween the Fe-protein and the MoFe-protein,and in Switch II

that affect the environment of the [4Fe–4S] cluster.

In nitrogenase, the [4Fe–4S] cluster of the Fe-protein ap-

proaches within ⬃14 Å of the P-cluster in the MoFe-protein,

whereas the P-cluster and the FeMo-cofactor are ⬃13 Å

apart. Hence, the sequence of the electron-transfer steps in

the nitrogenase reaction appears to be

It therefore seems that the role of ATP hydrolysis is to sta-

bilize a conformation in the Fe-protein that it cannot

achieve on its own and which facilitates electron transfer

from the [4Fe–4S] cluster on the Fe-protein to the P-cluster

on the MoFe-protein.

b. N

Reduction Is Energetically Costly

Nitrogen fixation requires two participants in addition

to N

and nitrogenase: (1) a source of electrons and (2) ATP.

Electrons are generated either oxidatively or photosyn-

thetically, depending on the organism. These electrons are

transferred to ferredoxin (Section 22-2C1a), a [4Fe–4S]-

containing electron carrier that transfers an electron to the

Fe-protein of nitrogenase, beginning the nitrogen fixation

process (Fig. 26-69). Two molecules of ATP bind to the

FeMo-cofactor

[4Fe–4s] cluster

P-cluster

reduced Fe-protein and are hydrolyzed as each electron is

passed from the Fe-protein to the MoFe-protein.The ATP

hydrolysis-induced conformational change in the Fe-protein

alters its redox potential from ⫺0.29 to ⫺0.40 V, making

the electron capable of N

reduction ( for

the half-cell N

⫹ 6H

⫹

⫹ 6e

⫺

34 2NH

The actual reduction of N

occurs on the MoFe-protein

in three discrete steps, each involving an electron pair:

An electron transfer must occur six times per N

molecule

fixed so that a total of 12 ATPs are required to fix one N

molecule. Although the N

binding site is almost certainly

the FeMo-cofactor, exactly how the N

is bound and re-

duced are largely a matter of speculation.Theoretical stud-

ies suggest that the FeMo-cofactor’s prismatically arranged

Fe atoms provide favorable interaction sites for N

and its

reduction products. Indeed, it seems highly likely that the

putative N atom that is liganded to the FeMo-cofactor

(Fig. 26-68c) participates in N

reduction.

Nitrogenase also reduces H

O to H

, which in turn re-

acts with diimine to reform N

The resulting futile cycle is favored when the ATP level is

low and/or the reduction of the Fe-protein is sluggish. Even

when ATP is plentiful, however, the cycle cannot be sup-

pressed beyond about one H

molecule produced per N

reduced and hence appears to be a requirement of the ni-

trogenase reaction. The total cost of N

reduction is there-

fore 8 electrons transferred and 16 ATPs hydrolyzed (phys-

iologically, 20–30 ATPs). Hence nitrogen fixation is an

energetically expensive process; indeed, the nitrogen-fixing

bacteria in the root nodules of pea plants consume nearly

20% of the ATP that the plant produces.

c. Leghemoglobin Protects Nitrogenase

from Oxygen Inactivation

Nitrogenase is rapidly inactivated by O

,so the enzyme must

be protected from this reactive substance. Cyanobacteria

HN“NH ⫹ H

⫹ 2H

+ 2e

–

+ 2e

–

+ 2e

–

NHH

N N 2NH

HydrazineDiimine

e°¿ ⫽⫺0.34 V

1082 Chapter 26. Amino Acid Metabolism

(Ferredoxin)

red

(Ferredoxin)

photosynthesis

or oxidative

electron

transport

(Fe-protein)

red

(MoFe-protein)

red

(MoFe-protein)

