Nevertheless, the stoichiometry of the metabolic pathway of CO2fixation is certain: 12 NADPH 12 H 18 ATP 6 CO2 12 H2O⎯⎯→ C6H12O6 12 NADP 18 ADP 18 Pi 21.3 A More Generalized Equation f
Trang 1CO2 by a series of enzymatic reactions found in the stroma (see Equation 21.3,
which follows)
Water Is the Ultimate eⴚDonor for Photosynthetic NADPⴙReduction
In green plants, water serves as the ultimate electron donor for the photosynthetic
generation of reducing equivalents The reaction sequence
nh
2 H2O 2 NADP x ADP x Pi⎯⎯→
O2 2 NADPH 2 H x ATP x H2O (21.2)
describes the process, where nh symbolizes light energy (n is some number of
pho-tons of energy h , where h is Planck’s constant and is the frequency of the light).
Light energy is necessary to make the unfavorable reduction of NADP by H2O
(Ᏹo 1.136 V; G° 219 kJ/mol NADP) thermodynamically favorable
Thus, the light energy input, nh , must exceed 219 kJ/mol NADP The
stoi-chiometry of ATP formation depends on the pattern of photophosphorylation
op-erating in the cell at the time and on the ATP yield in terms of the chemiosmotic
ratio, ATP/H, as we will see later Nevertheless, the stoichiometry of the metabolic
pathway of CO2fixation is certain:
12 NADPH 12 H 18 ATP 6 CO2 12 H2O⎯⎯→
C6H12O6 12 NADP 18 ADP 18 Pi (21.3)
A More Generalized Equation for Photosynthesis In 1931, comparative study of
photosynthesis in bacteria led van Niel to a more general formulation of the
over-all reaction:
Light
CO2 2 H2A ⎯⎯→ (CH2O) 2A H2O (21.4)
Hydrogen Hydrogen Reduced Oxidized
acceptor donor acceptor donor
In photosynthetic bacteria, H2A is variously H2S (photosynthetic green and
pur-ple sulfur bacteria), isopropanol, or some similar oxidizable substrate [(CH 2O)
sym-bolizes a carbohydrate unit.]
CO2 2 H2S⎯⎯→ (CH2O) H2O 2 S
In cyanobacteria and the eukaryotic photosynthetic cells of algae and higher
plants, H2A is H2O, as implied earlier, and 2 A is O2 The accumulation of O2to
constitute 21% of the earth’s atmosphere is the direct result of eons of global
oxy-genic photosynthesis
21.2 How Is Solar Energy Captured by Chlorophyll?
Photosynthesis depends on the photoreactivity of chlorophyll Chlorophylls are
magnesium-containing substituted tetrapyrroles whose basic structure is
reminis-cent of heme, the iron-containing porphyrin (see Chapters 5 and 20)
Chloro-phylls differ from heme in a number of properties: Magnesium instead of iron is
coordinated in the center of the planar conjugated ring structure; a long-chain
alcohol, phytol, is esterified to a pyrrole ring substituent; and the methine bridge
linking pyrroles III and IV is substituted and crosslinked to ring III, leading to the
formation of a fifth five-membered ring The structures of chlorophyll a and b are
shown in Figure 21.5a
Chlorophylls are excellent light absorbers because of their aromaticity That is,
they possess delocalized electrons above and below the planar ring structure The
(CH2O) H2O
O
Trang 2
634 Chapter 21 Photosynthesis
energy differences between electronic states in these orbitals correspond to the
energies of visible light photons When light energy is absorbed, an electron is pro-moted to a higher orbital, enhancing the potential for transfer of this electron to
a suitable acceptor Loss of such a photoexcited electron to an acceptor is an oxidation–reduction reaction The net result is the transduction of light energy into the chemical energy of a redox reaction
Chlorophylls and Accessory Light-Harvesting Pigments Absorb Light of Different Wavelengths
The absorption spectra of chlorophylls a and b (Figure 21.5b) differ somewhat Plants
that possess both chlorophylls can harvest a wider spectrum of incident energy Other
