STRUCTURAL REQUIREMENTS FOR THE ELECTRON-TRANSFER BETWEEN A FLAVOPROTEIN AND VIOLOGENS

The covalent binding of the viologen N-methyl-N'(aminopropyl)-4,4'-bipyridinium (APMV) to the flavoprotein ferredoxin-NADP(+) reductase, a photosynthetic enzyme from the cyanobacterium Anabaena PCC 7119, facilitates the transfer of electrons between the FAD group in the enzyme and oxygen molecules present in the solution. This oxidase activity, which is completely absent in the native protein, indicates that there is an efficient electron transfer between the FAD group in the enzyme and the viologen molecule. This reaction is much more effective than that where the viologen is free in solution since, in the latter case, higher concentration of the carrier is required for similar oxidase activity. The computer graphics model of the FNR-viologen adduct indicates that the interaction between both redox groups could take place when two aromatic rings (one the FAD group in the protein and the other belonging to the bipyridinium core in the viologen) are in the same plane and also when the methyl groups at positions 7-alpha and 8-alpha in the flavin ring are pointing to the viologen molecule. These type of studies could give important information on the structural requirements for the electron transfer reaction between redox groups present in biological molecules.