Electron Transfer From Ferredoxin and Flavodoxin to Ferredoxin:NADP+ Reductase

Abstract

An overview is presented of structure/function relationships in the interactions between the small electron transfer proteins ferredoxin (Fd) and flavodoxin (Fld) and the flavoprotein enzyme ferredoxin:NADP+ reductase (FNR), primarily emphasizing the proteins from the cyanobacterium, Anabaena, and the higher plant, spinach. Results are summarized from experiments utilizing rapid-reaction kinetic methods (stopped-flow spectrophotometry and laser flash photolysis) involving wild-type and site-specific mutants of these proteins, redox potential determinations, and X-ray crystallography, including the crystal structure of a Fd/FNR complex. These have provided detailed insights into the protein–protein recognition and electron transfer mechanisms utilized by these systems. Fd and Fld bind to FNR within a concave region of the FNR surface that contains the exposed dimethylbenzene ring of the FAD cofactor. In the Fd case, electron transfer between the iron–sulfur and flavin centers proceeds with a maximum rate constant of 5,500 sec−1 via a direct outer-sphere mechanism. Both electrostatic and hydrophobic interactions occur between the proteins, resulting in a precise surface complementarity. I.