Membrane bound protein complex

The development of efficient algorithms and the sophisticated description of long-range electrostatic effects allow calculations on systems with 100 000 atoms and more, which address biochemical problems like membrane-bound protein complexes or the action of molecular machines . 

In photosynthesis, water oxidation is accomplished by photosystem II (PSII), which is a large membrane-bound protein complex (158-161). To the central core proteins D1 and D2 are attached different cofactors, including a redox-active tyro-syl residue, tyrosine Z (Yz) (158-162), which is associated with a tetranuclear manganese complex (163). These components constitute the water oxidizing complex (WOC), the site in which the oxidation of water to molecular oxygen occurs (159, 160, 164). The organization is schematically shown in Fig. 18. 

All photosynthetic systems share common features. The conversion of light energy to a metabolically usable form is achieved by means of 1. lightabsorbing pigment-protein complexes, 2. electron transport components that produce vectorial electron transport within and around membrane boimd compartments, 3. membrane bound protein complexes, such as ATPases, that couple the relaxation of a transmembrane Ap to transport or synthesis, e.g. of ATP, and 4. reductases that reduce stable electron or hydrogen carriers that can be used to drive reductive... 

Marcus theory predicts that the nuclear factor in the electron transfer rate expression will be maximal when - AG° = X. Under these conditions, the electron transfer process will be temperature independent. The first two electron transfer steps in the RC approximately exhibit this behavior. The rate of the D 4>a step actually increases slightly (by a factor of 2-4) as the temperature decreases from 300 to 8 K (180). Consequently, X may be estimated to be in the range 0.3 to 0.5 V (—7-10 kcal/mol) from the A ° values for these two steps (Table IV). These values are approximately 4 times smaller than those observed for ruthenated proteins discussed previously. Sequestration of the redox groups in a membrane-bound protein complex, away from aqueous solution, may serve to decrease the value of X by minimizing the reorganization energy of a highly polar solvent. 

D. vulgaris and two ionizable centers in D. desulfuricans ATCC27774 that result in concerted two-electron/two-proton transfer steps has been called proton-thrusting, because it is akin to the redox-linked proton-transfer mechanisms seen in membrane-bound protein complexes involved in proton pumping. 

Biological proton transfers take place in proteins. These are either soluble enzymes or membrane-bound protein complexes, whose function requires proton exchanges with the solvent. Protons cannot diffuse freely in the protein medium but fast proton transfers, compatible with the millisecond timescale of most biological processes, can occur along chains of well-defined protonatable groups. These groups are either water molecules or ionisable residues like Asp, Glu, Ser and His. Their proton affinity is quantified by their pATa whose value may differ from that measured in water. 

In the respiratory chain, electrons from the powerful reducing agents NADH and FADH2 pass through four membrane-bound protein complexes and two mobile electron carriers before reducing O2 to H2O. We shall see that the electron transfer reactions drive the synthesis of ATP at three of the membrane protein complexes. 

The modem era of biochemistry and molecular biology has been shaped not least by the isolation and characterization of individual molecules. Recently, however, more and more polyfunctional macromolecular complexes are being discovered, including nonrandomly codistributed membrane-bound proteins [41], These are made up of several individual proteins, which can assemble spontaneously, possibly in the presence of a lipid membrane or an element of the cytoskeleton [42] which are themselves supramolecular complexes. Some of these complexes, e.g. snail haemocyanin [4o], are merely assembled from a very large number of identical subunits vimses are much larger and more elaborate and we are still some way from understanding the processes controlling the assembly of the wonderfully intricate and beautiful stmctures responsible for the iridescent colours of butterflies and moths [44]. 

Fig. 3 Binding and release of tropoelastin. The elastin receptor consists of a 67 kDa peripheral subunit (EBP) with two transmembrane proteins of 61 and 55 kDa. The 67 kDa protein binds tropoelastin and galactosugars through two separate sites, (a) Tropoelastin binds to the intact EBP complex, (b) Upon binding of a galactosugar, the EBP loses its affinity for both tropoelastin and the membrane-bound protein, which leads to the release of tropoelastin. Reproduced from [8] with permission from John Wiley and Sons, copyright 1998...

