Electron transfer azurin systems

Direct evidence for long range electron-transfer in biological systems was first observed by Gray et al.50,51) and Isied et al.481 using [Ru(NH3)5]3+ substituted metallo protein. Histidine-83 of blue copper (azurin) was labeled with Ru(III)(NH3)5 50). Flash photolysis reduction of the His-83 bound Ru(III) followed by electron-transfer from the Ru(II) to Cu2+ was observed with a rate constant of 1.9 s 1. The result shows that intramolecular long distance (approx. 1 nm) electron-transfer from the Ru(II) to the Cu2 + of the azurin takes place rapidly. 

We have used a range of different physical and chemical approaches in the effort to better understand how the different blue copper proteins function. With the relatively simpler, electron-mediating proteins like azurin, the ultraviolet chromophores were shown to be informative in terms of copper-protein interactions. These proteins are also a useful system for detailed examination of the electron transfer pathways to and from their single copper site. 

An enormous amount of electron-transfer chemistry goes on in biological systems, and nearly all of it critically depends on metal-containing electron-transfer agents. These include cytochromes (Fe), ferredoxins (Fe), and a number of copper-containing blue proteins, such as azurin, plastocyanin, and stellacyanin. 

Some of the first protein systems where pulse radiolysis was used to help determine mechanism were those of blue copper proteins. These are proteins that are blue in solution and contain what are known as type (I) and type (2) copper centers. Two of the most well-known and well-characterized examples of these are azurin and cytochrome c. It was the studies of these systems that opened up the field of long-distance electron transfer in proteins and, by using the protein structure as a framework for electron transfer through space and through bonds, allowed for the development of a broad theoretical basis and many fascinating experiments on long-range electron transfer. Here, I will limit the discussion to electron transfer studies in azurin as illuminated by pulse radiolysis studies. ... 

Jensen, P.S., Chi, Q., Zhang, J., and Ulstrup, J. (2009) Long-range interfadal electrochemical electron transfer of Pseudomonas aeruginosa azurin-gold nanoparticle hybrid systems. Journal of Physical Chemistry C, 113,13993-14000. 

Electron transfer copper proteins usually belong to the blue copper proteins (Type 1) azurin is a simple example. This family of proteins are also called cupredoxins, and they participate in many redox reactions involved in processes fundamental to biology, such as respiration or photosynthesis. The striking electron transfer capabilities of blue copper proteins have been studied extensively. Plastocyanin, with a tetrahedral CUN2S2 core, acts as the electron donor to Photo System I in photosynthesis in higher plants and some algae. 

Azurin as a Model System for Intramolecular Electron Transfer... 

Selectively modified and characterized ruthenated proteins have been prepared for cytochrome c (141-144), cytochrome c-551 (145), azurin (146, 147), plastocyanin (147), HiPlP (/4S), and myoglobin (77, 149-154). Intramolecular electron transfer rates, k, measured in these systems are listed in Table 111 along... 

Although electron transfer rates within myoglobin appear to follow an exponential dependence on distance, the derived rate expression is not directly transferable to other electron transfer proteins. A particularly striking comparison is between the c cytochromes and the copper proteins plastocyanin and azurin. Intramolecular electron transfer rates are at least 10-100 times slower in the copper proteins compared to the c cytochromes, even though the distances and driving forces for the reactions are comparable. The origin of this behavior is unclear, but it does suggest caution in the quantitative transfer of rate expressions between different systems. 

The application of direct electrochemistry of small redox proteins is not restricted to cytochrome c. For example, the hydroxylation of aromatic compounds was possible by promoted electron transfer from p-cresol methylhydroxylase (a monooxygenase from Pseudomonas putida) to a modified gold electrode [87] via the blue copper protein azurin. All these results prove that well-oriented non-covalent binding of redox proteins on appropriate electrode surfaces increases the probability of fast electron transfer, a prerequisite for unmediated biosensors. Although direct electron-transfer reactions based on small redox proteins and modified electrode surfaces are not extensively used in amperometric biosensors, the understanding of possible electron-transfer mechanisms is important for systems with proteins bearing catalytic activity. 

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