Mechanism of photoinduced electron transfer from tyrosine to the excited flavin in the flavodoxin from Helicobacter pylori. A comparative study with the flavodoxin and flavin mononucleotide binding protein from Desulfovibrio vulgaris (Miyazaki F)

Photoinduced electron transfer in the Helicobacter pylori flavodoxin (HPFD) was investigated using both experimental and theoretical techniques. Recombinant (r)HPFD was prepared by expression in Escherichia coli and the ultrafast fluorescence dynamics of the purified rHPFD was measured by means of fluorescence up-conversion. The fluorescence was found to decay with a lifetime of 173 fs (amplitude, 0.963) and 2.08 ps (0.037). These data were used in the Kakitani and Mataga theory to calculate the electron transfer (ET) rate inside the native HPFD. Molecular dynamics simulations of HPFD were performed over a 50 ns period to enable an inclusion of the protein dynamics effect on the ET process. The ET rate from Tyr91 to an excited isoalloxazine (Iso*) ranged from 6 to 23 ps−1, depending on the analysis method used, while the ET rates from Trp63 and Tyr120 to Iso* were negligibly slow. The logarithmic ET rate and net electrostatic (ES) energy between the photo-products and ionic groups inside HPFD both displayed a parabolic relationship with the total free energy gap, indicating that the net ES energy is one of the most influential factors upon the ultrafast ET rate. Although the logarithmic ET rate should theoretically be decreased with increasing center-to-center distances according to Duttonʹs law, this relationship was not observed in this system. One possible reason for this was elucidated by comparing our results with those from the flavin mononucleotide binding protein.