+2 H

+2ADP

2ATP

8 times

Figure 26-69 The flow of electrons in

the nitrogenase-catalyzed reduction of N

JWCL281_c26_1019-1087.qxd 4/20/10 9:26 AM Page 1082

fix their own nitrogen, a complex undertaking in which the

plant must be made to provide a hospitable environment

for nitrogen fixation as well as to acquire the enzymatic

machinery to do so. This would free farmers, particularly

those in developing countries, from the need for either pur-

chasing fertilizers, periodically letting their fields lie fallow

(giving legumes the opportunity to grow), or following the

slash-and-burn techniques that are rapidly destroying the

world’s tropical forests and contributing significantly to the

greenhouse effect (atmospheric CO

pollution causing

long-term global warming).

e. The Nitrogen Cycle Describes the Interconversion

of Nitrogen in the Biosphere

The ammonia produced by the nitrogenase reaction and

incorporated into amino acids is eventually recycled in the

biosphere as described by the nitrogen cycle (Fig. 26-70).

Nitrate is produced by certain bacteria that oxidize NH

⫺

and then NO

⫺

, a process called nitrification. Still

other organisms convert nitrate back to N

, which is known

as denitrification. In addition, nitrate is reduced to NH

plants, fungi, and many bacteria, a process called ammoni-

fication in which nitrate reductase catalyzes the two-electron

reduction of nitrate to nitrite (NO

⫺

and then nitrite reductase converts nitrite to ammonia,

The direct anaerobic oxidation of NH

back to N

without

the intermediacy of nitrate, the reverse of nitrogen fixa-

tion, has recently been discovered in certain bacteria.

⫺

⫹ 7H

⫹

⫹ 6e

⫺

⫹ 2H

⫺

⫹ 2H

⫹

⫹ 2e

⫺

⫹ H

Section 26-6. Nitrogen Fixation 1083

Atmospheric

–

Nitrite

Nitrication

Ammonication

Nitrogen xation

Nitrate

Ammonia

Assimilation

N-containing

biomolecules

Decomposition

nitrogenase

nitrite

reductase

nitrate

reductase

nitrogenase

nitrite

reductase

nitrate

reductase

Dentrication

Figure 26-70 The nitrogen cycle. Nitrogen

fixation by nitrogenase converts N

to the biologically

useful ammonia. Nitrate can also be converted to ammonia

by the sequential actions of nitrate reductase and nitrite reductase.

Ammonia is transformed to N

by nitrification followed by

denitrification.Ammonia may be assimilated into nitrogen-containing

biomolecules, which may be decomposed back to ammonia.

(photosynthetic oxygen-evolving bacteria; Section 1-1Ab)

provide protection by carrying out nitrogen fixation in spe-

cialized nonphotosynthetic cells called heterocysts, which

have Photosystem I but lack Photosystem II (Section 24-

2Ca). In the root nodules of legumes (Fig. 26-66), however,

protection is afforded by the symbiotic synthesis of leghemo-

globin. The globin portion of this ⬃145-residue monomeric

oxygen-binding protein is synthesized by the plant (an evolu-

tionary curiosity since globins are otherwise known to occur

only in animals), whereas the heme is synthesized by the Rhi-

zobium. Leghemoglobin has a very high O

affinity, thus

keeping the pO

low enough to protect the nitrogenase while

providing passive O

transport for the aerobic bacterium.

d. Installing the Nitrogen Fixation Machinery in

Nonleguminous Plants Would Revolutionize

Agriculture

Although atmospheric N

is the ultimate nitrogen source

for all living things, most plants do not support the symbi-

otic growth of nitrogen-fixing bacteria.They must therefore

depend on a source of “prefixed” nitrogen such as nitrate or

ammonia. These nutrients come from lightning discharges

(the source of ⬃10% of naturally fixed N

), decaying or-

ganic matter in the soil, or from fertilizer applied to it. The

Haber process, which was invented by Fritz Haber in 1910,

is a chemical process for N

fixation that is still widely used

in fertilizer manufacture. This direct reduction of N

by H

to form NH

requires temperatures of 300 to 500°C, pres-

sures of ⬎300 atm, and an Fe catalyst. Intriguingly, the spac-

ing of the Fe atoms on the surface of this catalyst resembles

that of the FeMo-cofactor’s central Fe atoms (Fig. 26-68c).

