Arginyl groups involved in the binding of Anabaena ferredoxin–NADP+ reductase to NADP+ and to ferredoxin

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Abstract
Chemical modification of ferredoxin–NADP+ reductase from the cyanobacteria Anabaena has been performed using the α‐dicarbonyl reagent phenylglyoxal. Inactivation of both the diaphorase and cytochrome‐c reductase activities, characteristic of the enzyme, indicates the involvement of one or more arginyl residues in the catalytic process of the enzyme. The determination of the rate constants for the inactivation process under different conditions, including those in which substrates, NADP+ and ferredoxin, as well as other NADP+ analogs were present, indicates the involvement of two different groups in the inactivation process, one that reacts very rapidly with the reagent (kobs= 8.3 M−1 min−1) and is responsible for the binding of NADP+, and a second less reactive group (kobs= 0.9 M−1 min−1), that is involved in the binding of ferredoxin. Radioactive labeling of the enzyme with [14C]phenylglyoxal confirms that two groups are modified while amino acid analysis of the modified protein indicates that the modified groups are arginine residues. The identification of the amino acid residues involved in binding and catalysis of the substrates of ferredoxin–NADP+ reductase will help to elucidate the mechanism of the reaction catalyzed by this important enzyme.