pigments in photosynthetic organisms, so-called accessory light-harvesting pigments
(Figure 21.6), increase the possibility for absorption of incident light of wavelengths not absorbed by the chlorophylls Carotenoids and phycocyanobilins, like chloro-phyll, possess many conjugated double bonds and thus absorb visible light Carotenoids have two primary roles in photosynthesis—light harvesting and photo-protection through destruction of reactive oxygen species that arise as by-products of photoexcitation
The Light Energy Absorbed by Photosynthetic Pigments Has Several Possible Fates
Each photon represents a quantum of light energy A quantum of light energy ab-sorbed by a photosynthetic pigment has four possible fates (Figure 21.7):
1 Loss as heat The energy can be dissipated as heat through redistribution into
atomic vibrations within the pigment molecule
CH2
CH3
3
H CH
H2C
(a)
H
CH3
CH2
H
H
O
O
H C
O OCH3 Mg
V
C C
H2C
CH2
H2C CH
H2C
CH2
H2C CH
H2C
CH2
H2C CH
H3C
CH3
CH3
CH3
R
Hydrophobic phytyl side chain
R=
Chlorophyll a —CH3 Chlorophyll b —CHO
(b)
Wavelength (nm)
a b
b a
FIGURE 21.5 Structures (a) and absorption spectra (b) of
chlorophyll a and b The phytyl side chain of ring IV
pro-vides a hydrophobic tail to anchor the chlorophyll in
membrane protein complexes.
Trang 3O
H
CH3
CH3
O
H
N
H
CH3
CH2
CH2
OH
CH2
CH2
OH
H3C
CH3
(a)
(b)
-Carotene
Phycocyanobilin
FIGURE 21.6 Structures of representative accessory light-harvesting pigments in photosynthetic cells.
(a)-Carotene, an accessory light-harvesting pigment
in leaves (b) Phycocyanobilin, a blue pigment found
in cyanobacteria.
e –
e –
e –
e –
e –
Heat
Photon of fluorescence
Thermal
dissipation
transfer
Transfer
Oxidized P (P+)
Energy transfer to neighboring P molecule
+
P*
h
+
+
Pigment molecule (P)
Light
energy (hv)
Excited state (P*)
Q–red
Qox
FIGURE 21.7 Possible fates of the quantum of light energy absorbed by photosynthetic pigments.
Trang 4636 Chapter 21 Photosynthesis
2 Loss as light Energy of excitation reappears as fluorescence (light emission); a
photon of fluorescence is emitted as the ereturns to a lower orbital This fate is common only in saturating light intensities For thermodynamic reasons, the photon of fluorescence has a longer wavelength and hence lower energy than the quantum of excitation
3 Resonance energy transfer The excitation energy can be transferred by
reso-nance energy transfer to a neighboring molecule if the energy level difference be-tween the two corresponds to the quantum of excitation energy In this process, the energy transferred raises an electron in the receptor molecule to a higher
energy state as the photoexcited ein the original absorbing molecule returns to
ground state This so-called Förster resonance energy transfer is the mechanism
whereby quanta of light falling anywhere within an array of pigment molecules can be transferred ultimately to specific photochemically reactive sites
4 Energy transduction The energy of excitation, in raising an electron to a higher
energy orbital, dramatically changes the standard reduction potential, Ᏹo, of the pigment such that it becomes a much more effective electron donor That is, the excited-state species, by virtue of having an electron at a higher energy level through light absorption, has become a more potent electron donor Reaction of this excited-state electron donor with an electron acceptor situated in its vicinity
leads to the transformation, or transduction, of light energy (photons) to
chem-ical energy (reducing power, the potential for electron-transfer reactions)
Trans-duction of light energy into chemical energy, the photochemical event, is the essence of photosynthesis.
