Electron transfer pathway model

Another report describes the 02 evolution catalyzed by a series of di- and oligonuclear structural models of PSII [169] and their uncharacterized hydrolysis products, in the presence of Ru(bpy)33+. The catalytic activity seems to be ligand independent (if the ligand is not oxidized) and boosted by dispersion of the catalyst in phospholipids. The exact nature of the catalyst is not known, but this work suggests the importance of creating lipophilic, membranelike catalyst environments. In the preceding cases Ru(bpy)3 is not covalently linked to the Mn centers, thus making an outer-sphere electron transfer pathway likely. 

It is clear from these results that the availability of additional electron transfer pathways in 40 has led to a substantial enhancement of the quantum yield of the final state relative to the two model systems which feature fewer electron transfer steps to attain related final states. This strategy should be useful in the design of other complex molecular devices. 

Figure 11 Molecular model of the complex between Ru-65-c) t bs and Cc. The geometry of the complex is the same as that of the complex involving native cytochrome bs proposed by Salemme. The heme groups (red), and the ruthenimn complex (green) are highhghted. The atoms forming an electron-transfer pathway between the ruthenimn complex and the heme group of Ru-65-c) t bs are colored yellow. The lysine and arginine residues are blue, while aspartate and glutamate residues are red ...

One peculiarity of PS I is that the two branches of electron cofactors converge at Fx so that no a priori reason exists for why electron transfer should use only one of them. On the other hand, no obvious need exists for both branches to be active. This problem has been addressed by a number of researchers in recent years by using the known structure of PS I to identify specific amino acid residues close to the cofactors in one branch or the other and making point mutations at these locations. Although no consistent picture of the electron transfer pathway in PS I has been developed yet, much of the data provide evidence that both branches may be active. If this model is correct, some data also suggest that it may be possible to influence the extent to which electron transfer occurs in a given branch (24). 

Figure 10. Proposed model for the iNOS dimer indicating domain swapping and electron transfer pathway. (Adapted from Ref. [98].)...

FIGURE 7.4 F roposed models depicting electron transfer pathways for Shewanella oneidensis MR-1 (a) and Geobacter sulfurreducens (b) during dissimilatory reduction of solid metal (hydr)oxides. (From Shi, Squier, Zachara i4 Fredrickson, 2007.)... 

It is of note that Kaminskaya, Konstantinov and Shuvalov also examined in detail the kinetics of low-temperature photooxidation of all four cytochromes in Rp. viridis reaction centers and established the complete heme sequence as HI, LI, H2, L2, as shown in Fig. 7, thus putting into doubt the earlier model involving two parallel H/L electron-transfer pathways, as suggested by the model in Fig. 2. 

Figure 6. Electron-transfer pathway in the azurin dimer mutant (43). The path connects Sy of Cys42 with Nj of His46, which is one of the copper ligands, and consists of 17 covalent bonds resulting in a very effective electronic coupling of the two redox centers. Calculations were based on the Beratan and Onuchic model (6, 7). Coordinates were taken from the PDB, code IJVO.

One of the most promising approaches for developing and testing models for electron transfer in the RC is the use of site-directed mutagenesis techniques to replace specific residues with other amino acids. These studies can serve to identify amino acid side chains that may be involved in electron transfer pathways, although it is more difficult to assess the role of main chain atoms in electron transfer by these methods. The usual strategy has been to replace conserved... 

One of the remarkable aspects of the hepatic CPR deletion in mice was the fact that they live and reproduce normally. This means that in adult mice at least, the hepatic P450 system is not essential for life, and indicates most strongly a fundamental role in providing protection against toxic environmental agents. The data from these transgenic models also demonstrate that potential alternative electron transfer pathways for P450s, such as the cytochrome b /b reductase systems that are discussed later, play only a minor role, if any, in vivo. 

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