Electron transfer center

The many redox reactions that take place within a cell make use of metalloproteins with a wide range of electron transfer potentials. To name just a few of their functions, these proteins play key roles in respiration, photosynthesis, and nitrogen fixation. Some of them simply shuttle electrons to or from enzymes that require electron transfer as part of their catalytic activity. In many other cases, a complex enzyme may incorporate its own electron transfer centers. There are three general categories of transition metal redox centers cytochromes, blue copper proteins, and iron-sulfur proteins. 

Achieving fast electron transfer to enzyme active sites need not be complicated. As mentioned above, many redox enzymes incorporate a relay of electron transfer centers that facilitate fast electron transfer between the protein surface and the buried active site. These may be iron-sulfur clusters, heme porphyrin centers, or mononuclear... 

Minteer and co-workers have also exploited the broad substrate specificity of PQQ-dependent alcohol dehydrogenase and aldehyde dehydrogenase from Gluconobacter species trapped within Nahon to oxidize either ethanol or glycerol at a fuel cell anode [Arechederra et al., 2007]. Although the alcohol dehydrogenase incorporates a series of heme electron transfer centers, it is unlikely that many enzyme molecules trapped within the mediator-free Nahon polymer are electronically engaged at the electrode. 

The need for oscillations can be seen in another way if we look at electron transfer processes in the mitochondrial or chloroplast reaction systems. In both there are series of electron transfer centers. By studying the spin state of metal ions in these centers it has been shown that the metal atoms can flip rapidly between different spin states. As these states must have some difference in conformation and as the metals are bound... 

A considerable number of crystal structures of type I copper sites in proteins are now available, so there may be no particular advantage in the synthetic model approach to prove the coordination structure of type I. Yet, inorganic chemists still have an opportunity to utilize the spectroscopic and structural bases established by model studies to understand the precise electronic structure of type I copper. One should keep in mind that the generally accepted interpretation derived from spectroscopic and theoretical studies on the proteins (47-49) has not been definitely proved experimentally. A systematic comparison of a series of copper(II) thiolate complexes having an unusual distorted coordination structure is required for a conclusive description of the electronic structure of the type I copper. The synthetic approach is ultimately the most adequate way to clarify how the ligand donors and geometry affect the electronic property and function of type I copper as an electron transfer center. 

E-VII) the latter is one of the most pervasive electron transfer centers in biology (Section 17-E-10). 

The type I copper sites function as electron transfer centers in the blue copper proteins and in multicopper enzymes, particularly oxidases (33). They are characterized by their intense blue color, their unusually small A values, and their very positive redox potentials (Table II). X-ray crystal structures of several blue copper proteins have been determined, notably plastocyanin (34), azurin (35), cucumber basic blue protein (36), and pseudoazurin (37). The active site structures show marked similarities but also distinct differences (Fig. 8). 

As an analytical spectroscopic technique, EPR is similar in concept to the more widely used nuclear magnetic resonance (NMR) spectroscopy [see NMR Overview of Applications in Chemical Biology]. In fact, EPR and NMR are complementary to each other. Both techniques detect magnetic moments, hut NMR determines the chemical stmctures in solution, whereas EPR describes more precisely the electronic and chemical structures of a particular region of the biological system, such as electron transfer centers, metal ions, and an intermediate state of the enzyme or substrate. It is not possible to present a full description of the theory of EPR in an article with this scope. Therefore, only sufficient information is provided here to enable the readers to understand the practical aspects of this analytical tool in enzymology. 

Crystallographic studies on the 343-residue Alcaligenes enzyme reveal two P barrel domains with the type 1 copper embedded in one of them and the type 2 copper in an interface between the domains. Studies of EPR541 ENDOR spectra and of various mutant forms have shown that, as for other copper enzymes, the type 1 copper is an electron-transferring center, accepting electrons from the pseudoazurin and passing them to the type 2 copper which binds and reduces nitrite. ... 

The answer is d. (Murray, pp 123—148. Scriver, pp 2367—2424. Sack, pp 159—175. Wilson, pp 287-317.) Some monooxygenases found in liver endoplasmic reticulum require cytochrome P450. This cytochrome acts to transfer electrons between NADPH, O2, and the substrate. It can be an electron acceptor from a flavoprotein. In the mitochondrial electron transport chain, flavoproteins donate electrons to coenzyme Q, which then transfers them to other cytochromes. Flavoproteins that are oxidases often react directly with molecular oxygen to form hydrogen peroxide. Flavoproteins can be NADH dehydrogenases that oxidize NADH and transfer the electrons to coenzyme Q. The electron transfer centers of flavoproteins in the electron transport chain contain nonheme iron and sulfur. 

We have no direct information concerning the nature of the slow reaction leading to stabilization of the radical. Reasonable mechanisms for a slow process leading to stabilization, such as positive charge diffusion away from the electron-transfer center... 

A group of proteins contain copper in sites in a permanently coordinated state. These sites may simply function as depository for copper, in molecules participating in copper trafficking, as electron transfer center or as substantial part of the active site of a copper enzyme. Research on copper proteins has been summarized in a three volume book by Lontie, in volume 2 of the Handbook of Copper Proteins, in the Handbook on Metalloproteins, in a book edited by Valentine and Gralla, and in several review articles (see, e.g., Abolmaali et al or Messerschmidt ). 

A residue that is conserved over the broader group of sequences is methionine 115 (129 in P. denitrificans CCP). This residue is located 60 residues C-terminal from the heme binding site characteristic of the class I cytochromes. This residue is also close to His 71. It was suggested that this methionine could be the distal ligand of the N-terminal heme domain in the E. coli and Mau G proteins and it should also be considered a candidate for the distal ligand in all the proteins in their mixed valence state [9]. If this would be the case, the roles of the peroxidase center and of the electron transfer center (see Model II Figure 6-8B), tentatively assigned to the N- and C-terminal hemes, respectively [10], may require reassessment once the structure of the mixed valence enzyme is known. This will be discussed in the next sections. 

Electrons from cytochrome c are first transferred to the binuclear Cua center which displays a delocalized mixed-valence (Cu(1.5)-Cu(1.5)) electronic structure in its oxidized state (see Chapter 8.4). Heme Fe is also another electron transfer center that is believed to be in rapid redox equilibrium with Cub. Subsequent electron transfer occurs to subunit I and the binuclear heme As-Cub center, where molecular oxygen reduction occurs. During this process, four protons are supplied from the inner membrane. The free energy made available by this exergonic reaction drives the translocation ( pump ) of four additional protons per O2 molecule reduced from the IN side to the OUT side. The active transport of protons generates an electrochemical gradient across the membrane that ultimately drives ATP synthesis. 

Type 1 Cu electron transfer centers Type 2 Cu catalytic centers... 

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