Voet D., Voet Ju.G. Biochemistry

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cleavage of the b-keto acid intermediate to yield an addi-

tional resonance-stabilized carboxylate group.

RuBP carboxylase activity requires a bound divalent

metal ion, physiologically Mg

2⫹

, which acts to stabilize the

developing negative charges during catalysis. The Mg

2⫹

is,

in part, bound to the enzyme by a catalytically essential

carbamate group that is generated by the reaction of a non-

substrate CO

with the ε-amino group of Lys 201 (R¬NH

⫹

S R¬NH¬COO

⫺

⫹ H

⫹

).Although the in vitro acti-

vation reaction occurs spontaneously in the presence of

2⫹

and HCO

⫺

, it is blocked in vivo by the particularly

tight binding of RuBP to active sites lacking carbamate.

This inhibition is relieved, however,by the release of RuBP

in an ATP-driven process catalyzed by RuBP carboxylase

activase.

d. Calvin Cycle Products Are Converted to Starch,

Sucrose, and Cellulose

The overall stoichiometry of the Calvin cycle is

GAP, the primary product of photosynthesis, is used in a va-

riety of biosynthetic pathways, both inside and outside the

chloroplast. For example, fatty acids and amino acids are

synthesized from GAP as is described, respectively, in Sec-

tions 25-4 and 26-5. GAP can also be converted to fructose-

GAP ⫹ 9ADP ⫹ 8P

⫹ 6NADP

⫹

3CO

⫹ 9ATP ⫹ 6NADPH

6-phosphate by the further action of Calvin cycle enzymes

and then to glucose-1-phosphate (G1P) by phosphoglucose

isomerase (Section 17-2B) and phosphoglucomutase

(Section 18-1B). G1P is the precursor of the higher carbohy-

drates characteristic of plants.

The polysaccharide ␣-amylose, a major component of

starch (Section 11-2D), is synthesized in the chloroplast

stroma as a temporary storage depot for glucose units. It is

also synthesized as a long-term storage molecule elsewhere

in the plant, including leaves, seeds, and roots. G1P is first

activated by its reaction with ATP to form ADP–glucose as

catalyzed by ADP–glucose pyrophosphorylase. Starch syn-

thase then transfers the glucose residue to a nonreducing

end of an ␣-amylose or amylopectin molecule, forming a

new glycosidic linkage (Fig. 24-35). The overall reaction is

driven by the exergonic hydrolysis of the PP

released in

the formation of ADP–glucose.A similar reaction sequence

occurs in glycogen synthesis, which uses UDP–glucose

(Section 18-2A). The ␣(1 S 6) branches of amylopectin

(Section 11-2D) are made by starch-branching enzyme,

which functions similarly to glycogen branching enzyme

(Section 18-2C).

Sucrose, a disaccharide of glucose and fructose (Section

11-2B), is the major transport sugar for delivering carbohy-

drates to nonphotosynthesizing cells and hence is the major

photosynthetic product of green leaves. Since sucrose is syn-

thesized in the cytosol, either glyceraldehyde-3-phosphate

Section 24-3. Dark Reactions 931

Figure 24-34 Probable mechanism of the carboxylation

reaction catalyzed by RuBP carboxylase. The reaction proceeds

via an enediolate intermediate that nucleophilically attacks CO

OPO

–

OPO

–

RuBP

Enz-B:

OPO

–

OPO

–

OPO

–

OPO

–

Enediolate ␤-Keto acid

–

OPO

–

OPO

–

OPO

–

OPO

–

HO CO

–

OPO

–

3PG

to form a ␤-keto acid.This intermediate reacts with water to

yield two molecules of 3PG.

See the Animated Figures

JWCL281_c24_901-939.qxd 3/25/10 12:05 PM Page 931

or dihydroxyacetone phosphate is transported out of the

chloroplast by an antiporter that exchanges phosphate for a

triose phosphate. Two trioses combine to form fructose-6-

phosphate (F6P) and subsequently glucose-1-phosphate

(G1P), which is then activated by UTP to form UDP–

glucose. Next, sucrose-6-phosphate is produced in a reaction

catalyzed by sucrose-phosphate synthase. Finally, sucrose-6-

phosphate is hydrolyzed by sucrose-phosphate phosphatase

to yield sucrose (Fig. 24-36), which is then exported to other

plant tissues.

Cellulose, which consists of long chains of ␤(1 S 4)-

linked glucose units and is the major polysaccharide of

plants, is also synthesized from UDP–glucose. Plant cell

walls consist of almost crystalline cables containing 36 par-

allel cellulose chains, each of 500 to 15,000 glucose units, all

embedded in an amorphous matrix of other polysaccha-

rides and lignin (Section 11-2C). Unlike starch in plants or

glycogen in mammals, cellulose is synthesized by multisub-

unit enzyme complexes in the plant plasma membrane and

extruded into the extracellular space.