One of the major long-term goals of genetic engineering

is to induce agriculturally useful nonleguminous plants to

JWCL281_c26_1019-1087.qxd 6/8/10 9:38 AM Page 1083

1084 Chapter 26. Amino Acid Metabolism

1 Amino Acid Deamination Amino acids are the precur-

sors for numerous nitrogen-containing compounds such as

heme, physiologically active amines, and glutathione. Excess

amino acids are converted to common metabolic intermedi-

ates for use as fuels. The first step in amino acid breakdown is

removal of the ␣-amino group by transamination. Transami-

nases require pyridoxal phosphate (PLP) and convert amino

acids to their corresponding ␣-keto acids. The amino group is

transferred to ␣-ketoglutarate to form glutamate, oxaloac-

etate to form aspartate, or pyruvate to form alanine. Gluta-

mate is subsequently oxidatively deaminated by glutamate de-

hydrogenase (GDH) to form ammonia and regenerate

␣-ketoglutarate. Hyperinsulinism/hyperammonemia (HI/HA),

a genetic disease, is caused by a mutation of the GDH gene

that decreases GTP’s ability to inhibit GDH.

2 The Urea Cycle In the urea cycle, amino groups from

and aspartate combine with HCO

⫺

to form urea. This

pathway takes place in the liver,partially in the mitochondrion

and partially in the cytosol. It begins with the ATP-dependent

condensation of NH

and HCO

⫺

by carbamoyl phosphate syn-

thetase, an enzyme with a 96-Å-long tunnel connecting its

three active sites through which its highly reactive intermedi-

ate products are channeled. The resulting carbamoyl phos-

phate then combines with ornithine to yield citrulline, which

combines with aspartate to form argininosuccinate, which in

turn is cleaved to fumarate and arginine. The arginine is then

hydrolyzed to urea, which is excreted, and ornithine, which

reenters the urea cycle. N-Acetylglutamate regulates the

urea cycle by activating carbamoyl phosphate synthetase

allosterically.

3 Metabolic Breakdown of Individual Amino Acids The

␣-keto acid products of transamination reactions are de-

graded to citric acid cycle intermediates or their precursors.

The amino acids leucine and lysine are ketogenic in that they

are converted only to the ketone body precursors acetyl-CoA

and acetoacetate. The remaining amino acids are, at least in

part, glucogenic in that they are converted to the glucose pre-

cursors pyruvate, oxaloacetate, ␣-ketoglutarate, succinyl-CoA,

or fumarate. Alanine, cysteine, glycine, serine, and threonine

are converted to pyruvate. Serine hydroxymethyltransferase

catalyzes the PLP-dependent C

␣

¬C

␤

bond cleavage of serine

to form glycine. This reaction requires the transfer of a meth-

ylene group from N

-methylene-tetrahydrofolate, which

the tetrahydrofolate (THF) obtains from the glycine cleavage

system, a multienzyme system. Asparagine and aspartate are

converted to oxaloacetate. ␣-Ketoglutarate is a product of

arginine, glutamate, glutamine, histidine, and proline degrada-

tion. Methionine, isoleucine, and valine are degraded to suc-

cinyl-CoA. Methionine breakdown involves the synthesis of

S-adenosylmethionine (SAM), a sulfonium ion that acts as a

methyl donor in many biosynthetic reactions. Hyperhomocys-

teinemia, a risk factor for cardiovascular disease, cognitive im-

pairment, and neural tube defects, is caused by a deficiency in

its folate-dependent degradation. Maple syrup urine disease

(MSUD) is caused by an inherited defect in branched-chain

amino acid degradation. Branched-chain amino acid degra-

dation pathways contain reactions common to all acyl-CoA

oxidations. Tryptophan is degraded to alanine and acetoac-

etate. Phenylalanine and tyrosine are degraded to fumarate

and acetoacetate. Most individuals with the hereditary disease

phenylketonuria lack phenylalanine hydroxylase (PAH), which

converts phenylalanine to tyrosine.

