Internal electron transfer sites

The studies of the Rh catalysts reveal that bpy/bpy redox couples can act as internal electron transfer sites for the ultimate delivery of two electrons to a coordinated CO molecule. In addition, the results show... 

The subunits of CODH/ACS have been isolated (see earlier discussion). The isolated a subunit contains one Ni and four Fe and has spectroscopic properties (186) similar to those of Cluster A, the active site of acetyl-CoA synthesis (212). Unfortunately, it has no ACS activity. Therefore, ACS activity may reside in the a subunit or it may require both the a and the fi subunits. If Clusters B and/or C of the B subunit are involved in acetyl-CoA synthesis, one possible role could be in electron transfer. Although acetyl-CoA synthesis and the CO/ exchange reactions do not involve net electron transfer, both of these reactions are stimulated by ferredoxin, indicating that internal electron transfer within CODH/ACS may be required during the reaction (121). Further studies with the isolated subunits and the reconstitu-... 

More subtle factors that might affect k will be the sites structures, their relative orientation and the nature of the intervening medium. That these are important is obvious if one examines the data for the two copper proteins plastocyanin and azurin. Despite very similar separation of the redox sites and the driving force (Table 5.12), the electron transfer rate constant within plastocyanin is very much the lesser (it may be zero). See Prob. 16. In striking contrast, small oxidants are able to attach to surface patches on plastocyanin which are more favorably disposed with respect to electron transfer to and from the Cu, which is about 14 A distant. It can be assessed that internal electron transfer rate constants are =30s for Co(phen)3+, >5 x 10 s for Ru(NH3)jimid and 3.0 x 10 s for Ru(bpy)3 , Refs. 119 and 129. In the last case the excited state Ru(bpy)3 is believed to bind about 10-12 A from the Cu center. Electron transfer occurs both from this remote site as well as by attack of Ru(bpy)j+ adjacent to the Cu site. At high protein concentration, electron transfer occurs solely through the remote pathway. 

The rate of electron transfer that occurs to/from the metal center is high. Structure based modeling of the active site of human MnSOD [40], which includes calculating the energies of both the oxidized and reduced states with either water or hydroxide bound to the metal, suggests the rate of this internal electron transfer is enhanced by electron-relaxation effects. In addition, a 0.17 V redox potential is calculated, a value that is low compared with the experimental values of 0.31 V fori . coli and 0.26 V for B. stearothermophilus, respectively. A potential of —0.30 V seems to be optimal as it lies midway between the redox potentials of the two half reactions of the dismutation process [41],... 

The reduction of CMPII(Fe = O, R) has been found to involve rapid internal electron transfer leading to formation of the radical cation in CMPII(Fe R +), which is then reduced by yCc in the high-affinity binding site (Scheme 4). ... 

Speculation about the precise roles of active-site sulfur is tempered by an appreciation of the redox versatihty and interplay of sulfur and molybdenum. This is evident from synthetic systems, where the catenation of sulfur (with attendant redox and/or ligand elaboration) and induced internal electron-transfer reactions are frequently observed. The redox interplay of Mo and S, reflected in undesirable synthetic outcomes, may prove crucial to a fifll description of enzyme behavior. see also Sulfur Inorganic Chemistry)... 

Copper- and heme-containing NiRs are both key enzymes in denitrification. They are both homooligomers and their subunits contain two distinct redox-active metal centers, an electron accepting site and a catalytic electron delivery center where the single electron reduction of nitrite to NO takes place. Thus, PR studies providing comparison of the two enzyme families are helping to resolve the different mechanisms of control of intramolecular ET reactivity. Internal electron transfer could be a rate-determining step in the catalytic cycle of both enzymes. 

Modification of larger proteins with ruthenium complexes, while possible, have proven difficult. Fortunately, a ruthenium labeled partner such as Cc can be used to rapidly inject or remove an electron from the large protein complex, and thus study internal electron-transfer reactions. In a complementary approach, Nilsson found that the excited state of Ru(bpy)3 + can inject an electron into CcO. The ruthenium complex binds electrostatically to the protein in a location similar to that occupied by Cc. The initial site of electron transfer is the Cua site, as it is when Cc is the electron donor. The simplicity of this technique makes it very attractive since it eliminates any modification of the protein which might alter the structure. Sadoski etal. showed that significant improvements in the yield of electron transfer could be obtained with ruthenium complexes of higher charge. One specific complex used was the dinuclear complex [(bpy)2Ru(qpy)Ru(bpy)2] + (Figure 10). ... 

There are two possible routes by which electron transfer could result in the oxidation of the methyl substituent on heme O. The first is via outer-sphere electron transfer, as depicted in Figure 5. In this mechanism, the cofactor heme B binds and activates O2 to form compound I, and then heme O is oxidized via a peroxidase-type mechanism. In the second, related mechanism, HAS oxidizes heme O via autoxidation. In this case, heme O binds and activates O2 to form compound I, while heme B is presumably involved in shuttling electrons from a putative ferredoxin to the active site. Heme O would then be oxidized by internal electron transfer, similar to the mechanism of heme cross-linking elucidated by Ortiz de Montellano and coworkers (22). While the labeling experiments of HAS strongly suggest that heme O is oxidized via electron transfer, they do not allow us to distinguish between these two possible scenarios, and additional experiments are required. 

Role of active site aspartate and histidine residues Internal electron transfer Mechanistic studies Model studies... 

The structure of the AfNiRH255 N mutant with both nitrite and H2O bound to the active site Cu has led to the proposal that a transient in catalysis may be a Cu OH NO species prior to this step. Protonation of the OH at the active site by His255 via the bridging water and release of NO regenerates the active site (Figure 6A). In a variant of this reaction scheme, a proton from Asp is transferred to nitrite to form a Cu NOOH intermediate. The subsequent internal electron transfer and protonation of this species by His255 yields NO and Cu H2O regenerating the type 2 center for further catalysis. Neither of these proposed mechanisms involves the putative nitrosyl intermediate. ... 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

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