Structure And Organization Of Chloroplasts

Chloroplasts are cytoplasmic organelles that are enclosed by a double membrane or envelope, which gives them an ellipsoidal or oval shape of about 1 by 5 (1,6-8). They are structurally sound entities that can be isolated in functional form from many plants (9). The interior of the organelle consists of an extensive membrane network (thylakoids), and an aqueous phase (stroma). Thylakoid membranes are interconnected series of disk-shaped, membrane-limited vesicles that encase a lumenal region. The pigments and proteins required for light harvesting and photosynthetic electron transport are associated with the thylakoid membrane (10).

The dark reactions of photosynthesis occur in the chloroplast stroma (1,11). This soluble phase contains the metabolic pathways that utilize the chemical energy (ATP) and reducing power (NADPH) generated by light reactions. The most conspicuous stromal process is the reductive pentose phosphate (RPP) cycle (Calvin-Benson cycle, photosynthetic carbon reduction cycle), which converts carbon dioxide to carbohydrate (11,12). Other important stromal biosynthetic pathways include amino acid synthesis (13), reduction and metabolism of nitrate and ammonia (14), and reduction of sulfate (13).

Photosynthetic Membranes and Photosystems

In oxygenic plants, algae and cyanobacteria, photosynthesis is carried out with the help of pigment-protein complexes associated with specialized structures called thylakoid membranes. Thylakoid membranes in plants and algae are located in the chloroplast, whereas in cyanobacteria these membranes arise from the inner cell membrane into the cytoplasm. Thylakoids from plants and algae, but not cyanobacteria, have two morphologies: stacked membranes (granal lamellae) and nonstacked membranes (stromal lamellae) (1,6-8). The thylakoid membrane contains four distinct complexes (Fig. 1)—photosystem II (PSII), photosystem I (PSI), cytochrome b6/f, and the ATP synthase—that are each composed of several protein sub-units, as well as chromophores and cofactors (eg, the chlorophylls, carotenoids, and hemes) (10,15,16). The complexes are spatially organized in the thylakoid membrane such that PSII is in the granal lamellae, and PSI and the ATP synthase are located only in the stromal lamellae. The cytochrome b6/f complex is distributed into both morphologies of thylakoids. Contrary to oxygenic photosynthetic organisms, the anoxygenic photosynthetic bacterial pigment-protein complexes are located either directly in the cell membrane or in the invaginations that arise from the membrane.

All oxygenic photosynthetic organisms, including plants, algae, and cyanobacteria, contain two photosystems, PS I and PS II (Fig. 2a). These photosystems are the sites for the photosynthetic electron transport and are embedded in the thylakoid membrane. Each photosystem is composed of pigments, proteins, and inorganic and organic cofactors that participate in the photosynthetic electron transport. Nonoxygenic photosynthetic bacteria contain only one photosystem (Figs. 2b and 2c), also composed of pigments, proteins, and other cofactors that participate in the electron transport.

Photosynthetic Pigments and Pigment-Protein Complexes

In oxygenic photosynthetic organisms, photosynthesis is initiated by visible radiation (400-700 nm) that is absorbed by pigments. The most abundant photosynthetic pigment is chlorophyll (Chi), a cyclic tetrapyrrole with a bound Mg+2 (Fig. 3a). Chi is utilized for both light collection and initiation of electron transport. In higher plants and green algae there are two forms of Chi, Chla and Chlb (17,18). They absorb in the blue (400-480 nm) and red (620-700 nm) spectral regions (Fig. 3b). Chla is involved in light capture and electron transport, while Chlb only has a light-harvesting role (15,19). Carotenoids, which absorb in the blue and green spectral regions, also serve as photosynthetic pigments in higher plants (19). Carotenoids, however, only play a modest role in light capture, because they are in relatively low concentration. Carotenoids also play a key role in protecting the chloroplast from excess light via the xanthophyll cycle (for more details see Refs. 20 and 21). All Chls and carotenoids used as pigments are bound to integral membrane proteins. The in

Stroma ATP Synthase

Figure 1. Organization of the four major thylakoid membrane complexes. PSII is constructed in two parts; the outer antenna (LHCII) and the core complex (the inner antenna [CP43 and CP47], cytochrome b559 and the D1/D2 reaction center). In PSII, electron flow is as indicated from water, through P680, to QB. The oxygen evolving Mn4 cluster in the reaction center is stabilized by the 33-, 23- and 17-kDa polypeptides on the lumenal face of PSII. Although not depicted here, PSII is the stacked thylakoids. The cytochrome b6/f complex is composed of four polypeptides. There are two electron pathways through this complex; one directly to PC and the other into the Q-cycle. PSI is also composed in two parts: the outer antenna (LHCI), and the core complex (CCI, which contains the inner antenna Chls, the reaction center Chls [P700], and electron acceptors). Electron flow is from PC to the Fd. CCI-III is a PC docking protein and CCI-II is an Fd docking protein. For both photosystems, light energy flows from the outer antenna, to the inner antenna, and finally to P680 or P700. The protons from water and PQH2 oxidation drive ATP synthesis through the ATP synthase. Pigments and cofactors are as defined in Fig. 5 and the text.

