Voet D., Voet Ju.G. Biochemistry

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901

CHAPTER 24

Photosynthesis

1 Chloroplasts

2 Light Reactions

A. Absorption of Light

B. Electron Transport in Purple Photosynthetic Bacteria

C. Two-Center Electron Transport

D. Photophosphorylation

3 Dark Reactions

A. The Calvin Cycle

B. Control of the Calvin Cycle

C. Photorespiration and the C

Cycle

Life on Earth depends on the sun. Plants and cyanobacteria

chemically sequester light energy through photosynthesis, a

light-driven process in which CO

is “fixed” to yield carbo-

hydrates (CH

O).

This process, in which CO

is reduced and H

O is oxidized

to yield carbohydrates and O

, is essentially the reverse of

oxidative carbohydrate metabolism. Photosynthetically

produced carbohydrates therefore serve as an energy

source for the organism that produced them as well as for

nonphotosynthetic organisms that directly or indirectly

consume photosynthetic organisms. In fact, even modern

industry is highly dependent on the products of photosyn-

thesis because coal, oil, and gas (the so-called fossil fuels)

are thought to be the remains of ancient organisms. It is es-

timated that photosynthesis annually fixes ⬃10

tons of

carbon, which represents the storage of over 10

kJ of en-

ergy. Moreover, photosynthesis, over the eons, has pro-

duced the O

in Earth’s atmosphere (Section 1-5Cb).

The notion that plants obtain nourishment from such in-

substantial things as light and air took nearly two centuries

to develop. In 1648, the Flemish physician Jean Baptiste

van Helmont reported that growing a potted willow tree

from a shoot caused an insignificant change in the weight

of the soil in which the tree had been rooted.Although an-

other century was to pass before the law of conservation of

matter was formulated, van Helmont attributed the tree’s

weight gain to the water it had taken up. This idea was ex-

tended in 1727 by Stephen Hales, who proposed that plants

extract some of their matter from the air.

⫹ H

light

(CH

O) ⫹ O

The first indication that plants produce oxygen was

found by the English clergyman and pioneering chemist

Joseph Priestley, who reported:

Finding that candles burn very well in air in which plants had

grown a long time, and having some reason to think, that there

was something attending vegetation, which restored air that had

been injured by respiration, I thought it was possible that the

same process might also restore the air that had been injured by

the burning of candles.Accordingly, on the 17th of August,

1771, I put a sprig of mint into a quantity of air, in which a wax

candle had burned out, and found that, on the 27th of the same

month, another candle burned perfectly well in it.

Although Priestley later discovered oxygen, which he

named “dephlogisticated air,” it was Antoine Lavoisier

who elucidated its role in combustion and respiration. Nev-

ertheless, Priestley’s work inspired the Dutch physician Jan

Ingenhousz, who in 1779 demonstrated that the “purify-

ing” power of plants resides in the influence of sunlight on

their green parts. In 1782, the Swiss pastor Jean Senebier

showed that CO

, which he called “fixed air,” is taken up

during photosynthesis. His compatriot Nicolas-Théodore

de Saussure found, in 1804, that the combined weights of

the organic matter produced by plants and the oxygen they

evolve is greater than the weight of the CO

they consume.

He therefore concluded that water, the only other sub-

stance he added to his system, was also necessary for pho-

tosynthesis. The final ingredient in the overall photosyn-

thetic recipe was established in 1842 by the German

physiologist Robert Mayer, one of the formulators of the

first law of thermodynamics, who concluded that plants

convert light energy to chemical energy.

1 CHLOROPLASTS

The site of photosynthesis in eukaryotes (algae and higher

plants) is the chloroplast (Section 1-2Ag), a member of the

membranous subcellular organelles peculiar to plants

known as plastids.The first indication that chloroplasts have

a photosynthetic function was Theodor Englemann’s obser-

vation, in 1882, that small, motile, O

-seeking bacteria con-

gregate at the surface of the alga Spirogyra, overlying its sin-

gle chloroplast, but only while the chloroplast is illuminated.

Chloroplasts must therefore be the site of light-induced O

evolution, that is, photosynthesis. Chloroplasts, of which

there are 1 to 1000 per cell, vary considerably in size and

JWCL281_c24_901-939.qxd 3/25/10 12:04 PM Page 901

shape but are typically ⬃5-␮m-long ellipsoids. Like mito-

chondria, which they resemble in many ways, chloroplasts

have a highly permeable outer membrane and a nearly im-

permeable inner membrane separated by a narrow inter-

membrane space (Fig. 24-1). The inner membrane encloses

the stroma, a concentrated solution of enzymes much like

the mitochondrial matrix,that also contains the DNA, RNA,

and ribosomes involved in the synthesis of several chloro-

plast proteins. The stroma, in turn, surrounds a third mem-

branous compartment, the thylakoid (Greek: thylakos, a sac

or pouch). The thylakoid is probably a single highly folded

vesicle, although in most organisms it appears to consist of

stacks of disklike sacs named grana, which are intercon-

nected by unstacked stromal lamellae. A chloroplast usually

contains 10 to 100 grana. Thylakoid membranes arise from

invaginations in the inner membrane of developing chloro-

plasts and therefore resemble mitochondrial cristae.

