Peptidylglycine alpha amidating enzyme
Release of peptides from large dense core vesicles (LDCVs; also known as secretory granules) occurs in dendrites, the cell soma, along the axon and at the nerve terminal (]) than release of small synaptic vesicles and many neuropeptides must diffuse some distance before encountering the receptors that identify their targets.Since the synthesis, storage and secretion of all neuropeptides require a common set of enzymes, manipulation of the genes encoding these enzymes has provided insight into the most sensitive elements of the peptidergic system.Like other m RNAs encoding secreted proteins, they include an N-terminal signal sequence that is removed co-translationally.The propeptide is then modified by the same machinery used for the synthesis of integral membrane proteins and other secreted proteins to ensure proper folding, disulfide bond formation, phosphorylation and glycosylation.Copper is the only metal that supports peptide amidation; no other metal can substitute.The N-terminal Cu site is formed from three His residues, two of which are adjacent, and most resembles copper binding sites used for electron transfer (Prigge et al., 1997; Prigge et al., 1999; Siebert et al., 2005). Despite its essential role, copper is not tightly bound to PHM (~0.06 μM); copper bound to PHM is lost during purification, necessitating the addition of exogenous copper to optimize catalytic activity (Bell et al., 2003).
Ca(II), which sits in the middle of the central cavity, plays a structural role.The active site sits in a cup formed by the loops connecting various β-strands.The active site includes a Zn(II) ion coordinated to three His residues.We review data indicating that genetic reduction of PAM expression and mild copper deficiency in mice cause similar alterations in several physiological functions known to be regulated by neuropeptides - thermal regulation, seizure sensitivity and anxiety-like behavior.(Asada et al., 2005; Grimmelikhuijzen et al., 1996; Kolhekar et al., 1997; Mair et al., 2004; Vos et al., 1995).