On the Structure and Function of Reduced Nicotinamide Adenine Dinucleotide Phosphate‐Cytochrome <i>f</i> Reductase of Spinach Chloroplasts

Abstract

NADPH‐eytochrome f reductase from spinach chloroplasts has been purified, and its amino acid composition is reported. The specificity of the enzyme for different electron acceptors has been studied. In addition to higher plant cytochrome f, Euglena gracilis cytochrome f and plastocyanin are reduced. The reaction with these acceptors occurs through a two‐step mechanism, whereas higher plant cytochrome f forms a ternary complex with NADPH and the enzyme. The K m values for NADPH in the reactions with the different electron acceptors, the K m 's for the electron acceptors and the turnover numbers are reported. The sulphydryl reagents slowly inactivate the enzyme and enhance the fluorescence of enzyme‐bound FAD and of the tryptophan residues of the protein. In the presence of NADPH, the inactivation is very rapid, and FADH 2 is split from the apoenzyme. Preincubation of the enzyme with NADPH inactivates reversibly all the reactions except cytochrome f reductase. The results are interpreted in terms of structure to function relationships and the significance of the NADPH‐cytochrome f reductase in photosynthetic electron transport is also discussed.