The lipids of the thylakoid membrane have a distinctive

composition. They consist of only ⬃10% phospholipids; the

majority, ⬃80%, are uncharged mono- and digalactosyl

diacylglycerols, and the remaining ⬃10% are the sulfolipids

sulfoquinovosyl diacylglycerols (quinovose is 6-deoxyglucose):

X ⫽ OH

X ⫽ SO

⫺

* ⫽ C atom inverted

X ⫽

HOH

Galactosyl diacylglycerol

Digalactosyl diacylglycerol

Sulfoquinovosyl diacylglycerol

The acyl chains of these lipids have a high degree of unsat-

uration, which gives the thylakoid membrane a highly fluid

character.

Photosynthesis occurs in two distinct phases:

1. The light reactions, which use light energy to gener-

ate NADPH and ATP.

2. The dark reactions, actually light-independent reac-

tions, which use NADPH and ATP to drive the synthesis of

carbohydrate from CO

and H

The light reactions occur in the thylakoid membrane and

involve processes that resemble mitochondrial electron

transport and oxidative phosphorylation (Sections 22-2

and 22-3). In photosynthetic prokaryotes, which lack

chloroplasts, the light reactions take place in the cell’s

plasma (inner) membrane or in highly invaginated struc-

tures derived from it called chromatophores (e.g., Fig. 24-2;

recall that chloroplasts evolved from cyanobacteria that as-

sumed a symbiotic relationship with a nonphotosynthetic

902 Chapter 24. Photosynthesis

Figure 24-2 Electron micrograph of a section through the

purple photosynthetic bacterium Rhodobacter sphaeroides. Its

plasma membrane invaginates to form externally connected

tubules known as chromatophores (arrows; seen here in circular

cross section) that are the sites of photosynthesis. [Courtesy of

Gerald A. Peters, Virginia Commonwealth University.]

Outer

membrane

Inner

membrane

Intermembrane

compartment

Thylakoid

compartment

(b)(a)

Granum

Stroma

Stromal

lamella

Figure 24-1 Chloroplast from corn. (a) An electron micrograph. (b) Schematic diagram.

[Electron micrograph courtesy of Lester Shumway, College of Eastern Utah.]

JWCL281_c24_901-939.qxd 6/16/10 11:54 AM Page 902

eukaryote; Section 1-2Ag). In eukaryotes, the dark reac-

tions occur in the stroma through a cyclic series of enzyme-

catalyzed reactions. In the following sections, we consider

the light and dark reactions in detail.

2 LIGHT REACTIONS

In the first decades of the twentieth century, it was gener-

ally assumed that light, as absorbed by photosynthetic pig-

ments, directly reduced CO

, which, in turn, combined with

water to form carbohydrate. In this view, CO

is the source

of the O

generated by photosynthesis. In 1931, however,

Cornelis van Niel showed that green photosynthetic bacte-

ria, anaerobes that use H

S in photosynthesis, generate

sulfur:

The chemical similarity between H

S and H

O led van Niel

to propose that the general photosynthetic reaction is

where H

A is H

O in green plants and cyanobacteria and

S in photosynthetic sulfur bacteria. This suggests that

photosynthesis is a two-stage process in which light energy

is harnessed to oxidize H

A (the light reactions):

and the resulting reducing agent [H] subsequently reduces

(the dark reactions):

Thus, in aerobic photosynthesis, H

O, not CO

, is pho-

tolyzed (split by light).

The validity of van Niel’s hypothesis was established un-

equivocally by two experiments. In 1937, Robert Hill dis-

covered that when isolated chloroplasts that lack CO

are

illuminated in the presence of an artificial electron accep-

tor such as ferricyanide O

is evolved with

concomitant reduction of the acceptor [to ferrocyanide,

in our example]. This so-called Hill reaction

demonstrates that CO

does not participate directly in the

-producing reaction. It was discovered eventually that

the natural photosynthetic electron acceptor is NADP

⫹

(Fig. 13-2), whose reduction product, NADPH, is utilized in

the dark reactions to reduce CO

to carbohydrate (Section

24-3A). In 1941, when the oxygen isotope

O became

available, Samuel Ruben and Martin Kamen directly

demonstrated that the source of the O

formed in photo-

synthesis is H

This section discusses the major aspects of the light reac-

tions.