B. Control of the Calvin Cycle

During the day, plants satisfy their energy needs via the

light and dark reactions of photosynthesis. At night, how-

ever, like other organisms, they must use their nutritional

reserves to generate their required ATP and NADPH

through glycolysis, oxidative phosphorylation, and the pen-

tose phosphate pathway. Since the stroma contains the en-

zymes of glycolysis and the pentose phosphate pathway as

well as those of the Calvin cycle, plants must have a light-

sensitive control mechanism to prevent the Calvin cycle

from consuming this catabolically produced ATP and

NADPH in a wasteful futile cycle.

As we saw in Section 17-4F,the control of flux in a meta-

bolic pathway occurs at enzymatic steps that are far from

equilibrium; that is, those that have a large negative value

of ⌬G. Inspection of Table 24-1 indicates that the four best

candidates for flux control in the Calvin cycle are the reac-

tions catalyzed by phosphoribulokinase, RuBP carboxylase,

FBPase, and SBPase (Reactions 1, 2, 7, and 10, Fig. 24-31).

In fact, the catalytic efficiencies of these four enzymes all

vary, in vivo, with the level of illumination.

932 Chapter 24. Photosynthesis

Figure 24-35 Starch synthesis. ADP–glucose

is formed from G1P and ATP in a

phosphoanhydride exchange reaction (1).The

product is rapidly hydrolyzed.ADP–glucose

is the substrate for starch synthase (2), which

adds the glucose residue to a nonreducing end

of an existing polysaccharide, releasing ADP.

Figure 24-36 The synthesis of sucrose.

Adenosine

ADP

ATP

-Amylose (n residues)

-Amylose (n + 1 residues)

starch synthase

–

␤␥␣

–

ADP – glucose

2 P

inorganic

pyrophos-

phatase

ADP–glucose

pyrophophorylase

–

G1P

–

...

UDP–glucose ⫹ F6P

Sucrose-6-phosphate

sucrose-phosphate

synthase

UDP

sucrose-phosphate

phosphatase

Sucrose

JWCL281_c24_901-939.qxd 3/25/10 12:05 PM Page 932

The activity of RuBP carboxylase responds to three

light-dependent factors:

1. It varies with pH. On illumination, the pH of the

stroma increases from around 7.0 to about 8.0 as protons

are pumped from the stroma into the thylakoid lumen.

RuBP carboxylase has a sharp pH optimum near pH 8.0.

2. It is stimulated by Mg

2⫹

. Recall that the light-induced

influx of protons to the thylakoid lumen is accompanied by

the efflux of Mg

2⫹

to the stroma (Section 24-2Db).

3. It is strongly inhibited by its transition state analog

2-carboxyarabinitol-1-phosphate (CA1P; Section 24-3Ac),

which many plants synthesize only in the dark. RuBP car-

boxylase activase (Section 24-3Ac) also facilitates the re-

lease of the tight-binding CA1P from RuBP carboxylase.

FBPase and SBPase are also activated by increased pH

and Mg

2⫹

,and by NADPH as well.The action of these factors

is complemented by a second regulatory system that re-

sponds to the redox potential of the stroma. Thioredoxin

(Trx), an ⬃105-residue protein that occurs in many types of

cells, contains a redox-active disulfide group. Reduced Trx ac-

tivates five Calvin cycle enzymes by disulfide interchange re-

actions (Fig. 24-37): phosphoribulokinase, glyceraldehyde-3-

phosphate dehydrogenase, FBPase, SBPase, and RuBP

carboxylase activase.This explains why these enzymes are ac-

tivated by reduced disulfide reagents such as dithiothreitol.

The redox level of Trx is maintained by ferredoxin–thiore-

doxin reductase (FTR), which contains a redox-active disul-

fide that is closely associated with a [4Fe–4S] cluster through

which the protein directly responds to the redox state of sol-

uble ferredoxin (Fd) in the stroma. This, as we have seen

(Section 24-2Ch), varies with the illumination level.

Section 24-3. Dark Reactions 933

Figure 24-37 Light-activation mechanism of chloroplast enzymes.

Photoactivated PSI reduces soluble ferredoxin (Fd), which reduces

ferredoxin–thioredoxin reductase, which, in turn, reduces the disulfide

linkage of thioredoxin. Reduced thioredoxin reacts with its target enzymes

by disulfide interchange, thereby activating or deactivating these enzymes.

In the dark, these processses are quickly reversed by reaction with oxygen.

Refer to Fig. 24-31.

Source: Bassham, J.A. and Buchanan, B.B., in Govindjee (Ed.), Photosynthesis, Vol. II, p. 155, Academic Press

(1982).