Carotenoids are also present in animals, including humans, where they are selectively absorbed from diet (Furr and Clark 1997). Because of their hydrophobic nature, carotenoids are located either in the lipid bilayer portion of membranes or form complexes with specific proteins, usually associated with membranes. In animals and humans, dietary carotenoids are transported in blood plasma as complexes with lipoproteins (Krinsky et al. 1958, Tso 1981) and accumulate in various organs and tissues (Parker 1989, Kaplan et al. 1990, Tanumihardjo et al. 1990, Schmitz et al. 1991, Khachik et al. 1998, Hata et al. 2000). The highest concentration of carotenoids can be found in the eye retina of primates. In the retina of the human eye, where two dipolar carotenoids, lutein and zeaxan-thin, selectively accumulate from blood plasma, this concentration can reach as high as 0.1-1.0mM (Snodderly et al. 1984, Landrum et al. 1999). It has been shown that in the retina, carotenoids are associated with lipid bilayer membranes (Sommerburg et al. 1999, Rapp et al. 2000) although, some macular carotenoids may be connected to specific membrane-bound proteins (Bernstein et al. 1997, Bhosale et al. 2004). 

G-proteins can be found membrane bound or free in the cytosol. The membrane bound proteins are trimeric complexes of a, [3 and y subunits. The [3 and y subunits may... 

In order to relate structure and function at a more direct level, it is necessary to focus on systems which have better characterized structure than the complex membrane bound proteins like cyt c oxidase. One particularly useful paradigm in this context is the cytochrome c-cytochrome c peroxidase couple [18]. Cep is not involved in electron transport, per se its apparent function [19] is detoxification of hydrogen peroxide via the sequence H2O2 -I- cep Fe(III) -> H2O -I- cep Fe(IV) O (protein) compound ES ... 

An important function of the PDZ domains hes in the formation of macromolecular associates at the cell membrane (review Pawson and Scott, 1997). PDZ proteins can also provide a framework for clustering of proteins, such as ion channels, at the cell membrane and they may help to recruit proteins into membrane-bound macromolecular complexes. 

Specific detection of nitrocellulose membrane-bound proteins using a conjugated enzyme. (1) Proteins are transferred from electrophoresis gel to nitrocellulose membrane. Blocker proteins bind to unoccupied sites on the membrane. (2) The membrane is incubated with a primary antibody directed against the protein of interest. (3) A secondary antibody is directed against the primary antibody. (4) The second antibody is conjugated with an enzyme to provide a detection mechanism. Substrate solution is added to the blot. The conjugated enzyme (HRP or AP) catalyzes the conversion of substrate (S) to product (P) to form a colored precipitate at the site of the protein-antibody complex. 

For many years, die nature and location of the complex of proteins (sometimes referred to as the engine of photosynthesis) were poorly understood. During the 1980s, much more was learned as the result of research carried out by Johann Deisenhofer (Howard Hughes Medical Institute), Robert Huber, and Harmut Michel (Max Planck Institute), and for this work the investigators were awarded the 1988 Nobel Prize for chemistry. The protein complex, called the membrane-bound proteins, are difficult to define structurally because they do not crystallize readily and thus could not be subjected to x-ray crystallography. However, over a period of three years, the researchers were able to create crystals and thus were able to determine precisely the position of some 10,000 atoms in the protein complex. 

The transport or release of iron has been much discussed. One view is that the iron is transferred from the siderophore complex at the outer membrane to another membrane-bound protein, for example in the uptake of iron by rhodotorulic acid in Rhodotorula. The other view is that the intact Fein-siderophore complex is taken up into the cell. This is supported by studies with inert chromium(III) complexes in E. coli and also by labelling studies. 

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