O ⫹ CO

light

(CH

O) ⫹

Fe(CN)

4⫺

[Fe(CN)

3⫺

4[H] ⫹ CO

(CH

O) ⫹ H

light

2A ⫹ 4[H]

⫹ 2H

light

(CH

O) ⫹ 2A ⫹ H

⫹ 2H

light

(CH

O) ⫹ 2S ⫹ H

A. Absorption of Light

The principal photoreceptor in photosynthesis is chloro-

phyll. This cyclic tetrapyrrole, like the heme group of

globins and cytochromes (Sections 10-1A and 22-2C), is

derived biosynthetically from protoporphyrin IX. Chloro-

phyll, however, differs from heme in four major respects

(Fig. 24-3):

1. Its central metal ion is Mg

2⫹

rather than Fe(II) or

Fe(III).

2. It has a cyclopentenone ring, Ring V, fused to pyrrole

Ring III.

3. Pyrrole Ring IV is partially reduced in chlorophyll a

(Chl a) and chlorophyll b (Chl b), the two major chloro-

phyll varieties in eukaryotes and cyanobacteria, whereas in

bacteriochlorophyll a (BChl a) and bacteriochlorophyll b

(BChl b), the principal chlorophylls of photosynthetic bac-

teria, Rings II and IV are partially reduced.

4. The propionyl side chain of Ring IV is esterified to a

tetraisoprenoid alcohol. In Chl a and b as well as in BChl b

it is phytol but in BChl a it is either phytol or geranylgeran-

iol, depending on the bacterial species.

In addition, Chl b has a formyl group in place of the

methyl substituent to atom C3 of Ring II of Chl a. Simi-

larly, BChl a and BChl b have different substituents to

atom C4.

a. Light and Matter Interact in Complex Ways

As photosynthesis is a light-driven process, it is worth-

while reviewing how light and matter interact. Electromag-

netic radiation is propagated as discrete quanta (photons)

whose energy E is given by Planck’s law:

[24.1]

where h is Planck’s constant (6.626 ⫻ 10

⫺34

J ⴢ s), c is the

speed of light (2.998 ⫻ 10

m ⴢ s

⫺1

in a vacuum), ␯ is the fre-

quency of the radiation, and ␭ is its wavelength (visible

light ranges in wavelength from 400 to 700 nm). Thus red

light with ␭⫽680 nm has an energy of 176 kJ ⴢ einstein

⫺1

(an einstein is a mole of photons).

Molecules, like atoms, have numerous electronic quan-

tum states of differing energies. Moreover, because mole-

cules contain more than one nucleus, each of their elec-

tronic states has an associated series of vibrational and

rotational substates that are closely spaced in energy (Fig.

24-4). Absorption of light by a molecule usually occurs

through the promotion of an electron from its ground

(lowest energy) state molecular orbital to one of higher

energy. However, a given molecule can only absorb pho-

tons of certain wavelengths because, as is required by the

law of conservation of energy, the energy difference be-

tween the two states must exactly match the energy of the

absorbed photon.

E ⫽ h␯⫽

␭

Section 24-2. Light Reactions 903

JWCL281_c24_901-939.qxd 6/8/10 8:53 AM Page 903

very rapidly, being complete in ⬍10

⫺11

s. Many molecules

relax in this manner to their ground states. Chlorophyll

molecules, however, usually relax only to their lowest ex-

cited states. Therefore, the photosynthetically applicable

excitation energy of a chlorophyll molecule that has ab-

sorbed a photon in its short-wavelength band, which corre-

sponds to its second excited state, is no different than if it

had absorbed a photon in its less energetic long-wavelength

band.

2. Fluorescence, in which an electronically excited mol-

ecule decays to its ground state by emitting a photon. Such

a process requires ⬃10

⫺8

s, so it occurs much more slowly

than internal conversion. Consequently, a fluorescently

emitted photon generally has a longer wavelength (lower

energy) than that initially absorbed. Fluorescence accounts

for the dissipation of only 3 to 6% of the light energy ab-

sorbed by living plants. However, chlorophyll in solution,

The various chlorophylls are highly conjugated mole-

cules (Fig. 24-3). It is just such molecules that strongly ab-

sorb visible light (the spectral band in which the solar radi-

ation reaching Earth’s surface is of peak intensity). In fact,

the peak molar extinction coefficients of the various

chlorophylls, ⬎10

⫺1

ⴢ cm

⫺1

, are among the highest

known for organic molecules (Fig. 24-5; absorbance spectra

are discussed in Section 5-3Ca). Yet the relatively small

chemical differences among the various chlorophylls

greatly affect their absorption spectra. These spectral dif-

ferences, as we shall see, are functionally significant.