Step

Enzyme ⌬G°¿ (kJ ⴢ mol

⫺1

) ⌬G (kJ ⴢ mol

⫺1

)

1 Phosphoribulokinase ⴚ21.8 ⴚ15.9

2 Ribulose bisphosphate carboxylase ⴚ35.1 ⴚ41.0

3 ⫹ 4 Phosphoglycerate kinase ⫹⫹18.0 ⫺6.7

glyceraldehyde-3-phosphate

dehydrogenase

5 Triose phosphate isomerase ⫺7.5 ⴚ0.8

6 Aldolase ⴚ21.8 ⴚ1.7

7 Fructose bisphosphatase ⴚ14.2 ⴚ27.2

8 Transketolase ⫹6.3 ⴚ3.8

9 Aldolase ⴚ23.4 ⴚ0.8

10 Sedoheptulose bisphosphatase ⴚ14.2 ⴚ29.7

11 Transketolase ⫹0.4 ⴚ5.9

12 Phosphopentose epimerase ⫹0.8 ⴚ0.4

13 Ribose phosphate isomerase ⫹2.1 ⴚ0.4

Table 24-1 Standard and Physiological Free Energy Changes for the Reactions

of the Calvin Cycle

red

Thioredoxin Thioredoxin

(Enzyme)

inactive

(Enzyme)

active

PSI

red

PSI

red

hν

red ox

Ferredoxin–

thioredoxin

reductase

Ferredoxin–

thioredoxin

reductase

SH S S

SH SH

SH SH S S

JWCL281_c24_901-939.qxd 3/25/10 12:05 PM Page 933

The X-ray structure of a Fd–FTR–Trx complex (Fig.

24-38), determined by Hans Eklund, reveals that redox-

active Cys residues of FTR and thioredoxin have formed

a disulfide bond. Moreover, the distance between this

disulfide bond and the [2Fe–2S] cluster of Fd is only ⬃20

Å as a consequence of the remarkably thin disklike cat-

alytic subunit of FTR, and that the [4Fe–4S] cluster of

FTR lies on a straight line between these two redox cen-

ters. This maximizes the efficiency of electron transfer

from the [2Fe–2S] cluster of Fd to the redox active disul-

fide of Trx.

Reduced Trx also deactivates the chloroplastic enzymes

phosphofructokinase (PFK), the main flux-generating en-

zyme of glycolysis (Section 17-4Fb), and glucose-6-phosphate

dehydrogenase, the first enzyme in the pentose phosphate

pathway (Section 23-4A), whose products, ATP and

NADPH, would otherwise be used by the Calvin cycle in a

futile cycle. Thus in plants, light stimulates the Calvin cycle

while deactivating glycolysis and the pentose phosphate

pathway, whereas darkness has the opposite effect (that is,

the so-called dark reactions do not occur in the dark). More-

over, chloroplast ATP synthase is activated by reduced Trx,

thus preventing it from uselessly hydrolyzing glycolytically

produced ATP in the dark. Indeed, the redox state of Trx

regulates a great variety of plant metabolic processes.

We have seen that ferredoxin reduces ferredoxin–

NADP

⫹

reductase (Section 24-2Ch) and FTR, as well as

supplying electrons to the cyclic pathway of chloroplast

photosynthesis (Section 24-2Ch). In addition, ferredoxin is

the reducing agent for three metabolically essential chloro-

plast enzymes: sulfite reductase (which reduces SO

2⫺

⫺

), nitrite reductase (which reduces NO

⫺

to ), and

glutamate synthase (which catalyzes the reaction of

␣-ketoglutarate and to form glutamate; Section 26-

5Aa).Thus Fd stands at the center of a complex web of en-

zymatic and regulatory processes.

C. Photorespiration and the C

Cycle

It has been known since the 1960s that illuminated plants con-

sume O

and evolve CO

in a pathway distinct from oxidative

phosphorylation. In fact, at low CO

and high O

levels, this

photorespiration process can outstrip photosynthetic CO

fixation. The basis of photorespiration was unexpected: O

competes with CO

as a substrate for RuBP carboxylase

(RuBP carboxylase is therefore also called RuBP carboxy-

lase–oxygenase or RuBisCO). In the oxygenase reaction, O

reacts with RuBisCO’s second substrate, RuBP, to form 3PG

and 2-phosphoglycolate (Fig. 24-39).The 2-phosphoglycolate

⫹

934 Chapter 24. Photosynthesis

Figure 24-38 X-ray structure of a Fd–FTR–Trx complex. The

subunits are drawn in ribbon form with Fd green, the catalytic

subunit of FTR blue, its variable subunit cyan, and Trx magenta.

