Electron transfer in proteins : theory and experiment

This thesis consists of two main parts and one appendix. The first part (Chapters 2 and 3) contains mainly experimental work on electron transfer in ruthenium-modified azurin mutants. The second part describes theoretical studies on the control of reduction potentials in cytochromes and feredoxins. The appendix presents calculations on the reaction mechanism for the GTP-hydrolysis in Ras p21. In the experimental part electron tunneling through the beta-sheet in pseudomonas aeruginosa azurin is discussed. In agreement with theoretical consideration it is found that electron tunneling through a beta-strand is quite efficient. It is argued that the efficiency is due to the highly covalent nature of the bridge. Furthermore, the effects of the weakly coordinated M121 ligand and the strongly coordinated C112 ligand on the coupling are compared in Chapters 2 and 3. It is found that indeed the C112 ligand facilitates electron transfer, although the rate enhancement in the experiments does not appear to be much more than an order of magnitude. The theoretical section (chapters 4-6) presents microscopical calculations of electrostatic effects found to be important for the control of reduction potentials. Chapter 4 analyzes the effect of the N521 mutation in cytochrome c. The calculations suggest that the change in reduction potential is due to an electrostatic effect and dominated by the loss of the N52 dipole, which compensates for the displacement of an internally bound water molecule. In chapter 5 it is concluded that the change in reduction potential for the M80H axial ligand replacement in cytochrome c is due to the effect of the ligand bonding rather than an electrostatic effect. The enormous variations in the reduction potentials of iron-sulfur clusters in several proteins are being investigated in chapter 6. The potentials mainly appear to be controlled by the number of hydrogen bonds. However, in some cases the effect of nearby water molecules need to be considered. The calculations presented in the appendix show that a previously proposed mechanism in Ras P21 does not appear energetically feasible (reprint 1). In reprint 2 an alternative mechanism is proposed.