DIRECT ELECTROCHEMICAL STUDIES OF CYTOCHROMES B(562)

The electrochemistry of cytochrome b(562) from E. coli has been studie d at graphite and gold electrodes. At edge plane graphite surfaces neo mycin promotes reversible, linear-diffusion limited electrochemistry a cross the pH range 4.5 to 8.0. At gold surfaces, a similar, but much m ore stable response is observed when the electrode is electrochemicall y modified with the cationic hexapeptide, KCTCCA. This reversible dire ct electrochemical response has been used to study the reduction poten tial of wild type cytochrome b(562) as a function of pH, ionic strengt h and temperature. The enthalpic and entropic parameters for the redox equilibrium show a dependence on pH consistent with the idea that the protonation of the haem propionate groups contribute significantly to the control of reduction equilibrium. The electrochemical response fr om the wild type protein is not reversible above pH 8 and is not detec table above pH 9. This is rationalised by the presence of a species wi th a deprotonated histidine ligand in the ferricytochrome but surprisi ngly, we have not observed any electrochemistry attributable to this s pecies under the conditions employed in this work. We have also studie d variants of this cytochrome which have bis-methionine ligation and u ndergo redox Linked ligation state changes between low-spin and high-s pin ferric species. The consequences of these changes for the electroc hemical response are examined.