The [2Fe–2S] cluster of Fd and the [4Fe–4S] cluster of FTR are

drawn in space-filling form with S yellow and Fe red-brown. The

two Cys side chains forming the disulfide bond between FTR

and Trx are shown in stick form with C green and S yellow.

[Based on an X-ray structure by Hans Eklund, Swedish Univer-

sity of Agricultural Sciences, Uppsala, Sweden. PDBid 2PVO.]

Figure 24-39 Probable mechanism of the

oxygenase reaction catalyzed by RuBP

carboxylase–oxygenase. Note the similarity

of this mechanism to that of the carboxylase

reaction catalyzed by the same enzyme

(Fig. 24-34).

OPO

–

OPO

–

RuBP

Enz-B:

OPO

–

OPO

–

OPO

–

OPO

–

Enediolate

–

OPO

–

OPO

–

OPO

–

OPO

–

3PG

2-Phosphoglycolate

OHO

–

JWCL281_c24_901-939.qxd 3/25/10 12:05 PM Page 934

is hydrolyzed to glycolate by glycolate phosphatase and, as

described below, is partially oxidized to yield CO

by a se-

ries of enzymatic reactions that occur in the peroxisome

and the mitochondrion. Thus photorespiration is a seem-

ingly wasteful process that undoes some of the work of

photosynthesis. In the following subsections we discuss the

biochemical basis of photorespiration, its significance, and

how certain plants manage to evade its deleterious effects.

a. Photorespiration Dissipates ATP and NADPH

The photorespiration pathway is outlined in Fig. 24-40.

Glycolate is exported from the chloroplast to the peroxi-

some (also called the glyoxisome, Sections 1-2Ad and 23-2),

where it is oxidized by glycolate oxidase to glyoxylate and

. The H

, a powerful and potentially harmful oxi-

dizing agent, is disproportionated to H

O and O

in the

peroxisome by the heme-containing enzyme catalase.

Some of the glyoxylate is further oxidized by glycolate ox-

idase to oxalate. The remainder is converted to glycine in a

transamination reaction, as discussed in Section 26-1A,and

exported to the mitochondrion. There, two molecules of

glycine are converted to one molecule of serine and one of

by a reaction described in Section 26-3B. This is the

origin of the CO

generated by photorespiration. The serine

is transported back to the peroxisome, where a transamina-

tion reaction converts it to hydroxypyruvate. This sub-

stance is reduced to glycerate and phosphorylated in the

cytosol to 3PG, which reenters the chloroplast, where it is

reconverted to RuBP in the Calvin cycle. The net result of

this complex photorespiration cycle is that some of the ATP

and NADPH generated by the light reactions is uselessly

dissipated.

Although photorespiration has no known metabolic

function, the RuBisCOs from the great variety of photo-

synthetic organisms so far tested all exhibit oxygenase ac-

tivity. Yet, over the eons, the forces of evolution must have

optimized the function of this important enzyme. It is

thought that photosynthesis evolved at a time when

Earth’s atmosphere contained large quantities of CO

and

very little O

, so that photorespiration was of no conse-

quence. It has therefore been suggested that the RuBisCO

reaction has an obligate intermediate that is inherently au-

tooxidizable. Another possibility is that photorespiration

protects the photosynthetic apparatus from photooxida-

tive damage when insufficient CO

is available to other-

wise dissipate its absorbed light energy. This hypothesis is

supported by the observation that when chloroplasts or

leaf cells are brightly illuminated in the absence of both

and O

, their photosynthetic capacity is rapidly and ir-

reversibly lost.

b. Photorespiration Limits the Growth

Rates of Plants

The steady-state CO

concentration attained when a

photosynthetic organism is illuminated in a sealed system

is named its CO

compensation point. For healthy plants,

this is the CO

concentration at which the rates of photo-

synthesis and photorespiration are equal. For many species

it is ⬃40 to 70 ppm (parts per million) CO

(the normal

Section 24-3. Dark Reactions 935

Figure 24-40 Photorespiration. This pathway metabolizes the

phosphoglycolate produced by the RuBP carboxylase-catalyzed

oxidation of RuBP. The reactions occur, as indicated, in the

chloroplast, the peroxisome, the mitochondrion, and the cytosol.

Note that two glycines are required to form serine ⫹ CO

(Section 26-3B).

