DNPEP

DNPEP Full Name

aspartyl aminopeptidase

DNPEP Introduction

DNPEP, also known as aspartyl aminopeptidase or aspartate aminopeptidase, is a cytosolic zinc-dependent aminopeptidase that specifically cleaves N-terminal acidic amino acids (aspartate and glutamate) from peptides and proteins. The human DNPEP gene is located on chromosome 2q35, spans approximately 15 kb, and contains 15 exons encoding a 436-amino-acid protein with a molecular weight of approximately 52 kDa. DNPEP belongs to the peptidase family M18, which includes aspartyl aminopeptidases from bacteria, archaea, and eukaryotes, and it shares structural homology with the bacterial enzyme peptidase A (PepA). The protein assembles into a homo-octameric complex (four dimers arranged in a ring-like structure) that is stabilized by zinc ions at the dimer interfaces. DNPEP is ubiquitously expressed in human tissues, with the highest levels detected in the kidney, liver, brain, and placenta, and lower expression in the heart, skeletal muscle, and lung. Within the cell, DNPEP is predominantly cytosolic, though a fraction has been reported in the nucleus and associated with the endoplasmic reticulum in some cell types. The enzyme is highly specific for N-terminal aspartate and glutamate residues, with a strong preference for aspartate over glutamate (approximately 5- to 10-fold higher catalytic efficiency for Asp versus Glu).

Figure 1. Schematic structure of DNPEP.

Enzymatic Properties and Catalytic Mechanism
DNPEP catalyzes the hydrolytic removal of N-terminal aspartate (Asp) and glutamate (Glu) from peptides and proteins, releasing the free acidic amino acid and a truncated peptide. The enzyme is a zinc-dependent metallopeptidase, containing two zinc ions per monomer that are coordinated by conserved histidine and glutamate residues (His-97, Glu-101, His-104, His-297, Glu-301, His-383 in the human sequence). The catalytic mechanism proceeds through a general base-general acid mechanism: one zinc ion activates a water molecule for nucleophilic attack on the carbonyl carbon of the peptide bond, while the other zinc ion stabilizes the tetrahedral transition state. A conserved tyrosine residue (Tyr-367) and a glutamate residue (Glu-383) participate in proton transfer and substrate positioning. The specificity for N-terminal acidic residues arises from a positively charged binding pocket that interacts with the side chain carboxyl groups of aspartate and glutamate. DNPEP has a pH optimum of 7.0-7.5, consistent with its cytosolic localization, and is inhibited by divalent metal chelators such as EDTA and 1,10-phenanthroline (which remove the essential zinc ions), as well as by the aminopeptidase inhibitor bestatin (IC50 approximately 10-50 μM).

Physiological Role in Intracellular Peptide Degradation
The primary function of DNPEP is thought to be the final step of the ubiquitin-proteasome pathway, in which proteins are degraded into short peptides (3-15 amino acids) by the proteasome, and these peptides are then hydrolyzed to free amino acids by a network of cytosolic aminopeptidases and endopeptidases. The proteasome generates peptides with diverse N-terminal residues, including a significant fraction (5-10%) with N-terminal aspartate or glutamate. These acidic N-terminal peptides are poor substrates for most aminopeptidases, which prefer hydrophobic or basic N-terminal residues. DNPEP is the only human enzyme that efficiently removes N-terminal aspartate and glutamate from such peptides, thereby allowing their complete degradation to free amino acids. In cells lacking DNPEP (knockdown or knockout models), peptides with N-terminal acidic residues accumulate in the cytosol, and the rate of amino acid recycling from proteasome-generated peptides is reduced.

Alternate Names for DNPEP

DNPEP; aspartyl aminopeptidase; DAP; ASPEP;