An essential lysyl residue (Lys208) in the substrate-binding site of porcine FAD-containing monooxygenase.

The substrate (amine)-binding site of porcine FAD-containing monooxygenase (FMO) (EC 1.14.13.8) was examined using pyridoxal 5'-phosphate (pyridoxal-P) to modify lysyl residues. The enzymic activity of the FMO was inhibited competitively by pyridoxal-P. Upon reduction of pyridoxal-P-treated FMO with NaBH4, a new characteristic absorption peak of substituted pyridoxal-P appeared at 325 nm. The amino acid residue compositions of the native and pyridoxal-P-treated FMOs indicated that the lysyl residues were modified by pyridoxal-P. The about 74% inactivation of the enzymic activity on covalent pyridoxal-P treatment of the FMO was nearly completely prevented in the presence of the substrate, N,N-dimethylaniline. The FMO covalently modified with pyridoxal-P in the presence or absence of N,N-dimethylaniline was digested with trypsin treated with tosylphenylalanylchloromethane and the resultant peptide fragments were separated with a reverse-phase high-performance liquid chromatography system; only one peptide was specifically labeled with pyridoxal-P and was detected at 325 nm in the absence of N,N-dimethylaniline. The modified peptide was analyzed and identified as that comprising the amino acid residues 186-208. These results suggest that Lys208 plays an important role in the substrate (amine)-binding site of FMO.