The Transduction of Light Energy into Chemical Energy Involves Oxidation–Reduction
The diagram presented in Figure 21.8 illustrates the fundamental transduction of light energy into chemical energy (an oxidation–reduction reaction) that is the ba-sis of photosyntheba-sis Chlorophyll (Chl) resides in a membrane in close association
with molecules competent in etransfer, symbolized here as A and B Chl absorbs
a photon of light, becoming activated to Chl* in the process Electron transfer from Chl* to A leads to oxidized Chl (Chl, a cationic free radical) and reduced
A (Ain the diagram) Subsequent oxidation of Aeventually culminates in re-duction of NADPto NADPH The electron “hole” in oxidized Chl (Chl) is filled
by transfer of an electron from B to Chl, restoring Chl and creating B Bis
re-stored to B by an edonated by water O2is the product of water oxidation Note that the system is restored to its original state once NADPH is formed and H2O is oxidized Proton translocations accompany these light-driven electron-transport
A Chl B
A 1
Chl*
B
A –
B
A
B
A Chl
B +
A Chl B
h
1
FIGURE 21.8 Model for light absorption by chlorophyll
and transduction of light energy into an oxidation–
reduction reaction I: Photoexcitation of Chl creates
Chl* II: Electron transfer from Chl* to A yields oxidized
Chl (Chl) and reduced A (A) III: An electron-transfer
pathway from Ato NADPleads to NADPH formation
and restoration of oxidized A (A) IV: Chlaccepts an
electron from B, restoring Chl and generating oxidized
B (B) V: Bis reduced back to B by an electron
origi-nating in H 2 O Water oxidation is the source of O 2
formation.
Trang 5reactions Such H translocations establish a chemiosmotic gradient across the
photosynthetic membrane that can drive ATP synthesis
Photosynthetic Units Consist of Many Chlorophyll Molecules
but Only a Single Reaction Center
In the early 1930s, Emerson and Arnold investigated the relationship between the
amount of incident light energy, the amount of chlorophyll present, and the amount
of oxygen evolved by illuminated algal cells Emerson and Arnold were seeking to
de-termine the quantum yield of photosynthesis: the number of electrons transferred
per photon of light Their studies gave an unexpected result: When algae were
illu-minated with very brief light flashes that could excite every chlorophyll molecule at
least once, only one molecule of O2 was evolved per 2400 chlorophyll molecules
This result implied that not all chlorophyll molecules are photochemically reactive,
and it led to the concept that photosynthesis occurs in functionally discrete units
Chlorophyll serves two roles in photosynthesis It is involved in light harvesting
and the transfer of light energy to photoreactive sites by exciton transfer, and it
par-ticipates directly in the photochemical events whereby light energy becomes
chem-ical energy A photosynthetic unit (Figure 21.9) can be envisioned as an antenna
of several hundred light-harvesting chlorophyll molecules (green) plus a special
pair of photochemically reactive chlorophyll a molecules called the reaction center
(orange) The purpose of the vast majority of chlorophyll in a photosynthetic unit
is to harvest light incident within the unit and funnel it, via resonance energy
trans-fer, to the reaction center chlorophyll dimers that are photochemically active Most
chlorophyll thus acts as a large light-collecting antenna, and it is at the reaction
cen-ters that the photochemical event occurs Oxidation of chlorophyll leaves a cationic
free radical,Chl, whose properties as an electron acceptor have important
con-sequences for photosynthesis Note that the Mg2ion does not change in valence
during these redox reactions
21.3 What Kinds of Photosystems Are Used
to Capture Light Energy?
All photosynthetic cells contain some form of photosystem Photosynthetic bacteria
have only one photosystem; furthermore, they lack the ability to use light energy to
split H2O and release O2 Cyanobacteria, green algae, and higher plants are oxygenic
phototrophs because they can generate O2from water Oxygenic phototrophs have two
distinct photosystems: photosystem I (PSI) and photosystem II (PSII) Type I
photo-systems use ferredoxins as terminal electron acceptors; type II photophoto-systems use
quinones as terminal electron acceptors PSI is defined by reaction center
chloro-phylls with maximal red light absorption at 700 nm; PSII uses reaction centers that
exhibit maximal red light absorption at 680 nm The reaction center Chl of PSI is
re-ferred to as P700 because it absorbs light of 700-nm wavelength; the reaction center
Chl of PSII is called P680 for analogous reasons Both P700 and P680 are chlorophyll
a dimers situated within specialized protein complexes A distinct property of PSII is
its role in light-driven O2evolution Interestingly, the photosystems of photosynthetic
bacteria are type II photosystems that resemble eukaryotic PSII more than PSI, even
though these bacteria lack O2-evolving capacity
Chlorophyll Exists in Plant Membranes in Association with Proteins
Detergent treatment of a suspension of thylakoids dissolves the membranes, releasing
complexes containing both chlorophyll and protein These chlorophyll–protein
com-plexes represent integral components of the thylakoid membrane, and their
organi-zation reflects their roles as either light-harvesting complexes (LHC), PSI complexes,
or PSII complexes All chlorophyll is apparently localized within these three
macro-molecular assemblies
Reaction center
Light-harvesting pigment (antenna molecules)
h
ANIMATED FIGURE 21.9 Schematic
diagram of a photosynthetic unit See this figure
animated at www.cengage.com/login.
