Complete assignment of disulfide bonds in bovine dopamine beta-hydroxylase

Peptide mapping, chemical sequencing, microbore HPLC/electrospray ionization mass spectrometry (LC/ESI/MS), and matrix-assisted laser desorption mass spectrometry (MALDI/MS) were used to identify the sites of intra- and intermolecular disulfide linkages in bovine dopamine beta-hydroxylase. The enzyme contains 14 cysteines and seven disulfides per monomer. Edman sequencing of tryptic and peptic peptides determined linkages at positions Cys140-Cys582, Cys218-Cys269, Cys255-Cys281, Cys452-Cys474, Cys514-Cys514, and Cys516-Cys516, where cysteines at positions 514 and 516 on one monomer disulfide pair with their homologs on a second monomer. These linkages were confirmed by LC/ESI/MS and MALDI/MS. Further analysis by LC/ESI/MS and MALDI/MS identified linkages at positions Cys376-Cys489 and Cys380-Cys551. Cysteines 140 and 582 form a disulfide linkage that folds the C-terminus back in proximity to the N-terminus. The remaining intramolecular disulfides occur along two separate internal regions of the protein. The density of histidine residues in these two regions suggests binding sites for two Cu2+ atoms per monomer. In addition, previously identified amino acids that react with mechanism-based inactivators occur in these two regions. We propose that these five internal disulfide bonds define two Cu2+ binding domains that make up the active site of a dopamine beta-hydroxylase monomer. Considering previous data on the location of glycosylation sites, mechanism-based inactivation sites, and the disulfide linkages presented here, the data suggest an overall topology were the N- and C-termini are in close proximity and are solvent exposed and where the Cu2+ binding sites are buried in two interior domains stabilized by five disulfide bonds.