3PG

Calvin

cycle

RuBP

carboxylase–

oxygenase

+ 3PG

2-Phosphoglycolate

phospho-

glycolate

phosphatase

Glycolate

Peroxisome

ADP

ATP

glycerate kinase

–

NAD

NADH

hydroxy-

pyruvate

reductase

Glycerate

–

Hydroxypyruvate

Glyoxylate

Glycine

catalase

O+O

()

Transamination

–

Serine

CH NH

Mitochondrion

RuBP

2–

glycolate

oxidase

2–

–

CHO

–

Cytosol

Chloroplast

JWCL281_c24_901-939.qxd 6/8/10 8:54 AM Page 935

atmospheric concentration of CO

is 330 ppm), so their

photosynthetic CO

fixation usually dominates their pho-

torespiratory CO

release. However, the CO

compensa-

tion point increases with temperature because the oxyge-

nase activity of RuBisCO increases more rapidly with

temperature than does its carboxylase activity. Thus, on a

hot bright day, when photosynthesis has depleted the level of

at the chloroplast and raised that of O

, the rate of pho-

torespiration may approach that of photosynthesis. This

phenomenon is, in fact, a major limiting factor in the growth

of many plants. Indeed, plants possessing a RuBisCO with

significantly less oxygenase activity would not only have

increased photosynthetic efficiency but would need less

water because they could spend less time with their stomata

(the pores leading to their internal leaf spaces) open ac-

quiring CO

and would have a reduced need for fertilizer

because they would require less RuBisCO. The control of

photorespiration is therefore an important unsolved agri-

cultural problem that is presently being attacked through

genetic engineering studies (Section 5-5).

c. C

Plants Concentrate CO

Certain species of plants, such as sugarcane, corn, and

most important weeds, have a metabolic cycle that concen-

trates CO

in their photosynthetic cells, thereby almost to-

tally preventing photorespiration (their CO

compensation

points are in the range 2 to 5 ppm). The leaves of plants that

have this so-called C

cycle have a characteristic anatomy.

Their fine veins are concentrically surrounded by a single

layer of so-called bundle-sheath cells, which in turn are sur-

rounded by mesophyll cells.

The C

cycle (Fig. 24-41) was elucidated in the 1960s by

Marshall Hatch and Roger Slack. It begins with the uptake

of atmospheric CO

by the mesophyll cells, which, lacking

RuBisCO in their chloroplasts, do so by condensing it as

HCO

⫺

with phosphoenolpyruvate (PEP) to yield oxaloac-

etate. The oxaloacetate is reduced by NADPH to malate,

which is exported to the bundle-sheath cells (the name C

refers to these four-carbon acids). There the malate is ox-

idatively decarboxylated by NADP

⫹

to form CO

, pyru-

vate, and NADPH. The CO

, which has been concentrated

by this process, enters the Calvin cycle. The pyruvate is re-

turned to the mesophyll cells, where it is phosphorylated to

again form PEP. The enzyme that mediates this reaction,

pyruvate-phosphate dikinase, has the unusual action of ac-

tivating a phosphate group through the hydrolysis of ATP

to AMP ⫹ PP

. This PP

is further hydrolyzed to two P

which is tantamount to the consumption of a second ATP.

is thereby concentrated in the bundle-sheath cells at

the expense of two ATPs per CO

. The dark reactions of

photosynthesis in C

plants therefore consume a total of five

ATPs per CO

fixed versus the three ATPs required by the

Calvin cycle alone. The additional ATP is presumably gen-

erated through the cyclic flow of electrons in the light reac-

tions (Section 24-2Ch).

plants, which comprise ⬃5% of terrestrial plants, oc-

cur largely in unshaded areas of tropical regions because

they grow faster under hot and sunny conditions than other,

so-called C

plants (so named because they initially fix CO

in the form of three-carbon acids). In cooler climates, where

photorespiration is less of a burden, C

plants have the ad-

vantage because they require less energy to fix CO

936 Chapter 24. Photosynthesis

Figure 24-41 The C

pathway. CO

is concentrated in the mesophyll cells and transported to

the bundle-sheath cells for entry into the Calvin cycle.

Malate

–

Oxaloacetate

Phosphoenol-

pyruvate (PEP)

OPO

–

–––

PEP carboxylase malate

dehydrogenase

pyruvate-

phosphate

dikinase

Pyruvate

–

ATP + PP

AMP +

2 P

HCO

–

NADPH NADP

malic enzyme

NADPH NADP

+CO

Calvin cycle

3PG

Mesophyll cell

Bundle-sheath cell

Air

carbonic

anhydrase

2–

JWCL281_c24_901-939.qxd 3/25/10 12:05 PM Page 936

d. CAM Plants Store CO

through a Variant

of the C

Cycle

Desert-adapted plants known as succulents (e.g., cacti)

employ a variant of the C

cycle that separates CO

acquisi-

tion and the Calvin cycle in time rather than in space. If, as

most plants, they opened their stomata by day to acquire

, they would simultaneously transpire (lose by evapora-

tion) what for them would be unacceptable amounts of wa-

ter. To minimize this loss, these succulents only absorb CO

at night when the temperature is relatively cool. They store

this CO

, in a process known as crassulacean acid

metabolism (CAM; so named because it was first discovered

in plants of the family Crassulaceae), by the synthesis of

malate through the reactions of the C

pathway (Fig. 24-41).

