Cross-reaction proteins, electron transfer

Tahie 9.6 Test of the Marcus cross-relation for electron transfer between proteins. Comparison of observed rate constants for protein-protein reactions with values calculated from the Marcus cross-relation. Data from Ref. [37]. The work terms are mostly neglected and values of tobs/ caIc rounded... 

Based on the Fe(EDTA)2- results, the blue copper center in stellacyanin appears to be much more accessible than that situated in either azurin or plastocyanin. Thus it should be profitable to compare the electron transfer reactivities of these three proteins with a variety of redox agents. Kinetic studies of the oxidation of the three blue proteins by Co(phen)33+ have been made (26), and the results together with those for other redox agents are set out in Table IV. The electrostatic corrections to the predicted kn values are modest both for the large charge on plastocyanin and the small one on azurin, as the protein selfexchange and the cross reaction work terms compensate. The reactivity... 

Table IV. Electron Transfer Cross-Reaction and Self-Exchange Rate Constants for Blue Copper Proteins (25°, /aO.IM, pH 7)a...

X 10 M s and was 3.1 x 10 M s" at 25°C, pH 7.0 and ionic strength of 1.0 . Kinetic data was interpreted in terms of a mechanism of electron transfer from chromium(II) involving attack of Cr(II) adjacent to the Fe(III) center Analysis of the one-to-one chromium(III) cytochrome c complex revealed that the chromium(III) cross-linked two peptide fragments located in the heme.crevice by binding to tyrosine 67 and asparagine 52 The chromium(III) bound to reduced cytochrome c did not affect the ability of the protein to be reoxidized with ferricyanide and then to be reduced with dithionite . The chromium complex was oxidized by cytochrome oxidase at the same rate as the untreated ferrocytochrome c, however, the rate of reduction of the chromium complex by bovine heart submitochondrial particles was slower than that of untreated ferricytochrome c Thus, the binding of chromium(III) to cytochrome c appears to selectively inhibit its function in certain electron transfer reactions. 

Test of the Marcus cross-relation for protein-protein reactions [37]. Some electron-transfer reactions between two metallic redox centres in different proteins can be examined to see how closely they follow the Marcus cross-relation (Section 9.2.2.2). This is possible in systems where the self-exchange rates can be determined by nmr or epr, i.e., where the metal ions are such that line-broadening techniques can be applied. (Such a test is not available for most of the donor-acceptor systems discussed above.) Some results are shown in Table 9.6. The observed and calculated values for the six reactions are seen to agree within... 

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