Redox Reactions Involving Metals in Other Biological and Model Systems

4 Redox Reactions involving Metals in other Biological and Model Systems 

It has recently become apparent that iron-sulphur, or non-haem iron, proteins occur widely in animals, plants, and bacteria, and that they play a very important role in such functions as respiration, photosynthesis, and nitrogen-fixation. Both iron-sulphur proteins and cytochromes are electron carriers containing iron, but their active centres are entirely different. Hall and Evans have recently reviewed iron-sulphur proteins, and a discussion on nitrogen fixation held in June 1968 has also been published.  

Hemerythrin is a non-haem iron protein which transports oxygen in a few simple invertebrates. The nature of the binding of the oxygen to the iron, and of the iron to the protein, is not fully understood, although the presence of histidine and tyrosine at the active site has recently been demonstrated. Magnetic and spectroscopic data have been used to obtain information about the oxidation states of the two iron atoms at the active site in the oxygenated and oxygen-free proteins. 

One of the simplest of the non-haem iron proteins is the rubredoxin from Clostridium pasteurianum, which contains one iron atom per molecule and undergoes a reversible one-electron oxidation and reduction. The 

Mestroni, and E. de Savorgnami, Inorg, Chim. Acta, 1969, 3, 323  

Synthetic iron-sulphur cluster compounds of general structure (11) serve as analogues of the active sites of (Fe4S4) ferredoxins and high-potential iron proteins 

The kinetics for ligand exchange of the water-soluble (11 R = CH2CH2C02 ) as a function of pH and thiol concentration are in accord with four reversible mer-captan-lyate species exchange reactions followed by product formation through specific acid and base catalysis. It has also been found that (11) (R = Me, Bu , or Bu ) represent the basic forms of weak acids, with pKa, comparable to those of carboxylic acids. 

The kinetics of reduction [K4Fe(CN)s, SO2 , Sa04 , sodium ascorbate, and cytochrome C2] and oxidation [K3Fe(CN)6 and cytochrome Cg] of HIPIP from Chro-matium vinosum have been reported, and it is concluded that HIPIP undergoes rapid outer-sphere electron transfer (there being no indication of complex formation with the various reactants). Moreover, steric restrictions and differences in oxidation-reduction potential are less important than electrostatic attraction and/or repulsion in determining the absolute rate constants (Table 2). Rawlings et al. have deter- 

Zumft and Mortenson have reviewed the bacterial nitrogenases and Lawrence and Spence have noted an interesting difference in the kinetics of reduction of ferricytochrome c (cyt c ) by two apparently similar molybdenum-containing enzyme models (12) and (13). The reduction by the dioxo-bridged compound is 

Plant ferredoxins are among the simpler iron-sulphur proteins, since their molecules contain only two Fe atoms and on reduction they take on only one electron. Their chemistry and function in photosynthesis, nitrogen fixation, and fermentative metabolism has recently been reviewed, and the results of several magnetic and spectroscopic studies have appeared which bear on the role of the metal in these non-haem macromolecules. 

Further magnetic and spectroscopic evidence suggests that the two iron atoms at the active site in hemerythrin are coupled, and it has been found that, at neutral pH, C104 has a profound effect on the oxygenbinding properties of this protein. Preliminary evidence on the reaction 

From steady-state studies on the inhibition of xanthine oxidase by methanol at high and low xanthine concentrations, it has been possible to show that Mo does not participate in the direct binding of the first substrate molecule to form the enzyme-substrate complex ES. However, inhibition by excess substrate and formation of a complex ESS are due to the disruption of the electron-transfer chain at the stage where the molybdenum atom takes part in the transfer process. 

Studies on the catalatic activity of additional Cu chelates confirm the previous conclusions that the complex must possess a suitable redox potential and two adjacent free sites to promote the efficient decomposition of H2O2. Malmstrom has discussed the asymmetric nature of oxidases which contain several copper atoms per molecule, including laccase and ceruloplasmin. The rate constant for the reaction of the hydrated electron 

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