Mn protein complex

This reaction can be simplified by the following three major processes (Eqs. (19.2) to (19.4)), where water is an electron donor which provides electrons to the whole system obtained at the Mn protein complex (Eq. (19.2)), the electrons from water are excited by solar photons at the chlorophyl reaction center (two ... 

Although catalytic water oxidation (dark reaction) is the first and important reaction of the electron flow in the photosynthesis represented by Fig. 19.1 whereby water is used as the source of electrons provided to the whole system, its catalyst and reaction mechanism are not yet established.10-13) In the photosynthesis Mn-protein complex works as a catalyst for the difficult four-electron oxidation of two molecules of water to liberate one 02 molecule (Eq. (19.2)). It is inferred that at least four Mn ions are involved in the active center, but its structure is not yet completely elucidated. 

Photosynthetic Oxygen-Evolving Center Composed of a Mn-Protein Complex... 

Figure Bl.17.6. A protein complex (myosin SI decorated filamentous actin) embedded in a vitrified ice layer. Shown is a defociis series at (a) 580 mn, (b) 1130 mn, (c) 1700 mn and (d) 2600 mn underfocus. The pictures result from averagmg about 100 individual images from one electron micrograph the decorated filament length shown is 76.8 nm.

The light-driven splitting of H20 is catalyzed by a Mn-containing protein complex 02 is produced. The reduced plastoquinone carries electrons to the cytochrome b6f complex from here they pass to plastocyanin, and then to P700 to replace those lost during its photoexcitation. 

When the system is saturated with either of these latter reactants, the resulting Mn + spectrum is so broad that it "disappears" and is essentially unobservable at the same conditions of concentration and spectrometer gain. The magnitude of this broadening at 35 GHz is unusual in the literature of Mn +-protein interactions. The spectra are similar to those observed by Villafranca et al. (32) for complexes of glutamine synthetase-Mn +-methionine(SR)-sulfoximine-Mg +-ATP, but even in this latter case, the broadened Mn2+ spectrum is still sufficiently narrow to be observed. 

While the stoichiometries of the Mn SOD enzymes appear to vary, the properties of the Mn-binding site do not. This is borne out in the electronic spectra of these proteins, which display a great degree of similarity despite the diversity of sources from which they have been isolated (Table II). This type of spectrum is distinctive for manganese in the trivalent oxidation state (3). The native enzymes are EPR silent, as might be anticipated if they contained Mn solely as the trivalent ion (S = 2) (1, 6,12,18-20, 24). However, when the enzymes are denatured, the characteristic six-line pattern of Mn(II) (I = 5/2) appears. Magnetic susceptibility studies with the E. coli SOD were consistent with the presence of a monomeric Mn(III) complex with a zero-field splitting of 1 to 2 cm-1 (4). The enzymes are additionally metal specific (however, see Refs. 36 and 37) metal reconstitution studies with E. coli and B. stearothermophilus revealed a strict requirement for Mn for superoxide dismutase activity (2, 22, 23). 

Mn, Mn , and Mn. Of these, Mn is by far the most common. Unfortunately, most Mn complexes, including Mn proteins, have very weak UV-visible transitions. When it is present in a high symmetry environment, Mn has an easily detectable EPR signal. In lower symmetry environments, the Mn EPR signal can become broad and hard to detect. Nevertheless, EPR studies have been extremely useful in characterizing the role of Mn in proteins. ... 

A second difficulty in studying Mn proteins is the lability of protein-bound Mn. Complexes of Mrf typically have binding constants that are somewhat smaller than those for the corresponding Fe complexes and substantially smaller than those for Cu and Zn complexes (this is the so-called Irving-Williams Series). For heme-iron proteins, the porphyrin cofactor see Iron Porphyrin Chemistry) increases the stability of the Fe-protein interaction. No such small molecule cofactors are known for Mn proteins. Small binding constants mean that Mn is readily lost from a protein during purification. 

Transferrin is a reversible iron-binding protein used in vertebrate iron-transport see Iron Proteins for Storage Transport their Synthetic Analogs). Manganese will bind to transferrin in vitro to form a Mn -transferrin complex. Transferrin appears to bind the majority of the serum Mn and may be important in Mn transport. 

Figure 22 Introducing normative prosthetic group into metalloproteins. (a) by chemical modification of heme propionate. (Reprinted with permission fi-om Ref. 25. 2002 the American Chemical Society) (b) by noncovalent addition strategy. The crystal stmcture of the Fe (3,3-Me2-salophen) incorporated into AlalTGlaMb. (Reprinted with permission from Ref 287. 2004 the American Chemical Society) (c) by a single attachment strategy. The computer model of adipoc)4e lipid binding protein-phenanthroline complex. (Reprinted with permission from Ref. 291. 1997 the American Chemical Society) (d) by a dual covalent attachment strategy. The computer model of Mb(L72C/Y 103C) with a Mn "-Salen complex covalently attached at two-points and overlayed with heme. (Reprinted with permission from Ref 288. 2004 the American Chemical Society)...

The electronic spectrum of the dimanganese(III) form of catalase from Thermus thermophilus shows an intense absorption at 450 nm with a shoulder at 500 nm. EPR studies show that Mn Mn , Mn Mn , and Mn Mn forms are also accessible and the Mn Mn protein exhibits a 16-line spectrum, characteristic of a p-oxo bridged system 202). Comparison with the model complexes leads to prediction of p-oxo-bis(/[i-carboxylato)dimanganese(III) structure for the catalase. X-ray crystal data (203) have indicated an Mn—Mn distance of 3.6 3 A for the enzyme this appears to be inconsistent with the above proposal but, as the X-ray data are at rather low (3 A) resolution, this may be the least reliable piece of data. 

The transition dipole moment of BChl c-744 is nearly parallel to the axis of the rod element, while that of BChl c-727 is more random. The presence of the longer-wave length BChl c-complex, BChl c-766, has been suggested by the results of deconvolution of the linear-dichroism spectra as well as by more recent measurements of time-resolved fluorescence spectra of oriented chlorosomes. The orientation of the transition moment of BChl c-766, determined from its fluorescence maximum at 778 mn, is intermediate between that of BChl c-744 and that of the baseplate BChl c-protein complex, B795. 

The splitting of H2Q, which provides the electrons for reduction of Peso in PSII, is catalyzed by a three-protein complex, the oxygen-evolving complex, located on the luminal surface of the thylakold membrane. The oxygen-evolving complex contains four manganese (Mn) ions as well as... 

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