Interactions between isoalloxazine and o-aminobenzoate in o-aminobenzoate−d-amino acid oxidase complex. Molecular dynamics and molecular orbital studies

Abstract

Molecular interactions between o-aminobenzoate (oAB) and isoallxazine (Iso) in o-aminobenzoate - d-amino acid oxidase complex (ODC) were studied by means of molecular dynamics simulation (MDS) and molecular orbital (MO) method. The distances between oAB and Iso were 0.60 in subunit A (Sub A) and 0.58 nm in Sub B. Four of six heteroatoms of Iso ring formed hydrogen bonding (HB) with nearby amino acids. The oxygen atom and nitrogen atom of oAB also formed HB with amino acids. The ODC displays a broad absorption band around 570 nm, which is considered to be charge transfer (CT) band between oAB and Iso in the complex. The ODC displayed fluorescence at around 520 nm, which is different from that of DAAO (emission peak at 535 nm) without oAB. The emission is from a local excited state (LE) without CT interaction. The CT interaction was studied by a semi-empirical molecular orbital (MO) method, using the structures of oAB and Iso obtained by MDS. The CT state did not form in the ground state of Iso, but formed in the excited state of Iso (Iso*), when the both cannot move appreciably from the MDS structures. The CT states formed both in the oAB – Iso and oAB – Iso* (excited state of Iso) systems when the both molecules can move so as to attain the minimum energy state. It is concluded that a part of oAB could move to form CT complex with Iso in the ground state, which displays the CT absorption band, but others could not move in the protein without CT interaction, which displays the fluorescence.