Structural Characterization and Mapping of the Covalently Linked FAD Cofactor in Choline Oxidase from Arthrobacter globiformis

The flavoenzyme choline oxidase catalyzes the oxidation of choline and betaine aldehyde to betaine. Earlier studies have shown that the choline oxidase from Arthrobacter globiformis contains FAD covalently linked to a histidine residue. To identify the exact type of flavin binding, the FAD-carrying amino acid residue was released by acid hydrolysis. The fluorescence excitation maxima of the isolated aminoacylriboflavin, showing a hypsochromic shift of the near-ultraviolet band relative to riboflavin, and the pH-dependent flavin fluorescence confirmed the presence of an 8α-substituted flavin linked to histidine. Similarly, MALDI-TOF mass spectrometry showed a molecular mass corresponding to histidylriboflavin. Classical experiments used to distinguish between the N(1) and N(3) isomers all indicated that the flavin was linked to the N(1) position of the histidine residue. The position of the FAD-carrying histidine residue in the choline oxidase polypeptide was identified by tryptic cleavage of the denatured enzyme, HPLC separation of the proteolytic peptide fragments, and characterization of the purified flavin-carrying peptide by mass spectrometry and spectroscopy. The FAD moiety was assigned to the tryptic peptide, His-Ala-Arg, corresponding to residues 87−89 in the open reading frame of the previously published cDNA sequence. Further analysis of the flavopeptide by collision-induced dissociation mass spectrometry confirmed that the flavin cofactor was attached to His87. We conclude that this variant of choline oxidase contains 8α-[N(1)-histidyl]FAD at position 87 in the polypeptide chain.