Identification of specific carboxyl groups on Anabaena PCC 7119 flavodoxin which are involved in the interaction with ferredoxin‐NADP+ reductase

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Abstract
Flavodoxin from the nitrogen‐fixing cyanobacteria Anabaena PCC 7119 forms an electron‐transfer complex with ferredoxin – NADP+ reductase (FNR) from the same organism. The complex is mainly governed by electrostatic interactions between side‐chain amino groups of the reductase and carboxyl residues of flavodoxin. In order to localize the binding site on flavodoxin, chemical modification of its carboxyl groups has been carried out. Treatment of flavodoxin with a water‐soluble carbodiimide, N‐ethyl‐3‐(3‐dimethylaminopropyl)carbodiimide (EDC), in the presence of a nucleophile, glycine ethyl ester, caused a time‐dependent modification of the protein that is responsible for the loss of its ability to participate as electron carrier in the photoreduction of NADP+ by chloroplast membranes, and also in NADPH – cytochrome‐c reductase activity, by about 85%. Nevertheless, the ability of flavodoxin to receive electrons from the reducing side of photosystem I was much less affected. The inhibition was enhanced at low pH, suggesting that carboxylic acid groups were the target of chemical modification. Treated flavodoxin failed to form covalent complexes with FNR and the dissociation constant for the non‐covalent complex with FNR was fourfold higher. After tryptic digestion of a sample of flavodoxin modified by EDC in the presence of [1‐14C]glycine ethyl ester, two major radioactive peptides were isolated. The first protein fragment contained three carboxylic residues (Asp123, Asp126 and Asp129), corresponding to the region where long‐chain flavodoxins show an insert compared to short‐chain flavodoxins. The second peptide corresponded to a similar region, either in the amino acid sequence or in the three‐dimensional structure of the protein and also containing three carboxyl groups (Asp144, Glu145 and Asp146). Four of these carboxyl groups (Asp123, Asp126, Asp144 and Asp146) are highly conserved in all long‐chain flavodoxins, suggesting that they could play an essential role in substrate recognition.