Trang 6638 Chapter 21 Photosynthesis
PSI and PSII Participate in the Overall Process of Photosynthesis
What are the roles of the two photosystems, and what is their relationship to each other? PSI provides reducing power in the form of NADPH PSII splits water, pro-ducing O2, and feeds the electrons released into an electron-transport chain that couples PSII to PSI Electron transfer between PSII and PSI pumps protons for chemiosmotic ATP synthesis As summarized by Equation 21.2, photosynthesis in-volves the reduction of NADP, using electrons derived from water and activated by
light, h ATP is generated in the process The standard reduction potential for the
NADP/NADPH couple is 0.32 V Thus, a strong reductant with an Ᏹo more neg-ative than 0.32 V is required to reduce NADPunder standard conditions By sim-ilar reasoning, a very strong oxidant will be required to oxidize water to oxygen be-causeᏱo( 1
2 O2/H2O) is 0.82 V Separation of the oxidizing and reducing aspects
of Equation 21.2 is accomplished in nature by devoting PSI to NADPreduction and PSII to water oxidation PSI and PSII are linked via an electron-transport chain
so that the weak reductant generated by PSII can provide an electron to reduce the
weak oxidant side of P700 (Figure 21.10) Thus, electrons flow from H 2O to NADP ,
driven by light energy absorbed at the reaction centers Oxygen is a by-product of
the photolysis, literally “light-splitting,” of water Accompanying electron flow is
pro-duction of a proton gradient and ATP synthesis (see Section 21.6) This
light-driven phosphorylation is termed photophosphorylation.
The Pathway of Photosynthetic Electron Transfer Is Called
the Z Scheme
Photosystems I and II contain unique complements of electron carriers, and these carriers mediate the stepwise transfer of electrons from water to NADP When the
individual redox components of PSI and PSII are arranged as an etransport chain according to their standard reduction potentials, the zigzag result resembles the
let-ter Z laid sideways (Figure 21.11) The various electron carriers are indicated as
fol-lows: “Mn complex” symbolizes the manganese-containing oxygen-evolving complex;
D is its eacceptor and the immediate edonor to P680; QAand QBrepresent spe-cial plastoquinone molecules (see Figure 21.13) and PQ the plastoquinone pool;
Fe-S stands for the Rieske iron–sulfur center, and cyt f, cytochrome f PC is the abbreviation for plastocyanin, the immediate edonor to P700; and FA, FB, and FX
represent the membrane-associated ferredoxins downstream from A0(a specialized
Chl a) and A1 (a specialized PSI quinone) Fd is the soluble ferredoxin pool that
serves as the edonor to the flavoprotein (Fp), called ferredoxin–NADPⴙreductase, which catalyzes reduction of NADPto NADPH Cyt(b6)N ,(b6)Psymbolizes the
cyto-chrome b6moieties of the cytochrome b6f complex PQ and the cytochrome b6f
com-plex also serve to transfer e from FA/FB back to P700 during cyclic photophos-phorylation (the pathway symbolized by the dashed arrow)
Overall photosynthetic electron transfer is accomplished by three
membrane-spanning supramolecular complexes composed of intrinsic and extrinsic poly-peptides (shown as shaded boxes bounded by solid black lines in Figure 22.11)
These complexes are the PSII complex, the cytochrome b6f complex, and the PSI
complex The PSII complex is aptly described as a light-driven water ⬊plastoquinone oxidoreductase;it is the enzyme system responsible for photolysis of water, and as
such, it is also referred to as the oxygen-evolving complex, or OEC PSII possesses a
°
°
°
°
+
PSII
“blue” light < 680 nm
PSI
“red” light 700 nm
Strong oxidant > +0.8 V
Weak reductant
Weak oxidant
≅ 0.45 V
1 2
2
FIGURE 21.10 Roles of the two photosystems, PSI
and PSII.