4 Amino Acids as Biosynthetic Precursors Heme is syn-

thesized from glycine and succinyl-CoA. These precursors

condense to form ␦-aminolevulinic acid (ALA), which cyclizes

to form the pyrrole porphobilinogen (PBG). Four molecules

of PBG condense to form uroporphyrinogen III, which then

goes on to form heme, with the final reaction, the insertion of

Fe(II) into protoporphyrin IX, catalyzed by ferrochelatase.

Defects in heme biosynthesis, which are known as porphyrias,

have a variety of bizarre symptoms. Heme is degraded to form

linear tetrapyrroles, which are subsequently excreted as bile pig-

ments.The hormones and neurotransmitters

L-DOPA, epineph-

rine, norepinephrine, serotonin, ␥-aminobutyric acid (GABA),

and histamine are all synthesized from amino acid precursors.

Glutathione, a tripeptide that is synthesized from glutamate,

cysteine, and glycine, is involved in a variety of protective,

transport, and metabolic processes.Tetrahydrofolate is a coen-

zyme that participates in the transfer of C

units.

5 Amino Acid Biosynthesis Amino acids are required

for many vital functions of an organism. Those amino acids

that mammals can synthesize from common ␣-keto acid car-

bon skeletons and preformed ␣-amino nitrogen such as that of

glutamate are known as nonessential amino acids; those that

mammals must obtain from their diets are called essential

amino acids. The biosynthesis of nonessential amino acids in-

volves relatively simple pathways, whereas those forming the

essential amino acids are generally more complex.

6 Nitrogen Fixation Although the ultimate source of ni-

trogen for amino acid biosynthesis is atmospheric N

, this

nearly inert gas must first be reduced to a metabolically useful

form, NH

, by nitrogen fixation. This process occurs only in

certain types of bacteria, one genus of which occurs in symbi-

otic relationship with legumes. N

is fixed in these organisms

by an oxygen-sensitive enzyme, nitrogenase, that consists of

two proteins: the Fe-protein dimer, which contains one

[4Fe–4S] cluster and two ATP binding sites, and the MoFe-

protein ␣

␤

tetramer, which contains one P-cluster (consist-

ing of two [4Fe–3S] clusters linked by a sulfide ion) and one

FeMo-cofactor (a [4Fe–3S] cluster and a [1Mo–3Fe–3S] clus-

ter bridged by three sulfide ions and coordinated with homo-

citrate) in each ␣␤ dimer.These cofactors each function as two-

electron carriers for the ATP-driven reduction of N

to NH

CHAPTER SUMMARY

JWCL281_c26_1019-1087.qxd 6/8/10 9:38 AM Page 1084

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(2001).

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1086 Chapter 26. Amino Acid Metabolism

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Problems 1087

1. Write the reaction for the transamination of an amino acid

in terms of Cleland notation (Section 14-5A).

2. Explain why the symptoms of the partial deficiency of a

urea cycle enzyme may be attenuated by a low-protein diet.

3. Why are people on a high-protein diet instructed to drink

lots of water?

4. A student on a particular diet expends 10,000 kJ ⴢ day

⫺1

while excreting 40 g of urea. Assuming that protein is 16% N by

weight and that its metabolism yields 18 kJ ⴢ g

⫺1

, what percentage

of the student’s energy requirement is met by protein?

5. Production of the enzymes that catalyze the reactions of the

urea cycle can increase or decrease according to the metabolic

needs of the organism. High levels of these enzymes are associ-

ated with high-protein diets as well as starvation. Explain this ap-

parent paradox.

6. Helicobacter pylori, the bacterium responsible for gastric

ulcers, can survive in the stomach (where the pH is as low as 1.5)

in part because it synthesizes large amounts of the enzyme urease.

(a) Write the reaction for urea hydrolysis by urease. (b) Explain

why this reaction could help establish a more hospitable environ-

ment for H. pylori, which tolerates acid but prefers to grow at

near-neutral pH.

7. Why are phenylketonurics warned against eating products

containing the artificial sweetener aspartame (NutraSweet; chem-

ical name

L-aspartyl-L-phenylalanine methyl ester)?