The large amount of PEP necessary to store a day’s supply

of CO

is obtained by the breakdown of starch via glycolysis.

During the course of the day, this malate is broken down to

, which enters the Calvin cycle, and pyruvate, which is

used to resynthesize starch.CAM plants are able, in this way,

to carry out photosynthesis with minimal water loss.

Chapter Summary 937

1 Chloroplasts Photosynthesis is the light-driven fixa-

tion of CO

to form carbohydrates and other biological mole-

cules. In plants, photosynthesis takes place in the chloroplast,

which consists of an inner and an outer membrane surround-

ing the stroma, a concentrated enzyme solution in which the

thylakoid membrane system is immersed. Photosynthesis oc-

curs in two stages, the so-called light reactions in which light

energy is harnessed to synthesize ATP and NADPH, and the

dark reactions in which these products are used to drive the

synthesis of carbohydrates from CO

and H

O. The thylakoid

membrane is the site of the photosynthetic light reactions,

whereas the dark reactions take place in the stroma.The coun-

terpart of the thylakoid in photosynthetic bacteria is a

specialized portion of the plasma membrane named the

chromatophore.

2 Light Reactions Chlorophyll is the principal photore-

ceptor of photosynthesis. Light is absorbed initially by light-

harvesting complexes (LHCs) that contain chlorophyll and ac-

cessory pigments such as carotenoids. The resulting excitation

then migrates via exciton transfer until it reaches the reaction

center chlorophyll, where it is trapped. LH2 from purple pho-

tosynthetic bacteria is a transmembrane protein that consists

of eight or nine rotationally related subunits that each bind

three BChl a molecules and one carotenoid. LH1, which is

similarly arranged but 16-fold symmetric, contains a central

hole that binds a photosynthetic reaction center (RC). Light

energy absorbed by LH2 is transmitted to LH1, which, in turn,

transmits it to the RC.

The purple photosynthetic bacterial RC (PbRC) is a pro-

tein that consists of three subunits and several redox-active

small molecules that are arranged as two pseudosymmetri-

cally related chains of electron carriers. The primary photon

absorbing species of the Rps. viridis bacterial reaction center is

a special pair of BChl b molecules known as P960. By rapid

measurement techniques it has been determined that the elec-

tron ejected by P960* passes by a third BChl b to a BPheo b

molecule located in only one of the two chains (the other

is apparently nonfunctional) and then sequentially to a

menaquinone (Q

) and a ubiquinone (Q

).The resulting is

subsequently further reduced in a second one-electron trans-

fer process and then takes up two protons from the cytosol to

form Q

. The electrons taken up by this species are re-

turned to P960 via a cytochrome bc

complex, cytochrome c

and, in some purple photosynthetic bacteria, a four-heme c-

type cytochrome associated with the photosynthetic reaction

center. This cyclic electron-transport process functions to

⫺

ⴢ

translocate protons, via a Q cycle mediated by the cytochrome

, from the cytoplasm to the outside of the cell.The resulting

proton gradient, in a process known as photophosphorylation,

drives the synthesis of ATP. Since bacterial photosynthesis

does not generate the reducing equivalents needed in many

biosynthetic processes, photosynthetic bacteria require an

outside source of reducing agents such as H

In plants and cyanobacteria,the light reactions occur in two

reaction centers, those of PSI and PSII, which are electrically

“connected” in series.This enables the system to generate suf-

ficient electromotive force to form NADPH by oxidizing H

in a noncyclic pathway known as the Z-scheme. PSI and PSII

both contain core antenna systems and their RCs are evolu-

tionarily related to each other and to the PbRC. PSII contains

an Mn

CaO

complex that oxidizes two H

O molecules to four

⫹

and O

in four one-electron steps.The electrons are passed

singly, through a Tyr side chain named Z, to photooxidized

P680, the reaction center’s photon-absorbing species, a special

pair that consists of two Chl a molecules. The electron previ-

ously ejected from P680* passes through a series of carriers re-

sembling those of the PbRC to a pool of plastoquinone mole-

cules. The electrons then enter the cytochrome b

f complex,

which transports protons, via a Q cycle, from the stroma to the

thylakoid space. These electrons are transferred individually,

by a plastocyanin carrier, directly to PSI’s photooxidized

photon-absorbing pigment, P700, a pair of Chl a’s that resem-

bles the PbRC’s special pair.The electron that had been previ-

ously ejected from P700* migrates through both sides of a

bifurcated chain of Chl a molecules and then through a chain

of three [4Fe–4S] clusters to a soluble ferredoxin (Fd) that

contains a [2Fe–2S] cluster. The electron then reduces NADP

⫹

in a noncyclic process mediated by ferredoxin–NADP

⫹

reductase. Alternatively, it may be returned, presumably via

ferredoxin–plastoquinone reductase, to the plastoquinone

pool in a cyclic process that does not require electron input

from PSII and only translocates protons across the thylakoid

membrane. ATP is synthesized by the CF

-ATP synthase,

which closely resembles the analogous mitochondrial com-

plex, in a reaction driven by the dissipation of the proton gra-

dient across the thylakoid membrane.

