Ligand-Induced Conformational Change of Insulin-Regulated Aminopeptidase: Insights on Catalytic Mechanism and Active Site Plasticity

Insulin-regulated aminopeptidase
(IRAP) is an enzyme with several
important biological functions that is known to process a large variety
of different peptidic substrates, although the mechanism behind this
wide specificity is not clearly understood. We describe a crystal
structure of IRAP in complex with a recently developed bioactive and
selective inhibitor at 2.53 Å resolution. In the presence of
this inhibitor, the enzyme adopts a novel conformation in which domains
II and IV are juxtaposed, forming a hollow structure that excludes
external solvent access to the catalytic center. A loop adjacent to
the enzyme’s GAMEN motif undergoes structural reconfiguration,
allowing the accommodation of bulky inhibitor side chains. Atomic
interactions between the inhibitor and IRAP that are unique to this
conformation can explain the strong selectivity compared to homologous
aminopeptidases ERAP1 and ERAP2. This conformation provides insight
on IRAP’s catalytic cycle and reveals significant active-site
plasticity that may underlie its substrate permissiveness.