8. Demonstrate that the synthesis of heme from PBG as la-

beled in Fig. 26-35 results in the heme-labeling pattern given in

Fig. 26-32.

9. Explain why certain drugs and other chemicals can precipi-

tate an attack of acute intermittent porphyria.

10. Heterozygotes for erythropoietic protoporphyria show

only 20 to 30% residual ferrochelatase activity rather than the

50% that is normally expected for an autosomal dominant inher-

ited disease. Provide a plausible explanation for this observation.

11. One of the symptoms of kwashiorkor, the dietary protein

deficiency disease in children, is the depigmentation of the skin

and hair. Explain the biochemical basis of this symptom.

12. What are the metabolic consequences of a defective uridylyl-

removing enzyme in E. coli?

13. Figure 26-60, Reaction 9, indicates that methionine is syn-

thesized in microorganisms by the methylation of homocysteine

in a reaction in which N

-methyl-THF is the methyl donor.Yet, in

the breakdown of methionine (Fig. 26-18), its demethylation oc-

curs in three steps in which SAM is an intermediate. Discuss why

this reaction does not occur via the simpler one-step reversal of

the methylation reaction.

*14. In the glucose–alanine cycle (Fig. 26-3), glycolytically de-

rived pyruvate is transaminated to alanine and exported to the

liver for conversion to glucose and return to the cell. Explain how

a muscle cell is able to participate in this cycle under anaerobic

(vigorously contracting) conditions. (Hint: The breakdown of

many amino acids yields NH

15. Draw the activated intermediates involved in (a) glutamine

and (b) asparagine biosynthesis from glutamate and aspartate, re-

spectively. (c) Provide an example of another metabolic activation

of a carboxylic acid group analogous to each of these reactions.

16. The ␣

␤

tetramer of tryptophan synthase catalyzes the

PLP-dependent reaction of indole-3-glycerol phosphate and ser-

ine to form tryptophan (Fig. 26-63, Reactions 5 and 6). Draw the

chemical reactions involved in this synthesis, including the partic-

ipation of PLP, and use curved arrows to show the flow of elec-

trons.What role does PLP play in the reaction?

17. Suggest a reason why the nitrogen-fixing heterocysts of

cyanobacteria have lost Photosystem II but retain Photosystem I.

PROBLEMS

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1088

NPY/

AgRP

POMC/

CART

–

CHAPTER 27

Energy Metabolism:

Integration and Organ

Specialization

1 Major Pathways and Strategies of Energy

Metabolism: A Summary

2 Organ Specialization

A. Brain

B. Muscle

C. Adipose Tissue

D. Liver

E. Kidney

3 Metabolic Homeostasis: Regulation of Appetite,

Energy Expenditure, and Body Weight

A. AMP-Dependent Protein Kinase Is the Cell’s Fuel Gauge

B. Adiponectin Regulates AMPK Activity

C. Leptin

D. Insulin

E. Ghrelin and PYY

3–36

F. Hypothalamic Integration of Hormonal Signals

G. Control of Energy Expenditure by Adaptive Thermogenesis

H. Did Leptin Evolve as a Thrifty Gene?

4 Metabolic Adaptation

A. Starvation

B. Diabetes Mellitus

At this point in our narrative we have studied all of the ma-

jor pathways of energy metabolism. Consequently, we are

now in a position to consider how organisms, mammals in

particular, orchestrate the metabolic symphony to meet

their energy needs. This chapter therefore begins with a re-

capitulation of the major metabolic pathways and their

control systems, then considers how these processes are ap-

portioned among the various organs of the body, and ends

with a discussion of metabolic adaptation, including how

the body maintains energy balance (homeostasis), how it

deals with the metabolic challenges of starvation and obe-

sity, and how it responds to the loss of control resulting

from diabetes mellitus.

1 MAJOR PATHWAYS AND

STRATEGIES OF ENERGY

METABOLISM: A SUMMARY

Figure 27-1 indicates the interrelationships among the ma-

jor pathways involved in energy metabolism. Let us review

these pathways and their control mechanisms.

