Light Absorption by Chlorophyll Induces Electron Transfer

II. Transducing and Storing Energy
19. The Light Reactions of Photosynthesis
19.2. Light Absorption by Chlorophyll Induces Electron Transfer
The trapping of light energy is the key to photosynthesis. The first event is the absorption of light by a photoreceptor
molecule. The principal photoreceptor in the chloroplasts of most green plants is chlorophyll a, a substituted tetrapyrrole
(Figure 19.5). The four nitrogen atoms of the pyrroles are coordinated to a magnesium ion. Unlike a porphyrin such as
heme, chlorophyll has a reduced pyrrole ring. Another distinctive feature of chlorophyll is the presence of phytol, a
highly hydrophobic 20-carbon alcohol, esterified to an acid side chain.
Chlorophylls are very effective photoreceptors because they contain networks of alternating single and double bonds.
Such compounds are called polyenes. They have very strong absorption bands in the visible region of the spectrum,
where the solar output reaching Earth also is maximal (Figure 19.6). The peak molar absorption coefficient (ε, Section
3.1) of chlorophyll a is higher than 105 M-1 cm-1, among the highest observed for organic compounds.
What happens when light is absorbed by a molecule such as chlorophyll? The energy from the light excites an electron
from its ground energy level to an excited energy level (Figure 19.7). This high-energy electron can have several fates.
For most compounds that absorb light, the electron simply returns to the ground state and the absorbed energy is
converted into heat. However, if a suitable electron acceptor is nearby, the excited electron can move from the initial
molecule to the acceptor (Figure 19.8). This process results in the formation of a positive charge on the initial molecule
(due to the loss of an electron) and a negative charge on the acceptor and is, hence, referred to as photoinduced charge
separation. The site where the separational change occurs is called the reaction center. We shall see how the
photosynthetic apparatus is arranged to make photoinduced charge separation extremely efficient. The electron, extracted
from its initial site by absorption of light, can reduce other species to store the light energy in chemical forms.
19.2.1. Photosynthetic Bacteria and the Photosynthetic Reaction Centers of Green
Plants Have a Common Core
Photosynthesis in green plants is mediated by two kinds of membrane-bound, light-sensitive complexes photosystem I
(PS I) and photosystem II (PS II). Photosystem I typically includes 13 polypeptide chains, more than 60 chlorophyll
molecules, a quinone (vitamin K1), and three 4Fe-4S clusters. The total molecular mass is more than 800 kd.
Photosystem II is only slightly less complex with at least 10 polypeptide chains, more than 30 chlorophyll molecules, a
nonheme iron ion, and four manganese ions. Photosynthetic bacteria such as Rhodopseudomonas viridis contain a
simpler, single type of photosynthetic reaction center, the structure of which was revealed at atomic resolution. The
bacterial reaction center consists of four polypeptides: L (31 kd), M (36 kd), and H (28 kd) subunits and C, a c-type
cytochrome (Figure 19.9). The results of sequence comparisons and low-resolution structural studies of photosystems I
and II revealed that the bacterial reaction center is homologous to the more complex plant systems. Thus, we begin our
consideration of the mechanisms of the light reactions within the bacterial photosynthetic reaction center, with the
understanding that many of our observations will apply to the plant systems as well.
19.2.2. A Special Pair of Chlorophylls Initiates Charge Separation
The L and M subunits form the structural and functional core of the bacterial photosynthetic reaction center (see Figure
19.9). Each of these homologous subunits contains five transmembrane helices. The H subunit, which has only one
transmembrane helix, lies on the cytoplasmic side of the membrane. The cytochrome subunit, which contains four c-type
hemes, lies on the opposite periplasmic side. Four bacteriochlorophyll b (BChl-b) molecules, two bacteriopheophytin b
(BPh) molecules, two quinones (QA and QB), and a ferrous ion are associated with the L and M subunits.
Bacteriochlorophylls are similar to chlorophylls, except for the reduction of an additional pyrrole ring and some other
minor differences that shift their absorption maxima to the near infrared, to wavelengths as long as 1000 nm.
Bacteriopheophytin is the term for a bacteriochlorophyll that has two protons instead of a magnesium ion at its center.
