Role of acidic amino acid residues of PsaD subunit on limiting the affinity of photosystem I for ferredoxin

The PsaD subunit of photosystem I is one of the central polypeptides for th e interaction with ferredoxin, its acidic electron acceptor. In the cyanoba cterium Synechocystis 6803, this role is partly performed by a sequence ext ending approximately from histidine 97 to arginine 119, close to the C-term inus. In the present work, acidic amino acids D100, E105, and E109 are show n to moderate the affinity of Photosystem I for ferredoxin. Most single rep lacements of these residues by neutral amino acids increased the affinity f or ferredoxin, resulting in a dissociation constant as low as 0.015 muM for the E105Q mutant (wild-type K-D = 04 muM). This is the first report on the limitation of photosystem I affinity for ferredoxin due to acidic amino ac ids from PsaD subunit. It highlights the occurrence of a negative control o n the binding during the formation of transient complexes between electron carriers. (C) 2001 Academic Press.