Binding ligand structures, electrochemical

Because of the exposed histidine ligands of the [2Fe-2S] cluster, the Rieske is capable of binding quinones in a redox-dependent manner. The variation of the hydrogen bond strength and of the electrostatic properties will control the movement of the catalytic domain of the Rieske protein. Therefore, the function depends on the unique structural and electrochemical properties of the Rieske cluster. 

Studies on the effect of pH on peroxidase catalysis, or the heme-linked ionization, have provided much information on peroxidase catalysis and the active site structure. Heme-linked ionization has been observed in kinetic, electrochemical, absorption spectroscopic, proton balance, and Raman spectroscopic studies. Kinetic studies show that compound I formation is base-catalyzed (72). The pKa values are in the range of 3 to 6. The reactions of compounds I and II with substrates are also pH-dependent with pKa values in a similar range (72). Ligand binding (e.g. CO, O2 or halide ions) to ferrous and ferric peroxidases is also pH-dependent. A wide range of pKa values has been reported (72). The redox potentials of Fe3+/Fe2+ couples for peroxidases measured so far are all affected by pH. The pKa values are between 6 and 8, indicative of an imidazole group of a histidine residue (6, 31-33),... 

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