The reaction begins with light absorption by a dimer of BChl-b molecules that lie near the periplasmic side of the
membrane. This dimer, called a special pair because of its fundamental role in photosynthesis, absorbs light maximally
at 960 nm, in the infrared near the edge of the visible region. For this reason, the special pair is often referred to as P960
(P stands for pigment). Excitation of the special pair leads to the ejection of an electron, which is transferred through
another molecule of BChl-b to the bacteriopheophytin in the L subunit (Figure 19.10, steps 1 and 2). This initial charge
separation, which yields a positive charge on the special pair (P960+) and a negative charge on BPh, occurs in less than
10 picoseconds (10-11 seconds). Interestingly, only one of the two possible paths within the nearly symmetric L-M dimer
is utilized. In their high-energy states, P960+ and BPh- could undergo charge recombination; that is, the electron on BPhcould move back to neutralize the positive charge on the special pair. Its return to the special pair would waste a valuable
high-energy electron and simply convert the absorbed light energy into heat. Three factors in the structure of the reaction
center work together to suppress charge recombination nearly completely (Figure 19.10, steps 3 and 4). First, another
electron acceptor, a tightly bound quinone (QA), is less than 10 Å away from BPh-, and so the electron is rapidly
transferred farther away from the special pair. Recall that electron-transfer rates depend strongly on distance (Section
18.2.3). Second, one of the hemes of the cytochrome subunit is less than 10 Å away from the special pair, and so the
positive charge is neutralized by the transfer of an electron from the reduced cytochrome. Finally, the electron transfer
from BPh- to the positively charged special pair is especially slow: the transfer is so thermodynamically favorable that it
takes place in the inverted region where electron-transfer rates become slower (Section 18.2.3). Thus, electron transfer
proceeds efficiently from BPh- to QA.
From QA, the electron moves to a more loosely associated quinone, QB. The absorption of a second photon and the
movement of a second electron down the path from the special pair completes the two-electron reduction of QB from Q
to QH2. Because the QB-binding site lies near the cytoplasmic side of the membrane, two protons are taken up from the
cytoplasm, contributing to the development of a proton gradient across the cell membrane (Figure 19.10, steps 5, 6, and
7).
How does the cytochrome subunit of the reaction center regain an electron to complete the cycle? The reduced quinone
(QH2) is reoxidized to Q by complex III of the respiratory electron-transport chain (Section 18.3.3). The electrons from
the reduced quinone are transferred through a soluble cytochrome c intermediate, called cytochrome c 2, in the periplasm
to the cytochrome subunit of the reaction center. The flow of electrons is thus cyclic. The proton gradient generated in
the course of this cycle drives the generation of ATP through the action of ATP synthase.
II. Transducing and Storing Energy
19. The Light Reactions of Photosynthesis
19.2. Light Absorption by Chlorophyll Induces Electron Transfer
Figure 19.5. Chlorophyll. Like heme, chlorophyll a is a cyclic tetrapyrrole. One of the pyrrole rings (shown in red) is
reduced. A phytol chain (shown in green) is connected by an ester linkage. Magnesium ion binds at the center of the
structure.
II. Transducing and Storing Energy
19. The Light Reactions of Photosynthesis
19.2. Light Absorption by Chlorophyll Induces Electron Transfer
Figure 19.6. Light Absorption By Chlorophyll A. Chlorophyll a absorbs visible light efficiently as judged by the
extinction coefficients near 105 M-1 cm-1.
II. Transducing and Storing Energy
19. The Light Reactions of Photosynthesis
19.2. Light Absorption by Chlorophyll Induces Electron Transfer
Figure 19.7. Light Absorption. The absorption of light leads to the excitation of an electron from its ground state to a
higher energy level.
II. Transducing and Storing Energy
19. The Light Reactions of Photosynthesis
19.2. Light Absorption by Chlorophyll Induces Electron Transfer
Figure 19.8. Photoinduced Charge Separation. If a suitable electron acceptor is nearby, an electron that has been
moved to a high energy level by light absorption can move from the excited molecule to the acceptor.
II. Transducing and Storing Energy
19. The Light Reactions of Photosynthesis
19.2. Light Absorption by Chlorophyll Induces Electron Transfer
Figure 19.9. Bacterial Photosynthetic Reaction Center. The core of the reaction center from Rhodopseudomonas
viridis consists of two similar chains: L (red) and M (blue). An H chain (white) and a cytochrome subunit (yellow)
complete the structure. A chain of prosthetic groups, beginning with a special pair of bacteriochlorophylls and
ending at a bound quinone, runs through the structure from top to bottom in this view.
II. Transducing and Storing Energy
19. The Light Reactions of Photosynthesis
19.2. Light Absorption by Chlorophyll Induces Electron Transfer