Electrochemical identification of intermediate forms of urea denaturation of horse heart cytochrome c

Chemical-induced unfolding of cofactor-free protein monitored by electrochemistry

Protein folding has been studied extensively with an aim to better understanding of the relationship between protein sequence, structure, and function. A large variety of techniques have been developed and utilized to probe protein conformation and folding/unfolding transition. In this report, electrochemical monitoring of urea-induced unfolding of a large cofactor-free protein, bovine serum albumin (BSA), is described. Enhanced electrochemical oxidation of tyrosine and tryptophan in free amino acids and in BSA was achieved on an indium tin oxide electrode by using an electron mediator, Os(bpy)2dppz (bpy = 2,2'-bipyridine, dppz = dipyrido[3,2-a:2',3'-c]phenazine). The oxidation current was used as a signal reporter in the monitoring of urea-induced BSA denaturation. At high urea concentrations, the electrochemical signal increased by 3-fold relative to the native protein. The increase is attributed to the closer contact between the oxidizable residues in the unfolded BSA and Os(bpy)2dppz. The degree of unfolding assessed by electrochemistry correlates well with the established fluorescence technique in the range of 0-10 M urea. The method can be used to investigate the unfolding process of other cofactor-free proteins.

Scanning tunnelling microscopy observation of cytochrome-c denaturation induced by bromopyrogal red on highly oriented pyrolytic graphite

The denaturation of cytochrome-c (cyt-c) induced by bromopyrogal red (BPR) was studied by scanning tunnelling microscopy (STM) on the electrochemically pretreated highly oriented pyrolytic graphite (HOPG) surface. STM images reveal that denatured cyt-c molecules exist in variable states including aggregates, globular compact, partially unfolded and combined with BPR molecule. The apparently low image contrast of denatured cyt-c observed in this experiment comparing to that of native cyt-c molecules, and the relative low image contrast of the unfolded part comparing with the compact globular part, are ascribed to the unfavourable tunnelling paths for the conformational variations of denatured cyt-c molecules.