Ferredoxin (Fd): A generic term for small
pro-teins possessing iron-sulfur clusters that
partici-pate in various electron-transfer reactions
Trang 7O2
2H +
4H +
Fp (FAD)
+1.60
+1.20
+0.80
+0.40
QA
QB
2 1 Protons released
in lumen
Protons taken up from stroma
h
Protons released into lumen
0
–0.40
Pheo
Chl a
P680*
Photosystem II
o '
–0.80
– 1.20
Mn complex
(Cyt b6)P
P680 D
PQ
Fe-S
Fd
Cyt f
h
A0
A1
FA
FB
FX
P700*
Photosystem I
P700 PC
(Cyt b6)N
H2O
H2O
(a)
NADPH +NADP+
H +
PQ
h
2
1
PC PC
Cyt b6
Cyt b6
Fd
4
+ +
Fd Photosystem I
NADP +
synthase Fp
(FAD)
Mn complex
H +
H +
+
PQ
h
Fe-S
Q B Fe
Q A
FeS A FeS B FeS X
A 1
A 0 Pheo
P680
Pheo
P700 Stroma
Lumen
(b)
NADPH
Cyt f
ACTIVE FIGURE 21.11 The Z scheme of photosynthesis (a) The Z scheme is a diagrammatic
rep-resentation of photosynthetic electron flow from H 2 O to NADP The energy relationships can be derived from the
Ᏹ o scale beside the Z diagram.Energy input as light is indicated by two broad arrows, one photon appearing in
P680 and the other in P700 P680* and P700* represent photoexcited states.The three supramolecular complexes
(PSI, PSII, and the cytochrome b6f complex) are in shaded boxes Proton translocations that establish the
proton-motive force driving ATP synthesis are illustrated as well (b) The functional relationships among PSII, the
cyto-chrome bf complex, PSI, and the photosynthetic CF1 CF 0–ATP synthase within the thylakoid membrane Test
yourself on the concepts in this figure at www.cengage.com/login.
Trang 8640 Chapter 21 Photosynthesis
metal cluster containing 4 Mn2atoms that coordinate two water molecules As P680 undergoes four cycles of light-induced oxidation, four protons and four electrons are removed from the two water molecules and their O atoms are joined to form O2
A tyrosyl side chain of the PSII complex (see following discussion) mediates electron transfer between the Mn2cluster and P680 The O2-evolving reaction requires Ca2 and Clions in addition to the (Mn2)4cluster
Oxygen Evolution Requires the Accumulation of Four Oxidizing Equivalents in PSII
When isolated chloroplasts that have been held in the dark are illuminated with very brief flashes of light, O2evolution reaches a peak on the third flash and every fourth flash thereafter (Figure 21.12a) The oscillation in O2 evolution dampens over repeated flashes and converges to an average value These data are interpreted
to mean that the P680 reaction center complex cycles through five different oxida-tion states, numbered S0to S4 One electron and one proton are removed photo-chemically in each step When S4 is attained, an O2molecule is released (Figure 21.12b) as PSII returns to oxidation state S0 and two new water molecules bind (The reason the first pulse of O2release occurred on the third flash [Figure 21.12a]
is that the PSII reaction centers in the isolated chloroplasts were already poised at
S1reduction level.)
