A molluscan peptide alpha-amidating enzyme precursor that generates five distinct enzymes.

Mechanisms underlying the specificity and efficiency of enzymes, which modify peptide messengers, especially with the variable requirements of synthesis in the neuronal secretory pathway, are poorly understood. Here, we examine the process of peptide alpha-amidation in individually identifiable Lymnaea neurons that synthesize multiple proproteins, yielding complex mixtures of structurally diverse peptide substrates. The alpha-amidation of these peptide substrates is efficiently controlled by a multifunctional Lymnaea peptidyl glycine alpha-amidating monooxygenase (LPAM), which contains four different copies of the rate-limiting Lymnaea peptidyl glycine alpha-hydroxylating monooxygenase (LPHM) and a single Lymnaea peptidyl alpha-hydroxyglycine alpha-amidating lyase. Endogenously, this zymogen is converted to yield a mixture of monofunctional isoenzymes. In vitro, each LPHM displays a unique combination of substrate affinity and reaction velocity, depending on the penultimate residue of the substrate. This suggests that the different isoenzymes are generated in order to efficiently amidate the many peptide substrates that are present in molluscan neurons. The cellular expression of the LPAM gene is restricted to neurons that synthesize amidated peptides, which underscores the critical importance of regulation of peptide alpha-amidation.