Protein-protein interaction in electron transfer reactions: The ferredoxin/flavodoxin/ferredoxin : NADP(+) reductase system from Anabaena

Electron transfer reactions involving protein-protein interactions require the formation of a transient complex which brings together the two redox ce ntres exchanging electrons. This is the case for the flavoprotein ferredoxi n:NADP(+) reductase (FNR) from the cyanobacterium Anabaena, an enzyme which interacts with ferredoxin in the photosynthetic pathway to receive the ele ctrons required for NADP(+) reduction. The reductase shows a concave cavity in its structure into which small proteins such as ferredoxin can fit. Fla vodoxin, an FMN-containing protein that is synthesised in cyanobacteria und er iron-deficient conditions, plays the same role as ferredoxin in its inte raction with FNR in spite of its different structure, size and redox cofact or. There are a number of negatively charged amino acid residues on the sur face of ferredoxin and flavodoxin that play a role in the electron transfer reaction with the reductase. Thus far, in only one case has charge replace ment of one of the acidic residues produced an increase in the rate of elec tron transfer, whereas in several other cases a decrease in the rate is obs erved. In the most dramatic example, replacement of Glu at position 94 of A nabaena ferredoxin results in virtually the complete loss of ability to tra nsfer electrons. Charge-reversal of positively charged amino acid residues in the reductase also produces strong effects on the rate of electron trans fer Several degrees of impairment have been observed, the most significant involving a positively charged Lys at position 75 which appears to be essen tial for the stability of the complex between the reductase and ferredoxin. The results presented in this paper provide a clear demonstration of the i mportance of electrostatic interactions on the stability of the transient c omplex formed during electron transfer by the proteins presently under stud y. (C) Societe francaise de biochimie et biologie moleculaire / Elsevier, P aris.