[5] Peptidylglycine α-amidating monooxygenase: An ascorbate-requiring enzyme

Publisher Summary Peptidylglycine α-amidating monooxygenase (PAM) is a bifunctional enzyme that catalyzes the carboxyterminal amidation of glycine-extended peptides. Amidation is a two-step reaction, with the first step being hydroxylation at the α-carbon of the carboxy-terminal glycine, catalyzed by peptidylglycine α-hydroxylating monooxygenase (PHM), a copper, ascorbate, and molecular oxygen-dependent enzyme. The second step of the reaction is dealkylation of the peptidyl α-hydroxyglycine intermediate, catalyzed by peptidyl α-hydroxyglycine α-amidating lyase (PAL), a divalent metal ion-dependent enzyme. Peptidylglycine α-amidating monooxygenase is the only enzyme known to catalyze the α-amidation of peptides. Although PAM is encoded by a single gene, soluble and membrane-bound monofunctional and bifunctional forms are generated by tissue-specific alternative splicing and endoproteolytic cleavage. The PHM-catalyzed reaction requires a reducing cofactor and ascorbate is the most likely physiological reductant, although the cofactor requirement in vivo can be cell-type specific. Overall, the enzyme converts 2 mol of reduced ascorbate into 2 mol of semidehydroascorbate, consuming 1 mol of ascorbate for each mole of α-amidated product peptide. The optimal ascorbate concentration is typically about 1 mM. Dopamine β-monooxygenase performs a similar reaction, using the same cofactors.