An electronically excited molecule can dissipate its ex-

citation energy in many ways. Those modes with the great-

est photosynthetic significance are as follows (Fig. 24-4):

1. Internal conversion, a common mode of decay in

which electronic energy is converted to the kinetic energy

of molecular motion, that is, to heat. This process occurs

904 Chapter 24. Photosynthesis

N N

Iron–protoporphyrin IX

–

IIIIV

Chlorophyll a

CH CH

P or G

Chlorophyll b

Bacteriochlorophyll a

Bacteriochlorophyll b

No double bond between positions C3 and C4.

P =

G =

Phytyl side chain

Geranylgeranyl side chain

–

N N

Chlorophyll

Figure 24-3 Chlorophyll structures. The

molecular formulas of chlorophylls a and b

and bacteriochlorophylls a and b are

compared to that of iron protoporphyrin IX

(heme).The starred atom has the opposite

stereochemistry in chlorophyll aⴕ (Chl aⴕ).

The isoprenoid phytyl and geranylgeranyl

tails presumably increase the chlorophylls’

solubility in nonpolar media.

JWCL281_c24_901-939.qxd 3/25/10 12:04 PM Page 904

Section 24-2. Light Reactions 905

where of course the photosynthetic uptake of this energy

cannot occur, has an intense red fluorescence.

3. Exciton transfer (also known as resonance energy

transfer), in which an excited molecule directly transfers its

excitation energy to nearby unexcited molecules with simi-

lar electronic properties (the basis of FRET; Section 9-1Cd).

This process occurs through interactions between the mo-

lecular orbitals of the participating molecules in a manner

analogous to the interactions between mechanically coupled

pendulums of similar frequencies. An exciton (excitation)

may be serially transferred between members of a group of

molecules or, if their electronic coupling is strong enough,

the entire group may act as a single excited “supermolecule.”

We shall see that exciton transfer is of particular importance

in funneling light energy to photosynthetic reaction centers.

4. Photooxidation, in which a light-excited donor mole-

cule is oxidized by transferring an electron to an acceptor

molecule, which is thereby reduced. This process occurs be-

cause the transferred electron is less tightly bound to the

donor in its excited state than it is in the ground state. In

photosynthesis, excited chlorophyll (Chl*) is such a donor.

The energy of the absorbed photon is thereby chemically

transferred to the photosynthetic reaction system. Photooxi-

dized chlorophyll,Chl

⫹

,a cationic free radical, eventually re-

turns to its ground state by oxidizing some other molecule.

b. Light Absorbed by Antenna Chlorophylls Is

Transferred to Photosynthetic Reaction Centers

The primary reactions of photosynthesis, as is explained

in Sections 24-2B and 24-2C, take place at photosynthetic

reaction centers (RCs). Yet photosynthetic organelles con-

tain far more chlorophyll molecules than RCs. This was

Figure 24-4 Energy diagram indicating the electronic states of

chlorophyll and their most important modes of interconversion.

The thin black lines denote different vibrational and rotational

substates of each electronic state. The wiggly arrows represent

the absorption of photons or their fluorescent emission. Excitation

energy may also be dissipated in radiationless processes such as

internal conversion (heat production) or chemical reactions.

See the Animated Figures

Figure 24-5 Absorption spectra of various photosynthetic

pigments. The chlorophylls each have two absorption bands, one

in the red and one in the blue. Phycoerythrin absorbs blue and

Second

excited

state

First

excited

state

Ground

state

Absorption

of blue

light

Absorption

of red

light

Fluor-

escence

Internal conversion

(radiationless)

Energy

Photooxidation

(chemical

reactions)

hν hν hν

400 500 600 700 800

Wavelength (nm)

Phycocyanin

Phycoerythrin

Solar spectrum

Bacteriochlorophyll a

Chlorophyll a

Absorbance

Chlorophyll b

Carotenoids

UV Violet Blue Green Yellow Orange Red IR

green light, whereas phycocyanin absorbs yellow light.Together,

these pigments absorb most of the visible light in the solar

spectrum. [After a drawing by Govindjee, University of Illinois.]

JWCL281_c24_901-939.qxd 3/25/10 12:04 PM Page 905

906 Chapter 24. Photosynthesis

demonstrated in 1932 by Robert Emerson and William

Arnold in their studies of O

production by the green alga

Chlorella (a favorite experimental subject), which had

been exposed to repeated brief (10-␮s) flashes of light.The

amount of O

generated per flash was maximal when the

interval between flashes was at least 20 ms. Evidently, this

is the time required for a single turnover of the photosyn-

thetic reaction cycle. Emerson and Arnold then measured

the variation of O

yield with flash intensity when the flash

interval was the optimal 20 ms. With weak flashes, the O

increased linearly with flash intensity such that about one

molecule of O

was generated per eight photons absorbed

(Fig. 24-6). With increasing flash intensity the efficiency of

this process fell off, no doubt because the number of pho-

tons began to approach the number of photochemical

units.What was unanticipated, however, was that each flash

of saturating intensity produced only one molecule of O

per ⬃2400 molecules of chlorophyll present. Since at least

eight photons must be sequentially absorbed to liberate

one O

molecule (Section 24-2C), these results suggest that

the photosynthetic apparatus contains ⬃2400兾8 ⫽ 300

chlorophyll molecules per RC.

