Proteins chlorophyll bound

Induced Circular Dichroism of Heme and Chlorophyll Bound to Proteins... 

The porphyrin dyes hemoglobinen and chlorophyll are biologically important. Both are octahedral metal complexes, the proteins being bound to the central atom. In the hemoglobin molecule there are five ligand positions occupied by histidine. 

In this contribution, we extend previous resonance Raman investigations of the Dl-D2-cytochrome bggg complex (3,4,5). Structural features of chlorophylls bound to Dl-D2-cytochrome bjs complexes and the chlorophyll-containing 47 kDa protein are determined by using both resonance and nearresonance conditions. 

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

The immediate electron acceptor for P700 is a special molecule of chlorophyll. This unique Chi a (Aq) rapidly passes the electron to a specialized quinone (Aj), which in turn passes the e to the first in a series of membrane-bound ferredoxins (Fd, Chapter 21). This Fd series ends with a soluble form of ferredoxin, Fd, which serves as the immediate electron donor to the fiavo-protein (Fp) that catalyzes NADP reduction, namely, ferredoxin NADP reductase. 

The structure of the major trimeric LHCII complex has been recently obtained at 2.72 A (Figure 7.3) (Liu et al., 2004). It was revealed that each 25kDa protein monomer contains three transmembrane and three amphiphilic a-helixes. In addition, each monomer binds 14 chlorophyll (8 Chi a and 6 Chi b) and 4 xanthophyll molecules 1 neoxanthin, 2 luteins, and 1 violaxanthin. The first three xanthophylls are situated close to the integral helixes and are tightly bound to some amino acids by hydrogen bonds to hydroxyl oxygen atoms and van der Waals interactions to chlorophylls, and hydrophobic amino acids such as tryptophan and phenylalanine. 

Mg is bound in a 6 coordinate site by four or less protein O-donors and several water molecules often in mixed a//3 proteins in machinery. (Note the extraordinary binding of Mg2+ to five-nitrogen donors in chlorophyll proteins.)... 

In plants, the photosynthesis reaction takes place in specialized organelles termed chloroplasts. The chloroplasts are bounded in a two-membrane envelope with an additional third internal membrane called thylakoid membrane. This thylakoid membrane is a highly folded structure, which encloses a distinct compartment called thylakoid lumen. The chlorophyll found in chloroplasts is bound to the protein in the thylakoid membrane. The major photosensitive molecules in plants are the chlorophylls chlorophyll a and chlorophyll b. They are coupled through electron transfer chains to other molecules that act as electron carriers. Structures of chlorophyll a, chlorophyll b, and pheophytin a are shown in Figure 7.9. 

Photosystem II (Fig. 1) bears many similarities to the much simpler reaction center of purple bacteria. Remarkable is, however, the increase in complexity at the protein level. In a recent review on the evolutionary development of the type 11 reaction centres340 this was attributed to the invention of water-splitting by PS II and the necessity to protect and repair the photosynthetic machinery against the harmful effects of molecular oxygen. The central part of PS II and the bRC show a highly conserved cofactor arrangement,19 see Fig. 1. These cofactors are arranged in two branches bound to two protein subunits, L/M and D1/D2 in bRC and PS II, respectively. On the donor side a closely related pair of chlorophylls or bacteriochlorophylls exists the acceptors comprise monomeric chlorophylls, pheophytins (Ph) and 2 quinones QA and QB. Qa and Qb are plas-... 

FIGURE 19-43 A phycobilisome. In these highly structured assemblies found in cyanobacteria and red algae, phycobilin pigments bound to specific proteins form complexes called phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (AP). The energy of photons absorbed by PE or PC is conveyed through AP (a phycocyanobilin-binding protein) to chlorophyll a of the reaction center by exciton transfer, a process discussed in the text. 