Figure 1. Organization of the four major thylakoid membrane complexes. PSII is constructed in two parts; the outer antenna (LHCII) and the core complex (the inner antenna [CP43 and CP47], cytochrome b559 and the D1/D2 reaction center). In PSII, electron flow is as indicated from water, through P680, to QB. The oxygen evolving Mn4 cluster in the reaction center is stabilized by the 33-, 23- and 17-kDa polypeptides on the lumenal face of PSII. Although not depicted here, PSII is the stacked thylakoids. The cytochrome b6/f complex is composed of four polypeptides. There are two electron pathways through this complex; one directly to PC and the other into the Q-cycle. PSI is also composed in two parts: the outer antenna (LHCI), and the core complex (CCI, which contains the inner antenna Chls, the reaction center Chls [P700], and electron acceptors). Electron flow is from PC to the Fd. CCI-III is a PC docking protein and CCI-II is an Fd docking protein. For both photosystems, light energy flows from the outer antenna, to the inner antenna, and finally to P680 or P700. The protons from water and PQH2 oxidation drive ATP synthesis through the ATP synthase. Pigments and cofactors are as defined in Fig. 5 and the text.

teractions with protein widen absorbance bands of the pigments into the green spectral region (500-600 nm) (22).

Oxygenic photosynthetic cyanobacteria use Chla for light capture and initiation of electron transport. However, in place of Chlb-containing light-harvesting antennae, they possess special pigment-proteins, phycoerythrin, phycocyanin, and allophycocyanin, which serve a light-harvesting role. These pigment-proteins are organized into phycobilisomes and harvest red, blue, and green light (23). Contrary to the plant light-harvesting pigments that are embedded in the thylakoid membrane, the phycobilisomes are attached on the stromal surface of the thylakoid membrane.

All chlorophyll is bound exclusively to integral thylakoid membrane proteins (22,24). There are two classes of Chl-containing complexes: (1) antenna, which hold the vast majority of pigment (22,24,25) and (2) reaction centers, which have much less Chi but are the site of primary photochemistry initiated by special chlorophylls, P680 (PS II reaction center chlorophyll) and P700 (PS I reaction center chlorophyll) (16,26). Several antenna complexes combine with one reaction center to form a single photosystem.

The light-harvesting Chl-binding proteins of higher plants may be classified as either outer or inner antenna depending on their position relative to a reaction center (24-26). The outer antenna apoproteins, termed light-harvesting chlorophyll proteins (LHC) (Fig. 1), each bind a total of 7 to 10 Chla plus Chlb molecules. Those associated with PSII (LHCII) range in size from 24 to 29 kDa, while their PSI counterparts (LHCI) are generally smaller at 17 to 24 kDa (27,28). Each LHC polypeptide is proposed to contain three membrane spanning helices (29), and most LHCs share at least some primary amino acid sequence homology (30).

The inner antenna of PSII (part of the core complex [CC] of PSII [19,22,24,25]) is contained on two homologous polypeptides (CP43 and CP47, numbers refer to their molecular weight in kDa). These proteins are tightly bound to the PSII reaction center complex (Fig. 1). Each polypeptide binds approximately 25 Chla molecules (31). The inner antenna of PSI is composed of 40 to 50 Chla molecules, which are bound to the two reaction center apoproteins (CCI-subunit I) that contain P700 (25,32). The total antenna size (outer and inner antenna combined) of the respective photosystems is approximately 250 Chls per P680 and 150 Chls per P700 (15,25). The relative amount of excitation energy funneled to the two reaction centers from the antenna can be balanced by a process involving state I-state II transitions (1,33). Reversible binding of LHCII to PSII is associated with this phenomenon.

A photon of light is absorbed by an antenna Chi, and the energy is passed by resonance transfer from the outer antenna to the inner antenna and, finally, to P680 or P700 (Fig. 1). The movement of excitation energy is always

(a) Plants, algae and cyanobacteria

P700*

P680*

2H20

X Mn4

Pheo

PQH2

PQH2

(b) Purple bacteria

P870*

(c) Heliobacteria P840* Ao

FNR-a

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