With such a great excess of chlorophyll molecules per

RC, it seems unlikely that all participate directly in photo-

chemical reactions. Rather,as subsequent experiments have

shown, most chlorophylls function to gather light; that is, they

act as light-harvesting antennas. These antenna chlorophylls

pass the energy of an absorbed photon, by exciton transfer,

from molecule to molecule until the excitation reaches an

RC (Fig. 24-7a).There, the excitation is trapped because RC

chlorophylls, although chemically identical to antenna

chlorophylls, have slightly lower excited state energies be-

cause of their different environments (Fig. 24-7b).

Transfer of energy from the antenna system to an RC

occurs in ⬍10

⫺10

s with an efficiency of ⬎90%.This high ef-

ficiency depends on the chlorophyll molecules having ap-

propriate spacings and relative orientations. Even in bright

sunlight, an RC intercepts only ⬃1 photon per second, a

metabolically insignificant rate, and hence, these light-

harvesting complexes (LHCs) serve an essential function.

c. The LHCs of Purple Photosynthetic Bacteria

Contain Multiple Symmetrically Arranged

Light-Absorbing Molecules

Most purple photosynthetic bacteria, which are among

the simplest photosynthetic organisms, have two types of

LHCs, LH1 and LH2, that are transmembrane proteins but

have different spectral and biochemical properties. LH2,

which absorbs light at shorter wavelengths than LH1,

rapidly passes the energy from the photons it absorbs to

Maximum ~ 1 O

/2400 chlorophylls

slope ~ 8 quanta

per

molecule evolved

Intensity of each flash

Evolved

per flash

Figure 24-6 The amount of O

evolved by Chlorella algae

versus the intensity of light flashes. Flashes are separated by dark

intervals of ⬎20 ms.

Figure 24-7 Flow of energy through a photosynthetic antenna

complex. (a) The excitation resulting from photon absorption

randomly migrates by exciton transfer among the molecules of

the antenna complex (light green circles) until it is either trapped

Photon

Photon(a)

Energy

Excited states

Ground states

Antenna pigment molecules

Exciton transfer

Reaction

center

chlorophyll

hν

(b)

by an RC chlorophyll (dark green circles) or, less frequently,

fluorescently reemitted. (b) The excitation is trapped by the RC

chlorophyll because its lowest excited state has a lower energy

than those of the antenna pigment molecules.

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The ␣ and ␤ subunits (56 and 45 residues, respectively)

both consist largely of single helices that are aligned nearly

perpendicularly to the plane of the membrane in which

they are embedded. The eight ␣ subunits pack side by side

to form a hollow cylinder of diameter ⬃31 Å (as measured

between helix axes). Each of the eight ␤ subunits occupies

a position radially outward from an ␣ subunit to form a

concentric cylinder of diameter ⬃62 Å.Sixteen of the BChl

a molecules are packed between these rings of helices in an

arrangement resembling a 16-bladed turbine: Successive

nearly parallel BChl a ring systems are in partial van der

Waals contact (their Mg

2⫹

ions are ⬃9 Å apart) with their

planes perpendicular to the plane of the membrane. Their

2⫹

atoms are each singly axially liganded by His side

chains [much like the Fe(II) in deoxyhemoglobin] that al-

ternately extend from an ␣ and a ␤ subunit around the

lower end of the cylinder.The remaining eight BChl a mol-

ecules, which are each singly axially liganded by a side

chain of Asp 6␣ near the upper end of the cylinder, are

arranged in an 8-fold symmetric ring between successive

␤ subunit helices and are oriented with the planes of their

ring systems tilted by ⬃35° relative to the plane of the mem-

brane. The eight lycopene molecules are sandwiched be-

tween the ␣ and ␤ subunits and extend along much of their

lengths, thereby contacting both sets of BChl a molecules.

The LH2 from Rhodopseudomonas (Rps.) acidophila, an-

other purple photosynthetic bacterium, is an ␣

␤

18-mer

but otherwise has a similar structure in its transmembrane

region to that of Rs. molischianum, even though their ␣ and

␤ subunits are only 26 and 31% identical.