Electrons Are Taken from H2O to Replace Electrons Lost from P680
The events intervening between H2O and P680 involve D, the name assigned to a specific protein tyrosine residue that mediates etransfer from H2O via the Mn com-plex to P680 (see Figure 21.11) The oxidized form of D is a tyrosyl free radical species, D To begin the cycle, an exciton of energy excites P680 to P680*,
where-upon P680* transfers an electron to a nearby Chl a molecule, which is the direct
elec-tron acceptor from P680* This Chl a then reduces a molecule of pheophytin,
sym-bolized by “Pheo” in Figure 21.11 Pheophytin is like chlorophyll a, except 2 H
replace the centrally coordinated Mg2 ion This special pheophytin is the direct electron acceptor from P680* Loss of an electron from P680* creates P680, the
electron acceptor for D Electrons flow from Pheo via specialized molecules of
plastoquinone,represented by “Q” in Figure 21.11, to a pool of plastoquinone (PQ) within the membrane Because of its lipid nature, plastoquinone is mobile within the membrane and hence serves to shuttle electrons from the PSII supramolecular
com-plex to the cytochrome b6f complex Alternate oxidation–reduction of
plasto-quinone to its hydroplasto-quinone form involves the uptake of protons (Figure 21.13) The asymmetry of the thylakoid membrane is designed to exploit this proton uptake and release so that protons (H) accumulate within the lumen of thylakoid vesicles, establishing an electrochemical gradient Note that plastoquinone is an analog of coenzyme Q, the mitochondrial electron carrier (see Chapter 20)
Electrons from PSII Are Transferred to PSI via
the Cytochrome b6 f Complex
The cytochrome b6f or plastoquinol⬊plastocyanin oxidoreductase is a large (210 kD)
multimeric protein possessing 26 transmembrane -helices This membrane protein
complex is structurally and functionally homologous to the cytochrome bc1complex (Complex III) of mitochondria (see Chapter 20) It includes the two heme-containing
electron transfer proteins for which it is named, as well as iron–sulfur clusters, which also
participate in electron transport The purpose of this complex is to mediate the trans-fer of electrons from PSII to PSI and to pump protons across the thylakoid membrane via a plastoquinone-mediated Q cycle, analogous to that found in mitochondrial
e transport (see Chapter 20) Cytochrome f (f from the Latin folium, meaning
“foliage”) is a c-type cytochrome, with a reduction potential of 0.365 V Cytochrome
b in two forms (low- and high-potential) participates in the oxidation of plastoquinol
O2
(a)
O2
Flash number
(b)
h
2 H2O
FIGURE 21.12 Oxygen evolution requires the
accumula-tion of four oxidizing equivalents in PSII (a) O2 evolution
after brief light flashes (b) The cycling of the PSII
reac-tion center through five different oxidareac-tion states, S 0 to S 4
One eis removed photochemically at each light flash,
moving the reaction center successively through S 1 , S 2 , S 3 ,
and S 4 S 4 decays spontaneously to S 0 by oxidizing 2 H 2 O
to O 2
e –
O
CH3
H +
+2 , 2 –2H +, 2e –
O H
CH3
O H
O
Plastoquinone A
Plastohydroquinone A
FIGURE 21.13 The structures of plastoquinone A and
its reduced form, plastohydroquinone (or plastoquinol).
Plastoquinone A has nine isoprene units and is the
most abundant plastoquinone in plants and algae.
Trang 9and the Q cycle of the b6f complex The cytochrome b6f complex can also serve in an
alternative cyclic electron transfer pathway Under certain conditions, electrons
de-rived from P700* are not passed on to NADPbut instead cycle down an alternative
path, whereby reduced ferredoxin contributes its electron to PQ This electron is then
passed to the cytochrome b6f complex, and then back to P700 This cyclic flow yields
no O2evolution or NADPreduction but can lead to ATP synthesis via so-called cyclic
photophosphorylation, discussed later
Plastocyanin Transfers Electrons from the Cytochrome b6 f
Complex to PSI
Plastocyanin (PC in Figure 21.11) is an electron carrier capable of diffusion along the
inside of the thylakoid and migration in and out of the membrane, aptly suited to its
role in shuttling electrons between the cytochrome b6f complex and PSI Plastocyanin
is a low-molecular-weight (10.4 kD) protein containing a single copper atom PC
func-tions as a single-electron carrier (Ᏹo 0.32 V) as its copper atom undergoes
alter-nate oxidation–reduction between the cuprous (Cu) and cupric (Cu2) states PSI is
a light-driven plastocyanin ⬊ferredoxin oxidoreductase When P700, the specialized
chlorophyll a dimer of PSI, is excited by light and oxidized by transferring its eto an
adjacent chlorophyll a molecule that serves as its immediate e acceptor, P700 is
formed (The standard reduction potential for the P700/P700 couple is about
0.45 V.) P700readily gains an electron from plastocyanin
The immediate electron acceptor for P700* is a special molecule of chlorophyll
This unique Chl a (A0) rapidly passes the electron to a specialized quinone (A1),
which in turn passes the eto the first in a series of membrane-bound ferredoxins This
Fd series ends with a soluble form of ferredoxin, Fds, which serves as the immediate
electron donor to the flavoprotein (Fp) that catalyzes NADPreduction, namely,
ferredoxin⬊NADPⴙreductase.
