1. Introduction
Bioactive peptides play a key role in controlling many complex pathways including blood glucose (insulin and glucagon, for example), salt and water balance (vasopressin, atrial natriuretic peptide), appetite (cholecystokinin, α-melanocyte stimulating hormone, leptin), reproduction (gonadotropin hormone releasing hormone, follicle stimulating hormone) and gastrointestinal function (gastrin, motilin). Each of these peptides is synthesized from a newly synthesized precursor protein as it moves from its site of synthesis in the endoplasmic reticulum, through the secretory pathway lumen and into the secretory granules from which it is released in response to the appropriate combination of stimuli (Chrétien & Mbikay, 2016; Clark & Lowry, 2016; Kumar, Mains, & Eipper, 2016). A limited set of subtilisin-like endoproteases, carboxypeptidase B-like exoproteases and other post-translational processing enzymes convert these precursors, which are often inactive, into active products as they move through the secretory pathway (Fig.1A ). The activity of many bioactive peptides requires α-amidation of their C-terminal residue; lacking an amidated C-terminus, their ability to bind to their receptors (generally a G Protein Coupled Receptor) is greatly reduced. Consistent with this, Peptidylglycine α-Amidating Monooxygenase (PAM), the only enzyme known to catalyze the formation of α-amidated peptides, is an essential enzyme. Mice lacking PAM survive only until mid-gestation (Czyzyk et al., 2005) and neither flies nor zebrafish lacking PAM are viable (Kolhekar, Roberts, et al., 1997; Kumar et al., 2018). Mice having a single functional Pam gene exhibit a wide variety of deficits, ranging from altered inhibitory synaptic neurotransmission to increased anxiety-like behavior and an inability to maintain body temperature in a cold environment (Bousquet-Moore et al., 2010; Gaier, Eipper, & Mains, 2014; Gaier et al., 2013). Genetic studies have identified PAM as a risk factor for type 2 diabetes (Steinhorsdottir et al., 2014; Thomsen et al., 2018).
PAM is a bifunctional enzyme: its monooxygenase domain (peptidylglycine α-hydroxylating monooxygenase, PHM: EC 1.14.17.3) catalyzes the copper- and ascorbate-dependent α-hydroxylation of its peptidylglycine substrates; its lyase domain (peptidyl-α-hydroxyglycine α-amidating lyase, PAL: EC 4.3.2.5; also called peptidylamidoglycolate lyase) then generates the α-amidated product peptide plus glyoxylate (Fig.1B ). C-terminal amidation can alter peptide structure, confer resistance to proteolytic degradation and reduce the effects of pH on receptor binding (Kahns & Bundgaard, 1991; Luxmi, Mains, King, & Eipper, 2021). Extensive mechanistic studies and structural studies facilitated the development of PAM inhibitors. In this brief review we will summarize several successful approaches taken to develop inhibitors of PHM and discuss observations indicating that PAM may serve as a useful therapeutic target or biomarker.