A LASER FLASH ABSORPTION-SPECTROSCOPY STUDY OF ANABAENA SP PCC-7119 FLAVODOXIN PHOTOREDUCTION BY PHOTOSYSTEM-I PARTICLES FROM SPINACH

Electron transfer from P700 in photosystem I (PSI) particles from spinach to Anabaena sp. PCC 7119 flavodoxin has been studied using laser flash absorption spectroscopy. A non-linear protein concentration dependence of the rate constants was obtained, suggesting a two-step mechanism involving complex formation (k=3.6 x 10(7) M-1.s-1) followed by intracomplex electron transfer (k=270 s-1). The observed rate constants had a biphasic dependence on the concentrations of NaCl or MgCl2, with maximum values in the 40-80 mM range for NaCl and 4-12 mM for MgCl2. To our knowledge, this is the first time that the kinetics of PSI-dependent flavodoxin photoreduction have been determined.