Superexchange mechanism proteins

Figure 8. Proposed electron transfer pathway in blue copper proteins. The plastocyanin wave function contours have been superimposed on the blue copper (type 1) site in ascorbate oxidase (40). The contour shows the substantial electron delocalization onto the cysteine Spir orbital that activates electron transfer to the trinuclear copper cluster at 12.5 A from the blue copper site. This low-energy, intense Cys Sp - Cu charge-transfer transition provides an effective hole superexchange mechanism for rapid long-range electron transfer between these sites (2, 3, 28).

A theory has been developed that suggests that ET in proteins is regulated by pathways that are optimal combinations of through-bond, hydrogen-bond, and through-space links (10, 11, 28). In this model, the molecular orbitals of the protein matrix mediate ET by a superexchange mechanism analogous to that proposed for donor-acceptor model compounds (see above). A computerized procedure (12) was employed to search for such pathways (see Fig. 16), and the ET data for His-33 of horse heart cytochrome c and His-62 of yeast iso-l-cytochrome c were found to... 

The results of the two P-N-T polypeptides indicate that the possible superexchange mechanism, if any, is small and does not play an important role in the ET processes in polypeptides and proteins. 

As with conductivity measurements, methods and results of theoretical treatments of CT in DNA have varied significantly. Mechanisms invoking hopping, tunneling, superexchange, or even band delocalization have been proposed to explain CT processes in DNA (please refer to other reviews in this text). Significantly, many calculations predicted that the distance dependence of CT in DNA should be comparable to that observed in the a-systems of proteins [26]. This prediction has not been realized experimentally. The dichotomy between theory and experiment may be related to the fact that many early studies gave insufficient consideration to the unique properties of the DNA molecule. Consequently, CT models derived for typical conductors, or even those based on other biomolecules such as proteins, were not adequate for DNA. 

The general framework of the quantum mechanical rate expression for long-range electron transfer processes in the very weak or non-adiabatic regime will be presented in Sect. 2 with an emphasis on the inclusion of superexchange interactions. The relation between the simplest case of direct donor-acceptor interactions, on the one hand, and long-range electronic interactions important in proteins, on the other, is considered in terms of the elements of electron transfer theory. 

Kinetics of electron tranter. The central problem in investigations of the kinetics of electron transfer is to establish the mechanism and pathways of electron transfer. Is the through space mechanism relevant to this process, or have specific electron conduction pathways (through bond or superexchange) been designed into electron transfer proteins Once this question is resolved, then a quantitative understanding of electron transfer rates may be feasible. 

The superexchange model. Evidence on electron-transfer mechanisms thus comes from a variety of donor-bridge-acceptor-medium systems ( supermolecules ) in which the bridge may be anything from a single covalently-bound link to a multi-path protein system, and the medium may be a rigid solvent, a fluid solvent, or the polymer cloud of a protein. The problem has been treated wave-mechanically by applying time-dependent... 

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