1. Glycolysis (Chapter 17) The metabolic degrada-

tion of glucose begins with its conversion to two molecules

of pyruvate with the net generation of two molecules each

of ATP and NADH. Under anaerobic conditions, pyruvate

is converted to lactate (or, in yeast, to ethanol) so as to re-

cycle the NADH. Under aerobic conditions, however,

when glycolysis serves to prepare glucose for further oxi-

dation, the NAD

⫹

is regenerated through oxidative phos-

phorylation (see below). The flow of metabolites through

the glycolytic pathway is largely controlled by the activity

of phosphofructokinase (PFK).This enzyme is activated by

AMP and ADP, whose concentrations rise as the need for

energy metabolism increases, and is inhibited by ATP and

citrate, whose concentrations increase when the demand

for energy metabolism has slackened. Citrate, a citric acid

cycle intermediate, also inhibits PFK and glycolysis when

aerobic metabolism takes over from anaerobic metabo-

lism, making glucose oxidation more efficient (the Pasteur

effect; Section 22-4C), and when fatty acid and/or ketone

body oxidation (which are also aerobic pathways) are

providing for energy needs (the glucose–fatty acid or

Randle cycle; Section 22-4Bb). PFK is also activated by

fructose-2,6-bisphosphate, whose concentration is regulated

by the levels of glucagon, epinephrine, and norepinephrine

through the intermediacy of cAMP (Section 18-3Fc). Liver

and heart muscle F2,6P levels are regulated oppositely: A

[cAMP] increase causes an [F2,6P] decrease in liver and an

[F2,6P] increase in heart muscle. However, skeletal muscle

[F2,6P] does not respond to changes in [cAMP].

2. Gluconeogenesis (Section 23-1) Mammals can syn-

thesize glucose from a variety of precursors, including

pyruvate, lactate, glycerol, and glucogenic amino acids (but

not fatty acids), through pathways that occur mainly in

liver and kidney. Many of these precursors are converted to

oxaloacetate which, in turn, is converted to phospho-

enolpyruvate and then, through a series of reactions that

largely reverse the path of glycolysis, to glucose. The irre-

versible steps of glycolysis, those catalyzed by PFK and

hexokinase, are bypassed in gluconeogenesis by hydrolytic

reactions catalyzed, respectively, by fructose-1,6-bisphos-

phatase (FBPase) and glucose-6-phosphatase. FBPase and

PFK may both be at least partially active simultaneously,

creating a substrate cycle.This cycle, and the reciprocal reg-

ulation of PFK and FBPase, are important in regulating

both the rate and direction of flux through glycolysis and

JWCL281_c27_1088-1106.qxd 4/22/10 9:58 AM Page 1088

gluconeogenesis (Sections 17-4F and 23-1B). Fatty acid and

ketone body oxidation can increase the rate of gluconeogen-

esis in liver by decreasing the concentration of F2,6P

(Section 18-3Fc). This occurs because the increased citrate

concentration accompanying activation of the citric acid

cycle during fatty acid oxidation inhibits PFK-2 as well as

PFK (Table 23-1). Phosphoenolpyruvate carboxykinase

(PEPCK) bypasses the third irreversible reaction of glycol-

ysis, that catalyzed by pyruvate kinase (PK), and is con-

trolled exclusively by long-term transcriptional regulation.

3. Glycogen degradation and synthesis (Chapter 18)

Glycogen, the storage form of glucose in animals, occurs

mostly in liver and muscle. Its conversion to glucose-6-

phosphate (G6P) for entry into glycolysis in muscle and its

conversion to glucose in liver is catalyzed, in part, by glyco-

gen phosphorylase, whereas the opposing synthetic path-

way is mediated by glycogen synthase. These enzymes are

reciprocally regulated through phosphorylation/dephos-

phorylation reactions as catalyzed by amplifying cascades

that respond to the levels of the hormones glucagon and

epinephrine through the intermediacy of cAMP, and by

insulin (Sections 18-3E and 19-4F). The glucagon–insulin

ratio is therefore a crucial factor in determining the rate and

direction of glycogen metabolism.