21.4 What Is the Molecular Architecture of Photosynthetic
Reaction Centers?
What molecular architecture couples the absorption of light energy to rapid
electron-transfer events, in turn coupling these etransfers to proton translocations
so that ATP synthesis is possible? Part of the answer to this question lies in the
membrane-associated nature of the photosystems A major breakthrough occurred
Structure of the cyanobacterial cytochrome b6f
com-plex The heme groups of cytochromes b6N , b6P , and f
are shown in red; the iron-sulfur clusters are blue (pdb id 1BF5).The upper bundle of -helices defines the transmembrane domain.
Trang 10642 Chapter 21 Photosynthesis
in 1984, when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein To the great benefit
of photosynthesis research, this protein was the reaction center from the
photosyn-thetic purple bacterium Rhodopseudomonas viridis This research earned these three
scientists the 1984 Nobel Prize in Chemistry
The R viridis Photosynthetic Reaction Center Is an Integral
Membrane Protein
R viridis is a photosynthetic prokaryote with a single photosystem that resembles PSII
(even though it lacks an OEC and the capacity to oxidize water) The reaction center
(145 kD) of the R viridis photosystem is localized in the plasma membrane of these photosynthetic bacteria and is composed of four different polypeptides, designated L (273 amino acid residues), M (323 residues), H (258 residues), and cytochrome (333 amino acid residues) (Figure 21.14a) L and M each consist of five
membrane-spanning-helical segments; H has one such helix, the majority of the protein
form-ing a globular domain in the cytoplasm (Figure 21.14b) The cytochrome subunit contains four heme groups; the N-terminal amino acid of this protein is cysteine This cytochrome is anchored to the periplasmic face of the membrane via the hydropho-bic chains of two fatty acid groups that are esterified to a glyceryl moiety joined via a
thioether bond to the Cys (Figure 21.14a) L and M each bear two bacteriochlorophyll molecules (the bacterial version of Chl) and one bacteriopheophytin L also has a bound
quinone molecule, QA Together, L and M coordinate an Fe atom The
photochemi-cally active species of the R viridis reaction center, P870, is composed of two
bacterio-chlorophylls, one contributed by L and the other by M.
Photosynthetic Electron Transfer by the R viridis Reaction Center
Leads to ATP Synthesis
The prosthetic groups of the R viridis reaction center (P870, BChl, BPheo, and
the bound quinones) are fixed in a spatial relationship to one another that favors
photosynthetic etransfer (Figure 21.14a,c) Photoexcitation of P870 (creation of
P870*) leads to eloss (P870) via electron transfer to the nearby
bacteriochloro-Cytochrome with
4 heme groups
hν
P870
BPheo
BPheo
<1 psec
20 psec
230 psec
QA
100
H
Note: The cytochrome subunit is membrane
associated via a diacylglycerol moiety on its
N-terminal Cys residue:
H
C
O
CH2
Membrane anchor +
FIGURE 21.14 The R viridis reaction center (RC) (a) Diagram of the RC showing light activation and path of
etransfer (b) Molecular graphic of the R viridis RC M and L are yellow and blue; H is orange; the cytochrome
is green (c) Deletion of the R viridis RC protein chains reveals the spatial relationship between its heme,
chlorophyll, and quinone prosthetic groups The iron atom is represented by a sphere (pdb id 1PRC).