3 Dark Reactions CO

is fixed in the photosynthetic

dark reactions of plants and cyanobacteria by the reactions of

the Calvin cycle.The first stage of the Calvin cycle, in sum, me-

diates the reaction 3RuBP ⫹ 3CO

S 6GAP with the con-

sumption of 9 ATP and 6 NADPH generated by the light reac-

tions. The second stage reshuffles the atoms of five GAPs to

CHAPTER SUMMARY

JWCL281_c24_901-939.qxd 3/25/10 12:05 PM Page 937

reform the three RuBPs with which the cycle began, a process

that requires no further input of free energy or reduction

equivalents. The sixth GAP, the product of the Calvin cycle, is

used to synthesize carbohydrates, amino acids, and fatty acids.

The flux-controlling enzymes of the Calvin cycle are activated

in the light through variations in the pH and the Mg

2⫹

and

NADPH concentrations, and by the redox level of thiore-

doxin.The central enzyme of the Calvin cycle, RuBP carboxy-

lase, catalyzes both a carboxylase and an oxygenase reaction

with RuBP. The latter reaction is the first step in the photores-

piration cycle that liberates CO

.The rate of photorespiration

increases with temperature and decreases with CO

concen-

tration, so photorespiration constitutes a significant energetic

drain on most plants on hot bright days. Calvin cycle products

are converted to sucrose, starch, and cellulose, as well as fatty

acids and amino acids. C

plants, which are most common in

the tropics, have a system for concentrating CO

in their pho-

tosynthetic cells so as to minimize the effects of photorespira-

tion but at the cost of 2 ATP per CO

fixed. Certain desert

plants conserve water by absorbing CO

at night and releasing

it to the Calvin cycle by day.This crassulacean acid metabolism

(CAM) occurs through a process similar to the C

cycle.

938 Chapter 24. Photosynthesis

General

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Nicholls, D.G. and Ferguson, S.J., Bioenergetics 3, Chapter 6,

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Light Reactions

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tosystem I at 3.4 Å resolution, Nature 447, 58–63 (2007).

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structure and evolution of biological solar power, Trends

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Jordan, P., Fromme, P., Witt, H.T., Klukas, O., Saenger, W., and

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tosystem I at 2.5 Å resolution, Nature 411, 909–917 (2001).

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crystal structure of the light-harvesting complex II (B800–850)

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Horthornthwaite-Lawless, A.M., Papiz, M.Z., Cogdell, R.J.,

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Problems 939

1. Why is chlorophyll green in color when it absorbs in the red

and the blue regions of the spectrum (Fig. 24-5)?

2. The “red tide” is a massive proliferation of certain algal

species that cause seawater to become visibly red. Describe the

spectral characteristics of the dominant photosynthetic pigments

in these algae.

3. H

O is added to a suspension of chloroplasts capable of

photosynthesis.Where does the label appear when the suspension

is exposed to light?

4. Indicate, where appropriate, the analogous components in

the photosynthetic electron-transport chains of purple photosyn-

thetic bacteria and chloroplasts.

5. Antimycin inhibits photosynthesis in chloroplasts. Indicate

its most likely site of action and explain your reasoning.

6. Calculate the energy efficiency of cyclic and noncyclic pho-

tosynthesis in chloroplasts using 680-nm light.What would this ef-

ficiency be with 500-nm light? Assume that ATP formation re-

quires 59 kJ ⴢ mol

⫺1

under physiological conditions.

*7. What is the minimum pH gradient required to synthesize

ATP from ADP ⫹ P

? Assume [ATP]/([ADP][P

]) ⫽ 10

,T ⫽ 25°C,

and that three protons must be translocated per ATP generated.

(See Table 16-3 for useful thermodynamic information.)

8. Indicate the average Calvin cycle labeling pattern in

ribulose-5-phosphate after two rounds of exposure to

9. Chloroplasts are illuminated until the levels of their Calvin

cycle intermediates reach a steady state. The light is then turned

off. How do the levels of RuBP and 3PG vary after this time?

