Flavin photochemistry in the analysis of electron transfer reactions: role of charged and hydrophobic residues at the carboxyl terminus of ferredoxin-NADP(+) reductase in the interaction with its substrates

The enzyme Ferredoxin-NADP(+) reductase participates in the reductive side of the photosynthetic chain transferring electrons from reduced Ferredoxin (Fd) (or Flavodoxin (Fld)) to NADP(+), a process that yields NADPH that can be used in many biosynthetic dark reactions. The involvement of specific amino acids in the interaction between the two proteins has been studied using site-directed mutagenesis. In the present study, the participation of charged (H299), polar (T302) or hydrophobic (V300) amino acid residues that are in the NADP(+)-binding domain of the reductase have been examined by analyzing its C-terminal region, which is located close to the active site. Stopped-flow and laser flash photolysis results of the reaction in which these mutant proteins participate show very little differences with respect to the wild-type protein. These results suggest that the NADPH-binding domain of the reductase has little effect on the processes of recognition and electron transfer to (and from) Fd or Fld, according to the recently reported crystallographic structure of the FNR/Fd complex.