Kinetics of some electron-transfer reactions in biological Photosystems II. Study of intramolecular electron-transfer rates between ferredoxin-NADP reductase, NADP radical and oxidized ferredoxin

It has been shown by the pulse radiolysis technique that radiation-generated NADP free radicals (NADP·) first combine with ferredoxin-NADP reductase and then transfer the odd electron by a fast intramolecular process to the enzyme flavin moiety yielding the semiquinone (ferredoxin-NADP reductase, FNR-FADH·). The corresponding first-order rate constant k₁₅ varies with ionic strength from 2.6·10³ s⁻¹ at I = 0.66 M to 2.3·10⁴ s⁻¹ at I = 0.005 M In the presence of ferredoxin-NADP reductase-bound oxidized ferredoxin, the electron cascades, thus further reducing the ferredoxin. The transfer of the electron from the flavin semiquinone (ferredoxin-NADP reductase, FNR-FADH·) to the bound oxidized ferredoxin proceeds at a rate of k₁₈ = 2.36 s⁻¹. This process approaches an equilibrium condition which is in favor of the reverse reaction suggesting that k₋₁₈ > k₁₈.