4. Fatty acid degradation and synthesis (Sections 25-1

through 25-5) Fatty acids are broken down in increments

of C

units through ␤ oxidation to form acetyl-CoA. They

are synthesized from this compound via a separate path-

way.The activity of the ␤-oxidation pathway varies with the

fatty acid concentration.This, in turn, depends on the activ-

ity of “hormone-sensitive” triacylglycerol lipase in adipose

tissue that is stimulated, through cAMP-regulated phos-

phorylation/dephosphorylation reactions, by glucagon and

epinephrine but inhibited by insulin.The fatty acid synthe-

sis rate varies with the activity of acetyl-CoA carboxylase,

Section 27-1. Major Pathways and Strategies of Energy Metabolism: A Summary 1089

glucose-6-

phosphatase

fructose-1,6-

bisphosphatase

phosphofructokinase

acetyl-CoA carboxylase

Glucose

Glycogen

hexo-

kinase

glycogen

synthase

glycogen

phosphorylase

Glucose-6-phosphate

pentose

phosphate

pathway

NADPH

Ribose-5-phosphate

gluconeo-

genesis

glycolysis

Phosphoenol-

pyruvate

kinase

Pyruvate

pyruvate

dehydro-

genase

Acetyl-CoA

Citric

acid

cycle

Glucogenic

amino

acids

Oxaloacetate

NADH + FADH

ATP

Oxidative

phosphorylation

ATP

Ketone bodies

Ketogenic amino acids

pyruvate

carboxylase

phosphoenol-

pyruvate

carboxykinase

Lactate

NADH

NAPDH

lactate

dehydrogenase

Triacylglycerols

Fatty acids

hormone-

sensitive

triacylglycerol

lipase

triacylglycerol

synthesis

oxidation

fatty acid

synthesis

FADH

ATP

Figure 27-1 The major energy metabolism pathways.

JWCL281_c27_1088-1106.qxd 6/8/10 8:48 AM Page 1089

which is activated by citrate and insulin-dependent de-

phosphorylation, and inhibited by the pathway product

palmitoyl-CoA and by cAMP- and AMP-dependent phos-

phorylation. Fatty acid synthesis is also subject to long-

term regulation through alterations in the rates of synthe-

sis of the enzymes mediating this process as stimulated by

insulin and inhibited by fasting. The glucagon–insulin ratio

is therefore of prime importance in determining the rate and

direction of fatty acid metabolism.

5. Citric acid cycle (Chapter 21) The citric acid cycle

oxidizes acetyl-CoA, the common degradation product of

glucose, fatty acids, ketone bodies, and ketogenic amino

acids, to CO

and H

O with the concomitant production of

NADH and FADH

. Many glucogenic amino acids can also

be oxidized via the citric acid cycle through their break-

down, ultimately to pyruvate and then to acetyl-CoA,

sometimes via the cataplerosis (using up) of a citric acid cy-

cle intermediate (Section 21-5). The activities of the citric

acid cycle regulatory enzymes citrate synthase, isocitrate

dehydrogenase, and ␣-ketoglutarate dehydrogenase are

controlled by substrate availability and feedback inhibition

by cycle intermediates, NADH, and ATP.

6. Oxidative phosphorylation (Chapter 22) This mito-

chondrial pathway oxidizes NADH and FADH

to NAD

⫹

and FAD with the coupled synthesis of ATP. The rate of

oxidative phosphorylation, which is tightly coordinated

with the metabolic fluxes through glycolysis and the citric

acid cycle, is largely dependent on the concentrations of

ATP, ADP, and P

, as well as O

7. Pentose phosphate pathway (Section 23-4) This

pathway functions to generate NADPH for use in reductive

biosynthesis, as well as the nucleotide precursor ribose-5-

phosphate, through the oxidation of G6P. Its flux-generating

step is catalyzed by glucose-6-phosphate dehydrogenase,

which is controlled by the level of NADP

⫹

. The ability of

enzymes to distinguish between NADH, which is mainly

utilized in energy metabolism, and NADPH permits energy

metabolism and biosynthesis to be regulated independently.