Spectroscopic measurements indicate that an LH2’s

His-liganded and closely associated BChl a molecules max-

imally absorb radiation at a wavelength of 850 nm (and

hence are called B850) and are strongly coupled, that is,

they absorb radiation almost as a unit. The other, more

loosely associated BChl a molecules (B800) maximally ab-

sorb radiation at 800 nm, largely as individual molecules

(BChl a’s local environment in the protein alters its spec-

trum from that in solution; Fig. 24-5). When a B800 BChl a

absorbs a photon, the excitation is rapidly [in ⬃700 fem-

toseconds (fs); 1 fs ⫽ 10

⫺15

s] transferred to a lower energy

B850 BCl a (which may independently absorb a photon),

LH1, which, in turn, passes it to the RC. The X-ray struc-

ture of LH2 from the purple photosynthetic bacterium

Rhodospirillum (Rs.) molischianum (Fig. 24-8),determined

by Hartmut Michel, reveals that this protein is an 8-fold ro-

tationally symmetric ␣

␤

16-mer that binds 24 bacteri-

ochlorophyll a (BChl a) molecules and 8 lycopene mole-

cules (a carotenoid; see below):

Lycopene

Section 24-2. Light Reactions 907

Figure 24-8 X-ray structure of LH2 from Rs. molischianum.

The ␣ subunits are purple and the ␤ subunits are pink. The

bound chromophores are drawn in stick form with the BChl a’s

green and the lycopenes yellow. The phytyl tails of the BChl a’s

have been truncated for clarity. (a) View perpendicular to the

bacterial membrane from the cytoplasm.The polypeptide chains

are drawn in worm form. (b) View parallel to the membrane with

the cytoplasm above.The protein subunits are represented by

only their helices, which are shown as cylinders.The Mg

2⫹

ions

are represented by white spheres. [Courtesy of Juergen Koepke

and Hartmut Michel, Max-Planck-Institut für Biochemie,

Frankfurt, Germany. PDBid 1LGH.]

See Interactive

Exercise 20

(a)

(b)

JWCL281_c24_901-939.qxd 10/19/10 9:35 AM Page 907

which even more rapidly (in ⬃100 fs) exchanges the excita-

tion among the other B850 BChl a molecules. Hence, the

B850 system acts as a kind of energy storage ring that delo-

calizes the excitation over a large region. The carotenoid

molecules in this system absorb visible light (⬍800 nm) and

may also facilitate the transmission of excitation between

the rather distantly separated (19 Å between Mg atoms)

nearest-neighbor B850 and B800 BChl a molecules.

LH1, like LH2, has ␣ and ␤ subunits of ⬃50 residues

each.The low (8.5-Å) resolution structure of LH1 from Rs.

rubrum, as determined by electron crystallography, reveals

that it resembles LH2 but with 16-fold rotational symme-

try, and forms a 116-Å-diameter cylinder with a 68-Å-

diameter hole down its center.This hole is of sufficient size

to contain an RC (see below), as electron microscopy stud-

ies indicate is, in fact, the case (Fig. 24-9). LH1’s BChl a

molecules absorb radiation at a longer wavelength than

those of LH2 and consequently, when these two assemblies

are in contact, excitation is rapidly [in 1–5 picoseconds (ps);

1 ps ⫽ 10

⫺12

s] transferred from LH2 to LH1 and then (in

20–40 ps) to LH1’s enclosed RC. Excitations may also be

rapidly exchanged between contacting LH2s. Thus, this an-

tenna system transfers virtually all of the radiation energy

it absorbs to the RC in far less than the few nanoseconds

(ns; 1 ns ⫽ 10

⫺9

s) over which these excitations would oth-

erwise decay. It should be noted that this complicated

arrangement of chromophores (light-absorbing molecules)

is among the simplest known; those of the light-harvesting

systems of plants are even more elaborate (see below).

d. LHCs Contain Accessory Pigments

Most LHCs contain organized arrays of other light-

absorbing substances in addition to chlorophyll. These

accessory pigments function to fill in the absorption spectra

of the antenna complexes in spectral regions where chloro-

phylls do not absorb strongly (Fig. 24-5). Carotenoids,

which are C

, largely linear polyenes such as lycopene and

␤-carotene,

are components of all green plants and many photosyn-

thetic bacteria and are therefore the most common acces-

sory pigments.They are largely responsible for the brilliant

fall colors of deciduous trees as well as for the orange color

of carrots (after which carotenoids are named).

The light-harvesting protein LHC-II of green plants

comprises ⬃30% of the protein in chloroplast membranes,

which makes it the most abundant membrane protein in

nature. Each subunit of this highly conserved, 232-residue,

trimeric protein binds eight Chl a’s, six Chl b’s, and four

carotenoids (Fig. 24-10), thereby accounting for around

half the chlorophyll in the biosphere. The orientations of

the 42 chlorophylls in each LHC-II trimer evenly sample

nearly all directions in space, thus maximizing the effi-

ciency of light harvesting.