Metal ion chelates of various porphyrins, differing in their substituents at positions 1-8, are intimately involved in a great number of life processes. Iron protoporphyrin (13) is the most common form and serves as the cofactor of a large number of enzymes. Usually (13) is non-covalently bound to its conjugate apoenzymes. Examples of covalently attached (13) are provided by c-type cytochromes, the attachment being between two vinyl side chains of (13) and two cysteine residues of the protein. Other biologically important derivatives of porphyrin include chlorophyll a (14), bacteriochlorophyll a and heme a (B-79MI11002). 

Figure 23-17 The zigzag scheme (Z scheme) for a two-quantum per electron photoreduction system of chloroplasts. Abbreviations are P680 and P700, reaction center chlorophylls Ph, pheophytin acceptor of electrons from PSII QA, Qg, quinones bound to reaction center proteins PQ, plastoquinone (mobile pool) Cyt, cytochromes PC, plastocyanin A0 and Aj, early electron acceptors for PSI, possibly chlorophyll and quinone, respectively Fx, Fe2S2 center bound to reaction center proteins FA, FB, Fe4S4 centers Fd, soluble ferredoxin and DCMU, dichlorophenyldimethylurea. Note that the positions of P682, P700, Ph, Qa/ Qb/ Ay and A, on the E° scale are uncertain. The E° values for P682 and P700 should be for the (chlorophyll / chlorophyll cation radical) pair in the reaction center environment. These may be lower than are shown.

Bacteriochlorophyll in Chromatium has three absorption bands with peak positions at 800, 850, and 890 nm. The last includes the reaction center bacteriochlorophyll and is the only form that fluoresces. Recent studies have established that most if not all chlorophyll is bound to specific proteins, a fact that can account for the various overlapping absorption bands. 

Eukaryotic plants and cyanobacteria. Photosynthetic dinoflagellates, which make up much of the marine plankton, use both carotenoids and chlorophyll in light-harvesting complexes. The carotenoid peridinin (Fig. 23-29), which absorbs blue-green in the 470- to 550-nm range, predominates. The LH complex of Amphidinium carterae consists of a 30.2-kDA protein that forms a cavity into which eight molecules of peridinin but only two of chlorophyll a (Chi a) and two molecules of a galactolipid are bound (Fig. 23-29).268... 

Figure 23-28 (A) Model of a light-harvesting chlorosome from green photosynthetic sulfur bacteria such as Chlorobium tepidum and species of Prosthecochloris. The chlorosome is attached to the cytoplasmic membrane via a baseplate, which contains the additional antenna bacteriochlorophylls (795 BChl a) and is adjacent to the trimeric BChl protein shown in (B) and near the reaction center. After Li et al.302 and Remigy et a/.304 (B) Alpha carbon diagram of the polypeptide backbone and seven bound BChl a molecules in one subunit of the trimeric protein from the green photosynthetic bacterium Prosthecochloris. For clarity, the magnesium atoms, the chlorophyll ring substituents, and the phytyl chains, except for the first bond, are omitted. The direction of view is from the three-fold axis, which is horizontal, toward the exterior of the molecule. From Fenna and Matthews.305 See also Li et al.302...

The reaction centers are embedded in the cytoplasmic membranes of the bacteria, with the bottom of the structure, as shown in Fig. 23-31, protruding into the cytoplasm and the heme protein at the top projecting out into the periplasm which lies within infoldings of the plasma membrane. Subunits L and M each contain five 4.0 nm long roughly parallel helices, which span the cytoplasmic membrane. Another membrane-spanning helix is contributed by subunit H, which is located mainly on the cytoplasmic side. An approximate twofold axis of symmetry relates subunits L and M and the molecules of bound chlorophyll and pheo-phytin. 

The Reactive Chlorophyll Is Bound to Proteins in Reaction Centers... 

The chlorophyll or bacteriochlorophyll that undergoes pho-tooxriiation is bound to a protein in a complex caffed a reaction center. Reaction centers have been purified by disrupt-... 

Photooxidation of P700 in photosystem I reduces a chlorophyll, which transfers electrons to a series of membrane-bound iron-sulfur centers, probably by way of a quinone. From the iron-sulfur centers, electrons move to the soluble iron-sulfur protein, ferre-... 

---