10. What is the energy efficiency of the Calvin cycle combined

with glycolysis and oxidative phosphorylation; that is, what per-

centage of the input energy can be metabolically recovered in syn-

thesizing starch from CO

using photosynthetically produced

NADPH and ATP rather than somehow directly storing these

“high-energy” intermediates? Assume that each NADPH is ener-

getically equivalent to three ATPs and that starch synthesis and

breakdown are energetically equivalent to glycogen synthesis and

breakdown.

11. Predict the effect of an uncoupler such as dinitrophenol

(Fig. 22-47) on production of (a) ATP and (b) NADPH in a

chloroplast.

12. Describe the effects of an increase in oxygen pressure on

the dark reactions of photosynthesis.

13. If a C

plant and a C

plant are placed together in a sealed

illuminated box with sufficient moisture, the C

plant thrives while

the C

plant sickens and eventually dies. Explain.

14. The leaves of some species of desert plants taste sour in

the early morning but, as the day wears on, they become tasteless

and then bitter. Explain.

PROBLEMS

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940

CHAPTER 25

Lipid Metabolism

1 Lipid Digestion, Absorption, and Transport

2 Fatty Acid Oxidation

A. Fatty Acid Activation

B. Transport Across the Mitochondrial Membrane

C.  Oxidation

D. Oxidation of Unsaturated Fatty Acids

E. Oxidation of Odd-Chain Fatty Acids

F. Peroxisomal  Oxidation

G. Minor Pathways of Fatty Acid Oxidation

3 Ketone Bodies

4 Fatty Acid Biosynthesis

A. Pathway Overview

B. Acetyl-CoA Carboxylase

C. Fatty Acid Synthase

D. Transport of Mitochondrial Acetyl-CoA Into the Cytosol

E. Elongases and Desaturases

F. Synthesis of Triacylglycerols

5 Regulation of Fatty Acid Metabolism

6 Cholesterol Metabolism

A. Cholesterol Biosynthesis

B. Control of Cholesterol Biosynthesis and Transport

C. Cholesterol Utilization

7 Eicosanoid Metabolism: Prostaglandins,

Prostacyclins, Thromboxanes, Leukotrienes,

and Lipoxins

A. Background

B. The Cyclic Pathway of Eicosanoid Metabolism:

Prostaglandins, Prostacyclins, and Thromboxanes

C. The Linear Pathway of Eicosanoid Metabolism: Leukotrienes

and Lipoxins

8 Phospholipid and Glycolipid Metabolism

A. Glycerophospholipids

B. Sphingophospholipids

C. Sphingoglycolipids

Lipids play indispensable roles in cell structure and metab-

olism. For example, triacylglycerols are the major storage

form of metabolic energy in animals; cholesterol is a vital

component of cell membranes and a precursor of the

steroid hormones and bile salts; arachidonate, a C

unsat-

urated fatty acid, is the precursor of the prostaglandins,

prostacyclins, thromboxanes, leukotrienes, and lipoxins, po-

tent intercellular mediators that control a variety of com-

plex processes; and complex glycolipids and phospholipids

are major components of biological membranes. We dis-

cussed the structures of simple and complex lipids in Sec-

tion 12-1. In the first half of this chapter, we consider the

metabolism of fatty acids and triacylglycerols, including

their digestion, oxidation, and biosynthesis. We then con-

sider how cholesterol is synthesized and utilized, and how

prostaglandins, prostacyclins, thromboxanes, leukotrienes,

and lipoxins are synthesized.We end by studying how com-

plex glycolipids and phospholipids are synthesized from

their simpler lipid and carbohydrate components.

1 LIPID DIGESTION, ABSORPTION,

AND TRANSPORT

Triacylglycerols (also called fats or triglycerides) constitute

⬃90% of the dietary lipid and are the major form of meta-

bolic energy storage in humans. Triacylglycerols consist of

glycerol triesters of fatty acids such as palmitic and oleic

acids

(the names and structural formulas of some biologically

common fatty acids are listed in Table 12-1). Like glucose,

they are metabolically oxidized to CO

and H

O.Yet, since

most carbon atoms of triacylglycerols have lower oxidation

states than those of glucose, the oxidative metabolism of

fats yields over twice the energy of an equal weight of dry

carbohydrate or protein (Table 25-1). Moreover, fats, being

nonpolar, are stored in an anhydrous state, whereas glyco-

gen, the storage form of glucose, is polar and is conse-

quently stored in a hydrated form that contains about twice

its dry weight of water. Fats therefore provide up to six

times the metabolic energy of an equal weight of hydrated

glycogen.

a. Lipid Digestion Occurs at Lipid–Water Interfaces

Since triacylglycerols are water insoluble, whereas di-

gestive enzymes are water soluble, triacylglycerol digestion

takes place at lipid–water interfaces. The rate of triacylglyc-

1-Palmitoyl-2,3-dioleoyl-glycerol

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