Intraprotein Electron Transfer Reactions

Many other papers on long-range electron transfer between two reactive sites of modified proteins were published [270-288] after the above mentioned pioneering works. Most of them dealt with photoinduced electron tunneling from triplet states of closed shell Mg(II) and Zn(II) porphyrins to Fe(III) or Ru(III). In agreement with the prediction of Marcus theory the rate constants for the majority of these intraprotein electron transfer reactions were found to increase as the free energy of reaction decreased. However for one of the reactions disagreement with this theory was observed [285],... 

Eleven 9,10-anthraquinones with various substituents, seven 1,4-naphthoquinones, 1,2-naphthaquinone and five 1,4-benzoquinones were used as QA. These quinones provide a series of RCs with a variation of the reaction exothermicity, - AG , from 0.11 to 0.9 eV. The rates of intraprotein electron transfer from various Qa to (BChl)J were found to be virtually temperature independent from 5 to 100 K and to decrease severalfold from 100 to 300 K. Only a small change of the rate upon the — AG° variation was found when reaction was made more exothermic than in the native RC. As the reaction was made less exothermic, the rate decreased notably without becoming temperature dependent. 

Smface modification with ruthenium complexes has proven valuable in studies of both interprotein and intraprotein electron transfer in systems that are difflcult to stndy by traditional kinetic tools. The choice of ruthenium complexes in these investigations stems from an extensive photochemistry as well as exceptional thermal stability. The photochemistry provides a means of examining reactions over a time range of nanoseconds to seconds by laser-flash photolysis and the thermal stability allows researchers to covalently bind a wide variety of complexes to proteins with... 

Voltammetry provides a useful tool for studying redox chemistry of the molecules relevant to biological redox reactions. Many proteins are exclusively involved in intraprotein electron transfer and typically function in ordered structures such as mitochondria. Under these circumstances, the redox... 

Light-induced electron transport in bacterial photosynthetic reaction centers leads to the creation of a charge-separated state stable for milliseconds to seconds. The structures provided by X-ray crystallography (Michel et aL, 1986 Allen et al., 1988 Deisenhofer Michel, 1989 El-Kabbani et al., 1991) constitute a unique guideline to address questions on how the function may be related to the arrangement of the cofactors and of specific amino acid residues in their vicinity. The sequence of electron transfer reactions, the identity of the reaction partners, and the reaction mechanisms have been characterized from static and time-resolved absorbance measurements (for a review, see Parson Ke, 1982). Transfer of the first electron to the primary (Q ) and secondary (Qg) quinone electron acceptors has received considerable attention, since it is associated with intraprotein protolytic reactions (for a recent review, see Okamura Feher, 1992), which have a potential role in electrostatic charge stabilization. 

Shown in Figures 5-7 are the redox pathways for xanthine oxidase, sulfite oxidase, and nitrate reductase (assimilatory and respiratory), respectively. These schemes address the electron and proton (hydron) flows. The action of the molyb-doenzymes is conceptually similar to that of electrochemical cells in which half reactions occur at different electrodes. In the enzymes, the half reactions occur at different prosthetic groups and intraprotein (internal) electron transfer allows the reactions to be coupled (i.e., the circuit to be completed). In essence, this is the modus operandi of these enzymes, which must be determined before intimate mechanistic considerations are seriously addressed. 

Dooley et al. first proposed an inner-sphere electron transfer mechanism where O2 binds directly to copper(I) 55 this was based upon precedence that suggested very rapid reactions between O2 and synthetic copper(I) compounds used as spectroscopic models for enzyme active sites. A three-coordinate copper(I) geometry in the CAOs had been demonstrated upon reduction with dithionite under anaerobic conditions.56 The reduction of the enzyme anaerobically with primary amine substrates actually produces two states, a copper(I)/semiquinone and a copper (II)/aminoquinol (Figure 9.11). In plant-derived and bacterial CAOs, these states have been demonstrated to exist in rapid equilibrium, due to intraprotein electron... 

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