Essential role of a single arginine of photosystem I in stabilizing the electron transfer complex with ferredoxin

PsaE is one of the photosystem I subunits involved in ferredoxin binding, The central role of arginine 39 of this 8-kDa peripheral polypeptide has been established by a series of mutations. The neutral substitution R39Q leads to a 250-fold increase of the dissociation constant K-d of the photosystem I-ferredoxin complex, as large as the increase induced by PsaE deletion. At pH 8.0, this K-d value strongly depends on the charge of the residue substituting Arg-39: 0.22 mu M for wild type, 1.5 mu M for R39K, 56 mu M for R39Q, and more than 100 mu M for R39D. The consequences of arginine 39 substitution for the titratable histidine were analyzed as a function of pH, The K-d value of R39H is increased 140 times at pH 8.0 but only 5 times at pH 5.8, which is assigned to the protonation of histidine at low pH. In the mutant R39Q, the association rate of ferredoxin was decreased 3-fold compared with wild type, whereas an 80-fold increase is calculated for the dissociation rate. We propose that a major contribution of PsaE is to provide a prominent positive charge at position 39 for controlling the electrostatic interaction and lifetime of the complex with ferredoxin.