Carotenoids serve an additional function besides that of

light-gathering antennas: Through electronic interactions,

they prevent their associated light-excited chlorophyll mol-

ecules from transferring this excitation to O

, which would

otherwise yield highly destructive reactive oxygen species

(ROS; Section 22-4Cg). This is particularly important un-

der full sunlight,when the rate that light energy is absorbed

exceeds the rate that it can be used in photosynthesis.Then,

the excess energy is dissipated as heat through internal

␤-Carotene

908 Chapter 24. Photosynthesis

Figure 24-9 Model of the light-absorbing antenna system of

purple photosynthetic bacteria. Several LH2s associate with each

other and with LH1, which surrounds the photosynthetic reaction

center (RC).The BChl a’s of LH2 B850 and LH1 are green,

those of LH2 B800 are purple, and the light-absorbing pigments

of the RC (see below) are red and black. Light absorbed by the

Bchl a and lycopene molecules of an LH2 is rapidly transferred

(curved arrows), often via other contacting LH2s, to LH1, which,

in turn, transfers the excitation to its enclosed RC. [From

Bhattarchardee, Y., Nature 412, 474 (2001).]

LH2

LH1

[Ross M. Horowitz/The Image Bank/Getty Images.]

JWCL281_c24_901-939.qxd 7/20/10 7:27 PM Page 908

(a)

(b)

conversion by carotenoids, thereby minimizing the irre-

versible damage to the photosynthetic system that would

otherwise occur. In fact, the acidification of the thylakoid

lumen resulting from high photosynthetic activity (see below)

induces a conformational change in LHC-II. This converts

it to a dissipative state by twisting one of its carotenoids

and hence changing its electronic properties.

Aquatic photosynthetic organisms, which are responsible

for nearly half of the photosynthesis on Earth, additionally

contain other types of accessory pigments. This is because

light outside the wavelengths 450 to 550 nm (blue and green

light) is absorbed almost completely by passage through

more than 10 m of water. In red algae and cyanobacteria,

Chl a is therefore replaced as an antenna pigment by a

series of linear tetrapyrroles, notably the red phycoeryth-

robilin and the blue phycocyanobilin:

The lowest excited states of these so-called bilins have

higher energies than those of the chlorophylls, thereby facil-

itating energy transfer to the RC. The bilins are covalently

linked via Cys S atoms to phycobiliproteins to form phyco-

erythrin and phycocyanin (spectra in Fig. 24-5). These, in

turn, are organized in high molecular mass particles called

phycobilisomes that are bound to the outer faces of photo-

synthetic membranes so as to funnel excitation energy to

RCs over long distances with ⬎90% efficiency.

B. Electron Transport in Purple

Photosynthetic Bacteria

Photosynthesis is a process in which electrons from excited

chlorophyll molecules are passed through a series of accep-

tors that convert electronic energy to chemical energy. Thus

two questions arise: (1) What is the mechanism of energy

transduction; and (2) how do photooxidized chlorophyll

molecules regain their lost electrons? We shall see that

photosynthetic bacteria solve these problems somewhat

differently from cyanobacteria and plants. We first discuss

these mechanisms in photosynthetic bacteria, where they

are simpler and better understood. Electron transport in

cyanobacteria and plants is the subject of Section 24-2C.

a. The Photosynthetic Reaction Center Is a

Transmembrane Protein Containing a Variety

of Chromophores

The first indication that chlorophyll undergoes direct

photooxidation during photosynthesis was obtained by

Louis Duysens in 1952. He observed that illumination of

membrane preparations from the purple photosynthetic

bacterium Rs. rubrum caused a slight (⬃2%) bleaching of

their absorbance at 870 nm, which returned to their origi-

nal levels in the dark. Duysens suggested that this bleach-

ing is caused by photooxidation of a bacteriochlorophyll

complex that he named P870 (P for pigment and 870 nm

for the position of the major long-wavelength absorption

band of BChl a; photosynthetic bacteria tend to inhabit

murky stagnant ponds, so that they require an infrared-

absorbing species of chlorophyll). The ability to detect the

presence of P870 eventually led to the purification and

characterization of the RC to which it is bound.

