Effects of protein association on the rates of photoinduced electron transfer from tryptophan residues to excited flavin in medium-chain acyl-Co A dehydrogenase. Molecular dynamics simulation

Abstract

Rates of photoinduced electron transfer (ET) from tryptophan residues (Trps) to excited isoalloxazine (Iso*) of flavin adenine dinucleotide (FAD) in medium-chain acyl Co A dehydrogenase from porcine kidney (MCAD) have been studied using its entire tetrameric structures obtained by molecular dynamics simulation (MDS) and experimental ultrafast fluorescence decays reported. The ET rates were fastest from Trp166 to Iso* among the four Trps. The emission-wavelength dependent decays were elucidated by introducing emission-wavelength dependent electron affinity of Iso*. The ET rates in Sub C were much slower than those in other subunits, which was ascribed to non-equivalent electrostatic interactions among the subunits. Effect of the protein association on the ET rates and related physical quantities were examined comparing those in the tetramer and monomer reported. The donor-acceptor distances in the tetramer were shorter than those in the monomer. The ET rates from Trp166 in the tetramer were slower than that in the monomer. The ET rates and related physical quantities were examined at seven emission wavelengths. The values of standard free energy gap between the photo-products and reactants displayed to increase as the emission wavelength becomes longer at all subunits. The ET rates displayed to decrease in Sub A, Sub C and Sub D, as the emission wavelength becomes longer.