Tyrosine residues of streptavidin were chemically converted to 3-aminotyrosine residues by nitration followed by reduction. Although the biotin binding activity of the modified streptavidin reduced by about a half, no conformational change was observed in CD spectroscopy. Furthermore, electron transfer between the modified streptavidin with 3-aminotyrosine residues and the biotin-modified gold electrode was detected by a voltammetry technique. This chemical conversion method will be a general and promising method for providing an electron transfer function to various proteins. (C) 2003 Elsevier Science B.V. All rights reserved.