–

OOC

Cys

COO

–

ethyl in

phycocyanobilin

dehydrogenated in

phycocyanobilin

Peptide-linked phycoerythrobilin and phycocyanobilin

Section 24-2. Light Reactions 909

Figure 24-10 X-ray structure of the homotrimeric protein

LHC-II from pea chloroplasts. The protein is drawn in ribbon

form (gray) viewed (a) perpendicular to the thylakoid membrane

from the stroma and along its 3-fold axis; and (b) parallel to the

membrane (gray band) with the stroma above. Its bound

carotenoids and chlorophylls are drawn in stick form with Chl a

cyan, Chl b green, carotenoids orange, and lipids magenta. Mg

2⫹

ions are represented by light pink spheres. Each subunit has

three transmembrane helices oriented with its N-terminus on the

stromal surface. Note the unusually high density of cofactors:

Nearly 40% of this protein’s nonhydrogen atoms comprise its

chlorophylls and carotenoids. [Courtesy of Werner Kühlbrandt,

Max Planck Institute of Biophysics, Frankfurt, Germany. PDBid

2BHW.]

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(a)

(b)

RC particles from several species of purple photosyn-

thetic bacteria (PbRCs) have similar compositions. That

from Rps. viridis consists of three hydrophobic subunits: H

(258 residues), L (273 residues), and M (323 residues). The

L and M subunits of this membrane-spanning protein col-

lectively bind four molecules of BChl b (which maximally

absorbs light at 960 nm), two molecules of bacteriopheo-

phytin b (BPheo b; BChl b in which the Mg

2⫹

is replaced

by two protons), one nonheme/non-Fe–S Fe(II) ion, one

molecule of the redox coenzyme ubiquinone (Fig. 22-17b),

and one molecule of the related menaquinone

(vitamin K

, a substance required for proper blood clot-

ting; Section 35-1Ba). In many PbRCs, however, the BChl

b, BPheo b, and menaquinone are replaced by BChl a,

BPheo a, and a second ubiquinone, respectively.

The RC of Rps. viridis, whose X-ray structure was deter-

mined by Johann Deisenhofer, Robert Huber, and Hart-

mut Michel in 1984, was the first transmembrane protein to

be described in atomic detail (Fig. 12-26). The protein’s

transmembrane portion consists of 11 ␣ helices that form a

45-Å-long flattened cylinder with the expected hydrophobic

surface. A c-type cytochrome containing four hemes, which

is an integral constituent of the PbRC complex in only

some photosynthetic bacteria, binds to the PbRC on the

external side of the plasma membrane. In fact, the PbRC from

another bacterial species, Rhodobacter (Rb.) sphaeroides,

whose X-ray structure (Fig. 24-11) was independently deter-

mined by Marianne Schiffer and by Douglas Rees and

George Feher, is nearly identical to that of Rps. viridis but

lacks such a bound cytochrome.

b. Two BChl Molecules Form a “Special Pair”

The most striking aspect of the PbRC is that its chro-

mophoric prosthetic groups are arranged with nearly perfect

2-fold symmetry (Fig. 24-12a). This symmetry arises be-

cause the L and M subunits, with which these prosthetic

groups are exclusively associated, have homologous se-

quences and similar folds. Two of the BChl b molecules in

the Rps. viridis PbRC, the so-called special pair, are closely

associated; they are nearly parallel and have an Mg¬Mg

distance of ⬃7 Å. The special pair occupies a predomi-

nantly hydrophobic region of the protein and each of its

2⫹

ions has a His side chain as a fifth ligand. Each mem-

ber of the special pair is in contact with another His-

liganded BChl b molecule, which, in turn, is associated with a

BPheo b molecule.The menaquinone is in close association

with the L subunit BPheo b (Fig. 24-12a, right), whereas

the ubiquinone, which is but loosely bound to the protein,

(CH

CH C CH

)

Menaquinone

910 Chapter 24. Photosynthesis

Figure 24-11 A ribbon diagram of the photosynthetic reaction

center (RC) from Rb. sphaeroides. (a) The H, M, and L subunits,

as viewed from within the plane of the plasma membrane with

the cytoplasm below, are magenta, cyan, and orange, respectively.

The prosthetic groups are drawn in stick form with C green,

N blue, and O red.The Fe(II) atom is represented by a yellow

sphere. The 11 largely vertical helices that form the central

portion of the protein constitute its transmembrane region.

Compare this structure with that of the RC from Rps. viridis

(Fig. 12-26), whose H, M, and L subunits are 39, 50, and 59%

identical to those of Rb. sphaeroides. Note that the Rb.

sphaeroides protein lacks the four-heme c-type cytochrome

(green in Fig. 12-26) on its periplasmic surface and that the Q

prosthetic group, whose quinone ring lies to the right of the

Fe(II), is ubiquinone in Rb. sphaeroides but menaquinone in Rps.

viridis.(b) View from the extracellular side of the membrane.

Note how the transmembrane portions of the M and L subunits

are related by a pseudo-2-fold axis passing through the Fe(II) ion

and that the prosthetic groups are sandwiched between these

two subunits. [Based on an X-ray structure by Marianne Schiffer,

Argonne National Laboratory. PDBid 2RCR.]

See

Interactive Exercise 21 and Kinemage Exercise 8-2

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