8. Amino acid degradation and synthesis (Sections 26-1

through 26-5) Excess amino acids may be degraded to com-

mon metabolic intermediates. Most of these pathways begin

with an amino acid’s transamination to its corresponding ␣-

keto acid with the eventual transfer of the amino group to

urea via the urea cycle. Leucine and lysine are ketogenic

amino acids in that they can be converted only to acetyl-CoA

or acetoacetate and hence cannot be glucose precursors. The

other amino acids are glucogenic in that they may be, at least

in part, converted to one of the glucose precursors pyruvate,

oxaloacetate, ␣-ketoglutarate, succinyl-CoA, or fumarate.

Five amino acids are both ketogenic and glucogenic. Essen-

tial amino acids are those that an animal cannot synthesize it-

self; they must be obtained from plant and microbial sources.

Nonessential amino acids can be synthesized by animals uti-

lizing preformed amino groups via pathways that are gener-

ally simpler than those synthesizing essential amino acids.

Two compounds lie at the crossroads of the foregoing

metabolic pathways: acetyl-CoA and pyruvate (Fig. 27-1).

Acetyl-CoA is the common degradation product of most

metabolic fuels, including polysaccharides, lipids, and pro-

teins. Its acetyl group may be oxidized to CO

and H

O via

the citric acid cycle and oxidative phosphorylation or used

to synthesize fatty acids. Pyruvate is the product of glycoly-

sis, the dehydrogenation of lactate, and the breakdown of

certain glucogenic amino acids. It may be oxidatively decar-

boxylated to yield acetyl-CoA, thereby committing its

atoms either to oxidation or to the biosynthesis of fatty

acids.Alternatively, it may be carboxylated via the pyruvate

carboxylase reaction to form oxaloacetate, which, in turn,

either replenishes citric acid cycle intermediates or enters

gluconeogenesis via phosphoenolpyruvate, thereby bypass-

ing an irreversible step in glycolysis. Pyruvate is therefore a

precursor of several amino acids as well as of glucose.

The foregoing pathways occur in specific cellular com-

partments. Glycolysis, glycogen synthesis and degradation,

fatty acid synthesis, and the pentose phosphate pathway

are largely or entirely cytosolically based, whereas fatty

acid degradation, the citric acid cycle, and oxidative phos-

phorylation occur in the mitochondrion. Different phases

of gluconeogenesis and amino acid degradation occur in

each of these compartments. The flow of metabolites across

compartment membranes is mediated, in most cases, by spe-

cific carriers that are also subject to regulation.

The enormous number of enzymatic reactions that simul-

taneously occur in every cell (Fig. 16-1) must be coordinated

and strictly controlled to meet the cell’s needs. Such regula-

tion occurs on many levels. Intercellular communications

regulating metabolism occur via certain hormones, including

epinephrine, norepinephrine, glucagon, and insulin, as well as

through a series of steroid hormones known as glucocorti-

coids (whose effects are discussed in Section 19-1Ga). These

hormonal signals trigger a variety of cellular responses, in-

cluding the synthesis of second messengers such as cAMP in

the short term and the modulation of protein synthesis rates

in the long term. On the molecular level, enzymatic reaction

rates are controlled by phosphorylation/dephosphorylation

via amplifying reaction cascades, by allosteric responses to

the presence of effectors, which are usually precursors or

products of the reaction pathway being controlled, and by

substrate availability. The regulatory machinery of opposing

catabolic and anabolic pathways is generally arranged such

that these pathways are reciprocally regulated.

2 ORGAN SPECIALIZATION

Different organs have different metabolic functions and

capabilities. In this section we consider how the special

needs of the mammalian body organs are met and how

their metabolic capabilities are coordinated to meet these

needs. In particular, we discuss brain, muscle, adipose tis-

sue, liver, and kidney (Fig. 27-2).

A. Brain

Brain tissue has a remarkably high respiration rate. For in-

stance, the human brain constitutes only ⬃2% of the adult

1090 Chapter 27. Energy Metabolism: Integration and Organ Specialization

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